The effects of root growth control using root restricting bags on the

growth, fruitfulness, and fruit calcium concentrations and contents

of apple (Malus x domestica Borkb. cv. 'Fuji').

A thesis

submitted in partial fulfilment

of the requirements for the degree

of

Master of Horticulture Science

at

Lincoln University

by

M.D. WHITE

Lincoln University

1995

Abstract of thesis submitted in partial fulfilment of the requirements for

the degree of Master of Horticultural Science

The effects of root growth control using root restricting bags

on the growth, fruitfulness, and fruit calcium concentrations and contents

of apple (Malus x domestica Borkh. cv. 'Fuji').

by

M.D. WHITE

ii

A field trial was planted in the spring of 1989 to investigate the effects of root growth

control using root restricting bags on the growth, fruitfulness, and fruit calcium concentrations

and contents of apple (Malus x domestica Borkh. cv. 'Fuji').

The trial was planted as a 2 x 5 factorial randomised complete block design. Trees were

conventionally planted or planted in in-ground fabric containers of 15, 34, 65 or 139 litre

volume on MMI06 or MM115 rootstocks. The fabric container used was the Root Control

BagSM , (Root Control Inc., Oklahoma City, OK). These containers are cylindrical, with one end

closed with clear plastic to form the bottom of the bag and the wall material is porous which

allows new root growth in the form of fme roots around the periphery of the container.

Trees were planted in two rows at a spacing of 3.0 x 5.0 metres. Throughout the

experiment, trees were untrained, un pruned , and allowed to develop natural crown form and

size. Fruit thinning was not carried out in any year to avoid obscuring treatment effects. The

within-row and inter-row area was maintained weed free using an overall herbicide spray

programme. Irrigation water was applied through drippers with all trees receiving the same

iii

frequency of irrigation and amount of water. In other respects, normal orchard management

practices were followed.

Root restriction reduced tree trunk cross-sectional area (TCSA) in the years 1991 to

1994, and canopy volume and tree height in 1995 when compared with the unconfmed treatment.

Throughout the experimental period, TCSA decreased linearly as bag volume was reduced.

However, growth differences in TCSA within bag sizes were not reflected in canopy volumes.

Trees grown on MMI 06 rootstock compared with MM115 had smaller TCSA and canopy

volumes. However, root restriction reduced the invigorating effect of MM115 as the difference

in TCSA or canopy volume (the absolute difference or expressed as a percentage) between the

two rootstocks was considerably greater in the unconfined than in the bagged treatments.

The smaller trees resulting from root restriction had lower yields when compared with

the unconfined treatment. Throughout the experiment, there were no differences in yields per

tree within bag volumes or between rootstocks.

However, the cumulative yields per unit of TCSA were higher where roots were

restricted, indicating that these trees had a higher productive efficiency. The effect of root

restriction across all bag volumes was to increase cumulative yield efficiency by between 215

to 1589 grams per cm2 of TCSA at the 95% confidence interval.

Treatment differences in mean fruit weight in 1992 to 1994 and fruit size distribution in

1994 were attributable to their effect on crop density. Crop densities (fruit number/TCSA) and

yield efficiencies (yield per tree/TCSA) between the bagged and unconfined treatments varied

between years indicating the trees experienced an 'over charge' of fruiting. Confirming this

view, all trees showed a biennial bearing trend although deviations in yield from constant

cropping were not strong. The propensity towards biennial bearing did not differ between

treatments.

iv

Root restriction increased apple fruit calcium concentrations and contents. This effect was

independent of fruit size and was not adequately explained by differences in crop density

between treatments.

KEYWORDS: Apple (Malus x domestica Borkh. cv. 'Fuji'); root restriction; rootstocks;

yield efficiency; biennial bearing; fruit calcium.

· To Mum and Dad.

vi

ACKNOWLEDGEMENTS

I would like to acknowledge the contribution of Gilbert Wells who generously granted

me access and the use of all data collected prior to 1992 presented within this thesis.

I am indebted to the New Zealand Apple and Pear Marketing Board for their generous

fmancial support.

I am sincerely grateful for the time and guidance of my supervisors, Professor R.N.

Rowe and Dr M.e.T. Trought. Without their advice, editorial assistance, and encouragement

this project would still be down in the orchard.

Again I must thank Gilbert Wells for the many hours spent pouring over the figures,

sharing his invaluable knowledge of statistics, and the time spent proofmg parts of this thesis.

I dedicate this work to my parents and family. Without their support, encouragement and

understanding none of this would have been possible. Yes I'm fmished Dad. I would like to

thank all my friends for their encouragement and support, which sometimes went beyond the call

of duty. As he won't let me forget, your apple picking skills are unbelievable Simon. Finally

a thank you to all those who have helped me and not received a mention.

MI)'1L M.D. White

vii

CERTIFICATE

I hereby certify that the experimental work contained in this thesis was planned, executed and

described by the candidate, under the direct supervision of Professor R.N. Rowe and Dr M.C.T

Trought. Data presented within this thesis prior to 1992 was collected by Mr G. Wells.

R.N.Rowe

(Supervisor)

viii

TABLE OF CONTENTS

HEADING PAGE

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ii

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

List of figures and plates .................................. xiii

CHAPTER 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1

CHAPTER 2: Review of the literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

2.1. Malus x Domestica Borkh. ............................... 3

2.1.1. Growth habit and phenology . . . . . . . . . . . . . . . . . . . . . . . .. 3

2.1.1.1. Shoot growth ............................ 3

2.1.1.2. Root growth . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4

2.1.1. 3. Bud development and fruit growth ............... 5

2.2. Adverse effects of vegetative growth on fruit production ............. 7

2.2.1. Controlling vegetative growth . . . . . . . . . . . . . . . . . . . . . . .. 8

2.2.2. Physiological control ............................. 8

2.2.3. Cultural control ................................ 9

2.3. Coordination of growth between the root and shoot ............... 12

2.4. Manipulation of shoot growth by root treatments . . . . . . . . . . . . . . . .. 14

2.4.1. Root pruning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14

2.4.1.1. Effects on plant growth . . . . . . . . . . . . . . . . . . . .. 15

2.4.1.2. Effect on root:shoot ratio . . . . . . . . . . . . . . . . . . .. 16

ix

2.4.1.3. Fruiting and fruit quality .................... 19

2.4.2. Root restriction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 20

2.4.2.1. Effects on plant growth . . . . . . . . . . . . . . . . . . . .. 21

2.4.2.2. Effect on root:shoot ratio . . . . . . . . . . . . . . . . . . .. 23

2.4.2.3. Fruiting and fruit quality .................... 26

2.4.2.4. Conclusion .......... ~ . . . . . . . . . . . . . . . . .. 28

CHAPTER 3: The effects of root growth control using root restricting bags on the growth and

fruitfulness of apple (Malus x domestica Borkh. cv. 'Fuji') .................. 29

3.1. Introduction ...................................... . . 29

3.2. Materials and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

3.2.1. Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

3.2.2. Experimental design . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 33

3.2.3. Measurement of tree growth and fruitfulness . . . . . . . . . . . . .. 34

3.2.3.1. Measurement of vegetative growth and fruiting . . . . . .. 34

3.2.3.2. Productivity comparisons . . . . . . . . . . . . . . . . . . .. 34

3.2.4. Description of terminology used and statistical analysis . . . . . . .. 35

3.3. Results .......................................... 38

3.3.1. Vegetative growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 38

3.3.2. Fruiting .................................... 39

3.3.3. Productivity comparisons . . . . . . . . . . . . . . . . . . . . . . . . .. 40

3.4. Discussion ........................................ 59

3.4.1. Vegetative growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59

3.4.2. Fruiting .................................... 61

3.4.3. Tree productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 64

x

CHAPTER 4: The effects of root growth control using root restricting bags on fruit

calcium concentrations and contents of apple (Malus x domestica Borkh. CV. 'Fuji') 67

4.1. Introduction ....................................... 67

4.2. Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 70

4.2.1. 1993 experimental sampling and analysis ................ 70

4.2.2. 1994 experimental sampling and analysis ................ 71

4.2.3. Statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 72

4.3. Results .......................................... 73

4.4. Discussion ....................... ~ . . . . . . . . . . . . . . .. 79

CHAPTER 5: Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 84

CHAPTER 6: Agronomic implications and further research . . . . . . . . . . . . . . . . .. 86

6.1. Agronomic implications ......... . . . . . . . . . . . . . . . . . . . . . .. 86

6.2. Further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 88

Bibliography .............................................. 90

Appendices ............................................... 113

xi

LIST OF TABLES

TABLE PAGE

Table 3.1. The effect of bag volume and rootstock on tree trunk

cross-sectional area growth (cm2) in the years 1991,

1992, 1993, and 1994 ........................... 45

Table 3.2. The effect of bag volume and rootstock on tree height

Table 3.3.

and canopy volume in 1995 . . . . . . . . . . . . . . . . . . . . . . .. 46

The effect of bag volume and rootstock on yield per

tree in the years 1991, 1992, 1993, 1994 and cumulated

yield per tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 48

Table 3.4. The effect of bag volume and rootstock on mean fruit

Table 3.5.

Table 3.6.

weight (grams) in the years 1991, 1992, 1993, and 1994 ...... 49

The effect of bag volume and rootstock on yield efficiency

and crop density in the years 1991, 1992, 1993, and

1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 52

The effect of bag volume and rootstock on bienniality

indices calculated from yield and yield efficiency

figures in 1991, 1992, 1993, 1994 . . . . . . . . . . . . . . . . . . .. 53

Table 3.7.

Table 3.8.

Table 4.1.

Table 4.2.

The effect of bag volume and rootstock on cumulative

yield efficiency and the slope ,of lines of cumulative

xii

yield plotted against trunk cross-sectional area . . . . . . . . . . . .. 54

Number of trees by treatment used to calculate the frequency

polygons of figures 3.6 and 3.7, and the mean fruit size

calculated from fruit size distributions of the treatments . . . . . .. 58

The effect of bag volume, rootstock and fruit size on apple

fruit calcium concentration in the years 1993 and 1994 ....... 75

The effect of bag volume, rootstock and fruit size on apple

fruit calcium content in the years 1993 and 1994 ........... 76

xiii

LIST OF FIGURES AND PLATES

FIGURE PAGE

Figure 3.1. The effect of bag volume on tree trunk cross-sectional

area in the years 1991, 1992, 1993, and 1994 . . . . . . . . . . . .. 44

Figure 3.2. The effect of root restriction on the relationship between

tree trunk cross-sectional area in 1994 and canopy volume

in 1995 .................................... 47

Figure 3.3. The effect of crop density on mean fruit weight in 1991

and 1992 ................................... 50

Figure 3.4. The effect of crop density on mean fruit weight in 1993

and 1994 ................................... 51

Figure 3.5. The effect of bag volume and rootstock on the relationship

between cumulated yield and tree trunk cross-sectional area

growth over the years 1991 to 1994 . . . . . . . . . . . . . . . . . .. 55

Figure 3.6. Frequency polygons of the fruit size distribution of trees

in an 'on phase' in 1994 on A. MM106 and B. MMl15

rootstock ................................... 56

xiv

Figure 3.7. Frequency polygons of the fruit size distribution of trees

in an 'off phase' in 1994 on A. MM106 and B. MM115

rootstock ................................... 57

Figure 4.1. The effect of crop density on apple fruit ca:Icium concentration

in 1993 .................................... 77

Figure 4.2. The effect of crop density on apple fruit calcium concentration

PLATE

Plate 3.1.

Plate 3.2.

Plate 3.3.

Plate 3.4.

in 1994 .................................... 78

10 litre root control bagSM in situ in 1994 . . . . . . . . . . . . . . .. 37

Malus x domestica Borkh. cv. 'Fuji' growing in 10 litre root

control bagSM on MMI06 rootstock in 1993 .............. 42

Malus x domestica Borkh. cv. 'Fuji' growing in 102 litre root

control bagSM on MMI06 rootstock in 1993 .............. 42

Malus x domestica Borkh. cv. 'Fuji' growing with unconfmed root

zone volume on MM106 rootstock in 1993 . . . . . . . . . . . . . .. 43

1

CHAPTER 1

Introduction.

Achieving the proper balance between vegetative and reproductive growth has long been

viewed as a major factor for maximising yields throughout the life of the orchard. Vegetative

growth is essential to maintain the bearing surface for flowers and fruit and to maintain tree

vigour (Elfving, 1988). However, excessive vegetative growth can reduce flower initiation

(Mika, 1986), fruit-set (Forshey and Elfving, 1989), fruit growth (Williamson et al., 1992), and

light penetration into the tree canopy (Jackson, 1968). The characteristics of many growing

regions such as high soil fertility, mild growing conditions and moderate to high rainfall or

irrigation ensures that vigour control is a essential management practice.

A further consequence of excessive vegetative vigour is the development over time of

large tree size. Increasing the planting density of fruit tree crops has become common place in

many countries throughout the world and has attracted further evaluation in New Zealand as

orchardists seek to become more efficient and produce more fruit per area of orchard. However,

reduced light penetration, internal shading, and poor manageability (Boswell et al., 1975;

Golomb, 1988) have been associated with the failure to control tree size in planting systems

incorporating high tree density.

Genetic control through the introduction of size-controlling rootstocks and scions is

viewed by many authors (eg. Ferree, 1988; Martin, 1989) as the long-term solution to

controlling vegetative growth in fruit tree crops because of its lasting effects and low

environmental impact. However, the nature of such work is long term and other techniques for

controlling vegetative growth will continue to be important in the immediate future (Ferree,

1988). Furthermore the integration of various growth control techniques is likely to achieve the

best results for specific locations.

2

The majority of research directed at controlling tree growth has focused on managing the

above-ground portions of the tree (Ferree et al., 1992). The existence of the close coordination

of growth between the root and shoot opens up the possibility that manipulation of the root

system could be used to control growth and performance of above-ground portions. Confining

root volumes by using barriers that physically restrict the root system, root competition between

plants or through partial irrigation to the soil is one option to achieve control of tree growth.

Root restriction has been reported to result in a reduction in top growth of deciduous

tree crops (Myers, 1992; Bravdo et al., 1992; Williamson et al., 1992). However, few studies

have reported the effects of long-term root restriction in the field. Furthermore the effects of

root restriction on fruit quality has received scant attention. Lower foliar concentrations of

calcium have been reported for peach seedlings (Richards and Rowe, 1977a) and Euonymous

kiautschovica (Dubik et al., 1990) subjected to root restriction. AI-Sabaf (1984) and Aloni

(1986) concluded from pot experiments that root restriction impairs the translocation of calcium

to the shoots. Mineral concentrations, particularly calcium, are important determinants of apple

fruit quality during storage (Ferguson and Watkins, 1989).

This study investigates the effect of root restriction using root restricting bags on the

growth and fruitfulness of Malus x domestica Borkh. cv. 'Fuji' over a range of bag volumes.

Root growth is not completely restricted with the fabric bags used in this trial as the bags

material is porous, and allows new root growth in the form of [me roots around the periphery

of the container. The effect of root restriction on apple fruit calcium concentrations and contents

was also investigated.

3

CHAPTER 2

Review of the literature.

2.1. Malus x domestica Borkh.

Malus x domestica Borkh. is likely a hybrid between Malus sylvestris (crab apple), Malus

dasyphylla, and Malus praecox, all native to Europe, and some Asiatic species (Hickey and

King, 1988). The commercial apple tree is a small to medium sized composite, woody perennial.

Growth habit ranges from erect to spreading depending on rootstock, scion, and training system.

The fruit of Malus x domestica Borkh. can be sweet with a wide range of colours (Hickey and

King, 1988) providing a valuable food source to attract birds, the main vector for seed dispersal.

2.1.1. Growth habit and phenolo2Y

2.1.1.1. Shoot lUowth

Shoots originate from dormant vegetative or mixed buds. Apple trees produce three types

of shoots: [1] terminal shoots which develop from terminal buds on the previous season's shoots,

[2] lateral shoots which develop from lateral buds on the previous season's shoots, and [3]

bourse shoots which develop from buds in leafaxils at the base of flower clusters (Abbott, 1960;

Forshey et al., 1987; Forshey and Elfving, 1989).

The shoot system consists of long (extension) shoots and short shoots (spurs or bourses)

(Hansen, 1967; Pratt, 1990) with distinctive differences in appearance. Bland (1978) (as cited

by Pratt, 1990) concluded that long shoots have longer, but not more internodes than short

shoots. Leaves are ovate-elliptical, serrate, usually rounded at the base (Hickey and King, 1988)

differing in thickness and size depending on position within the canopy. Leaves of extension and

4

bourse shoots are thicker and larger than those of fruiting spurs (Cowart, 1935; Struckmeyer and

Roberts, 1942) and they contribute proportionately more to leaf area than to leaf numbers

(Maggs, 1960; Forshey et aI., 1983; Forshey et al., 1987).

With increasing temperatures, buds break to give rise to shoots or flowers. Shoot growth

begins in early spring in the period just before to just after full bloom and is made at the

expense of reserve carbohydrates accumulated in the woody tissue the previous year. The shoots

become self-supporting after five to eight leaves develop and only about 20% of the

carbohydrates used in shoot growth comes from reserves (Hansen, 1967; Johnson and Lakso,

1986). Growth is very rapid in the 3-4 weeks after full bloom (Forshey et aI., 1983) and is

usually complete by mid summer (Forshey and Elfving, 1989). Trees continue to increase in dry

weight after shoot growth ceases (Maggs, 1960) due in part to root growth and in part to

secondary thickening of the trunk and branches (Forshey and Elfving, 1989). In response to

shorter days and lower temperatures, leaves senesce and the tree enters a period of dormancy.

2.1.1.2. Root 2fowth

The root system of the apple tree arises adventitiously producing a shallow, horizontal

scaffold of roots with vertical sinkers (Atkinson, 1980; Pratt, 1990). The main scaffold roots

spread and branch extensively (Coker, 1958). Young roots are initially white and succulent with

short root hairs. After a period of one to four weeks (Atkinson, 1980), browning as a result of

death and shedding of the cortex, spreads from the older regions towards the root tip. After the

loss of the cortex, secondary thickening occurs in some roots which then become part of the

permanent root system. Other roots usually laterals, either remain unthickened or disappear

completely (Atkinson and Wilson, 1979; Atkinson, 1980). Individual roots may continue growth

for several months, producing lateral roots, or they may cease growth after only a few weeks

(Head, 1966).

5

Generally the root system extends horizontally beyond the spread of the branches (Rogers

and Vyvyan, 1934; Atkinson, 1980; Pratt, 1990) and can reach considerable depths depending

on soil type, tree density, rootstock, and scion cultivar. Apple roots have been reported at depths

of over eight metres although generally most roots are found at 0 to 80 centimetres and

approximately 70% of the root weight occurs at a 0 to 30 centimetre depth (Atkinson, 1980).

As a result of low root densities, apples are liable to be susceptible to adverse soil conditions

or plant competition (Atkinson and Wilson, 1979; Atkinson, 1980).

Generally root extension begins in spring before shoot development and continues through

the growing season concurrently with shoot growth (Cripps, 1970; Atkinson, 1980). Very little

root growth takes place during the winter months (Head, 1966), probably due to low soil

temperatures (Cripps, 1970). However, continued root growth throughout winter has been

reported in warmer temperate regions (Zhang, 1993). Studies have shown a peak of root growth

in early summer ending at the time of vigourous shoot growth and a second smaller peak in

early autumn beginning after shoot growth has ended (Head, 1967; Rogers and Head, 1969).

This bimodal periodicity for apple root growth has been associated with competition between

shoots and roots for carbohydrate reserves (Atkinson, 1980).

2.1.1.3. Bud development and fruit KI"owth

Apple fruit buds are mixed buds borne terminally on fruiting spurs and/or terminally or

laterally on extension shoots (Buban and Faust, 1982; Pratt, 1988). Most commercially important

cultivars produce the bulk of their crops on spurs although young trees and trees in high vigour

may produce a large part of the crop on last year's shoots (Forshey and Elfving, 1989). The

apple inflorescence is determinate consisting of 6-8 white to pink flowers. Flowers are epigynous

and hermaphroditic (Pratt, 1988). The ovaries of the flowers are imbedded in tissue which as

well as the carpel tissues become fleshy to form the fruit, a pome. The accessory tissue has been

6

suggested to form from the receptacle or from the fused bases of the sepals, petals, and stamens

(Pratt, 1988). Fruit vary in size, colour, shape, and flavour with cultivar.

Vegetative or mixed buds form in the season prior to their emergence. However, some

buds after they have formed may remain dormant or grow out in subsequent seasons (Maggs,

1965). The time terminal buds form can vary considerably with weather, vigour, crop, cultivar,

and rootstock (Forshey and Elfving, 1989). Transformation of the vegetative apex to a

reproductive structure takes place only when the structure of the vegetative bud is complete

(Bub an and Faust, 1982). Consequently, flower induction occurs only when a critical number

of internodes have developed in the bud (Buban and Faust, 1982). Generally initiation of flower

primordia occurs 3-6 weeks after bloom on fruiting spurs or after cessation of shoot growth on

current season's growth (Buban and Faust, 1982; Forshey and Elfving, 1989). Thus the period

from full bloom to fruitlet abortion (June drop) is a period of intense competition between fruit

set, shoot growth, and flower bud initiation. Terminal flowers develop more rapidly than lateral

flowers and by the end of the growing season; sepals, petals, stamens and carpels have began

to develop (Pratt, 1988). Early flower development in late summer and autumn is followed by

a period of dormancy during the winter months (Sedgley, 1990).

Flower development precedes vegetative growth in the next growing season and is

completed between bud break and anthesis (Pratt, 1988). Most cultivars require cross-pollination

for consistent commercial crops and effective pollination is dependent on a supply of suitable

pollen, an abundance of pollinating insects, and weather conductive to insect flight (Forshey and

Elfving, 1989). When expressed in terms of volume or weight, the rate of fruit growth is

generally uniform throughout the season. Fruit growth rates may be slightly depressed early and

late in the season, so that the growth curve is slightly sigmoid in shape. The rate of fruit growth

is affected by such factors as nutritional status, soil moisture supply, but the major influence is

cropload (Forshey and Elfving, 1989).

7

2.2. Adverse effects of ve2etative lUowth on fruit production

Achieving the proper balance between vegetative and reproductive growth is necessary

to enhance early orchard production and to maximise yields during the life of the orchard

(Williamson and Coston, 1990). The control of vigour has long been viewed as a major

consideration in realising these aims. The term vigour has been used ambiguously in the past but

should only be used to derme the growth rate of the plant (Forshey and Elfving, 1989; Martin,

1989).

Vegetative growth in tree crops is essential to maintain the bearing surface for flowers

and fruit and to maintain tree vigour (Elfving, 1988). Additionally adequate leaf area is a

decisive factor for flower bud formation (Bub an and Faust, 1982) and fruit growth (Forshey and

Elfving, 1989). However, many authors have reported reductions in flower initiation (Mika,

1986; Williamson and Coston, 1990), fruit set (Forshey and Elfving, 1989), and fruit growth

(Williamson et al., 1992) with excessive vegetative vigour. The detrimental effect of vegetative

growth on fruiting has been associated with competition for substrates (Quinlan and Preston,

1971; Quinlan, 1975; Forshey and Elfving, 1989) and the presence of growth-promoting

substances (gibberellins and auxins) in growing shoots which suppress fruit bud formation (Mika,

1986). Additionally excessive vegetative growth may reduce light penetration and cause internal

shading which has been shown to reduce flower initiation, fruit set, and fruit growth (Jackson,

1968; Forshey and Elfving, 1989).

A further consequence of excessive vegetative vigour is the development over time of

large tree size. The problem of controlling tree size has become more acute as economics force

growers to become more efficient and produce more fruit per area of orchard (Ferree,. 1988).

With a specific training system, early production increases with increasing tree density

(Wertheim et al., 1986). However, reduced light penetration, internal shading, and poor

manageability (Boswell et al., 1975; Golomb, 1988) have been associated with the failure to

control tree size in planting systems incorporating high tree density.

8

2.2.1. Controlline veeetative erowth

Elfving (1988) divided management of vegetative growth into two categories:

physiological and cultural. Physiological factors are those inherent in the plant genome (Martin,

1989) and encompass the genetic potential of rootstock and scion to control vegetative growth.

Many cultural techniques have been applied to trees to modify growth including shoot pruning,

plant growth regulators, deficit irrigation, root restriction, root pruning and girdling. Root

pruning and root restriction are reviewed in chapter 2, section 2.4. More than one method of

growth control will likely be used on the same trees in a modem orchard emphasising the

importance of comparing all means of growth control to assess how techniques can be best

combined (Ferree, 1988).

2.2.2. Physioloeical control

Cultivar differences in tree size and vigour have been recognised for many years (Faust

and Zagaja, 1984; Martin, 1989). In the last thirty years, rootstocks have been extensively used

to limit scion growth (Faust and Zagaja, 1984). A wide range of proven size-controlling

rootstocks are available to apple producers and new rootstock candidates should expand this

range and extend choices within a given tree size class (Ferree, 1988). In addition to providing

size control, dwarfmg rootstocks have been reported to increase production efficiency (Granger,

1984; Ferree, 1988) and precocity (Ferree, 1988; Barritt, 1992). In contrast, other studies have

shown reduction in tree size by rootstocks is not necessarily inversely associated with production

efficiency (McKenzie, 1985) or precocity (Kosina, 1988). There is a more limited range of

proven size-controlling rootstocks for fruit crops other than apple and pear (Ferree, 1988; Martin

1989; Ferree et al., 1992). Progress has been made in improving the range of size-controlling

rootstocks in fruit crops such as cherry (Webster, 1980; Sansavini, 1984); plum (Webster, 1980)

and peach (Layne, 1987).

9

Selection and breeding of dwarf and 'spur type' cultivars has also been shown to

successfully reduce tree size (Hansche and Beres, 1980; Looney and Lane, 1984; Fideghelli et

al., 1984; Ferree et al., 1992). 'Spur type' mutations of the apple cultivars 'Delicious' and

'McIntosh' have been reported to be less vigorous in regards to overall tree height and average

shoot length, and have higher spur densities (Looney and Lane, 1984; Ferree, 1988). However,

considerable overlap exists when compared with trees of standard growth habits, and reference

to a 'spur type' does not necessarily imply a given degree of size control or productive

efficiency (Looney and Lane, 1984; Ferree, 1988). Similarly, a considerable variation in tree

vigour has been reported in plum and prune selections comprising of both spur and non-spur

types (Faust and Zagaja, 1984).

Hansche and Beres (1980) reported selection of dwarf genotypes with shorter shoot

internodes in peach resulted in trees with a plant stature of about one-third that of standard peach

cultivars. In a subsequent field trial, Hansche et al. (1986) concluded that dwarf genotypes were

more precocious and at higher tree densities would out produce standard peach cultivars based

on comparisons with previous trials. Fideghelli et al. (1984) reported preliminary selections of

promising dwarf genotypes in other fruit crops such as cherries, apricots and plums. Ferree

(1988) concluded that although there is a considerable amount of rootstock and scion growth

habit evaluation, the nature of testing required is long term and other techniques for controlling

vegetative growth will continue to be important in the immediate future.

2.2.3. Cultural control

Shoot pruning has been advocated to improve fruit quality, light penetration, renew fruit­

bearing surface, increase the ease of harvest and management, and to regulate tree size (Mika,

1986; Martin, 1989). The response of trees to dormant and summer pruning can be variable

depending on types of cuts, timing, tree vigour, and cultivar (Mika, 1986). Maggs (1965)

concluded that generally despite the faster growth of individual shoots, in most cases, dormant

10

pruned trees remain smaller than unpruned trees. In contrast, summer pruning has been reported

to reduce shoot growth of young trees (Maggs, 1965, Mika et aI., 1983) but has been found to

be ineffective in reducing the rate of shoot growth on older fruit bearing trees (Taylor and

Ferree, 1984). Despite differences in the response of shoot growth with tree age, the above

studies reported that summer pruning restricted canopy dimensions.

Martin (1989) concluded while pruning is essential for orchard productivity, it is not the

best means for maintaining tree size as it is more economical to work closer to the natural

growth habit of the tree. Mika (1986) agreed and concluded that the main reason for pruning

mature trees is to maintain high fruit quality. Additionally, pruning is an integral component in

conjunction with training and trellising. systems, by which tree growth may be redirected

(Martin, 1989). Reducing the angle of shoot growth from more vertical to horizontal is well

known to reduce shoot growth rates (Lakso and Corelli Grappadelli, 1992) and has been reported

to increase fruiting and yield (Preston, 1974; Dozier et al., 1982). Tromp (1973) (as cited by

. Forshey and Elfving, 1989) concluded that increased bloom in horizontally-positioned branches

is not directly correlated with reduced vegetative growth and other factors are involved in such

responses.

Many studies have promoted the potential for control of vegetative growth using plant

growth regulators. Chemicals such as daminozide (Batjer et al., 1964; Volz and Knight, 1986),

ethephon (Volz and Knight, 1986), paclobutrazol (Quinlan, 1980; Williams, 1984), and

chlormequat (Deckers, 1992) have been shown to reduce vegetative growth in a range of fruit

crops. In addition to controlling vegetative growth, plant growth regulators have been reported

to increase (Williams, 1984) or have no adverse effect on tree yield (Mika et al., 1983; Quinlan

and Richardson, 1984).

While originally foreseen as the ultimate solution (Martin, 1989), the use of plant growth

regulators is not without difficulties. Control of vegetative growth using daminozide has not been

successful in fruit species other than apples (Martin, 1989) and when used at concentrations

11

sufficient to control vegetative growth, daminozide leads to detrimental effects on apple fruit size

and shape (Williams, 1984; Martin, 1989). Additionally environmental concerns have lead to

the discontinued use of daminozide in some countries. In contrast, paclobutrazol has been

reported to effectively control vegetative growth in a range of fruit crops without adverse effects

on fruit size (Quinlan, 1980; Williams, 1984; Qu~an and Richardson, 1984). However, the

compound has a six to twelve-month half-life in the soil and has carry over effects on vegetative

growth the following year (Curry and Williams, 1983). Martin (1989) suggests that future

studies with plant growth regulators must concentrate on accurate chemical delivery to the target

tissue to achieve maximum results and minimise environmental contamination.

Withholding water may also restrict vegetative growth (Martin, 1989). Deficit irrigation

(irrigation at less than 100% of the plants full water requirement) during early stages of fruit

growth has been shown to reduce vegetative vigour in peach and pear without detrimental effects

on final yields (Chalmers et aI., 1981; Mitchell and Chalmers, 1982; Mitchell et al., 1986). The

effectiveness of such responses depends on the time of year that vegetative and fruit growth

occur (Chalmers et al., 1984) and have been associated with an increased supply of solutes to

the fruit due to reduced competition from vegetative growth (Mitchell et aI., 1986).

Additionally, a number of studies have implicated restricted root zone volume (Chalmers et al.,

1981; Chalmers et al., 1984) as a controlling variable in reductions in tree vigour and increased

fruitfulness in response to deficit irrigation (Chapter 2, section 2.4.2).

However, exacting water management is needed for deficit irrigation and it is only likely

to succeed in regions of infrequent precipitation and shallow soils where root spread is not

extensive. Clearly many fruit growing regions do not fulfil these conditions. Cultural techniques

which restrict root growth may improve the application of deficit irrigation in the field and allow

for greater control of vegetative growth (Williamson and Coston, 1990).

12

Girdling, the practice of removing part of the limb bark was a common horticultural

practice to induce bearing and control growth before the introduction of precocious rootstocks

and chemical growth regulators (Hoying and Robinson, 1992). Numerous studies have reported

reductions in vegetative growth in response to girdling and scoring (a single circumferential cut

through the bark) at various times of the growing season (Batjer and Westwood, 1963; Greene

and Lord, 1978; Greene and Lord, 1983; Hoying and Robinson, 1992). Furthermore yields have

been reported to be unaffected (Hoying and Robinson, 1992) or increased (Batjer and Westwood,

1963; Greene and Lord, 1983) in response to such treatments. However, repeated (annual)

girdling of the same tree results in progressively less response (Grierson et al., 1982) and

successful control of vegetative vigour depends upon properly timing the girdling treatment

(Martin, 1989). Batjer and Westwood (1963) found that despite early yield increases, total yield

over three years was unaffected or reduced by scoring.

2.3. Coordination of lWowth between the root and shoot

Because of the dependent nature of the root for carbohydrates from the shoot and the

shoot for water and nutrients from the root, it is not surprising that there appears to be a close

coordination of growth and activity between root and shoot systems. Coordination of root and

shoot growth is often expressed as the ratio of their sizes (i.e., root:shoot ratio). Root:shoot

ratios tend to show considerable genetic variability (Wareing, 1970) and change with plant age

and size (Kramer and Kozolowski, 1979). Once, established in a given environment, however,

plants appear to retain their root:shoot ratio even following cultural treatments that may initially

disrupt it (Richards, 1983).

This has been shown where the root: shoot ratio was altered through root (Buttrose and

Mullins, 1968; Richards and Rowe, 1977a) or shoot pruning (Randolph and Wiest, 1981).

Richards and Rowe (1977a) reported that the immediate response of peach seedlings to root

pruning was rapid root growth and a depression in shoot extension and leaf emergence.

13

However, after 25 days pruned plants had total dry weights and root:shoot ratios comparable

with control plants. The redistribution of assimilates was evidenced by a 20% increase in root

dry weight and a 23 % reduction in top dry weight. Water and nutrient uptake declined following

root pruning and hence may have also influenced growth. Brouwer and Dewit (1969) referred

to the ability of the plant to adjust and re-establish its root:shoot ratio as its 'functional

equilibrium' .

Conversely, where the root: shoot ratio has been unaltered but a change in the

environment induced, the rate of growth of either the root or shoot has been shown to change

relative to the other so that the 'functional equilibrium' is maintained (Milligan and Dale, 1988).

Chung (1983) reported that subjecting hydroponically grown cucumber plants to high root zone

temperatures, low light intensities, and low nutritional solutions caused assimilate partitioning

to be altered in favour of the organ under stress. It was concluded that this change in partitioning

allowed the plant to maintain its 'functional equilibrium'.

It appears that the 'functional equilibrium' entails more than a maintenance of dry weight

ratios but instead involves a precise balance between the outputs (size and activity) of the root

and shoot systems. Davidson (1969) expressed this 'functional equilibrium' as root mass x root

activity (absorption) ex shoot mass x shoot activity (photosynthesis). Subsequently this equation

has been reorganised in various ways (Hunt, 1975; Thornley, 1975).

Recently, Chung (1983) proposed that expressing the 'functional equilibrium' as total

piantweight/(leaJnumber/leajarea) ex total "k"/(root number/root length), where "k" represents

the total contents of elements or compounds was more accurate in biological terms. Chung

(1983) suggested this expression of the relationship more adequately accounts for morphological

changes in organs and could be applied to a wider range of situations.

14

Plant growth and function has been shown to conform well with these expressions of the

'functional equilibrium' for variety of plant species under different experimental and

environmental conditions (Davidson, 1969; Hunt, 1975; Richards and Rowe, 1977b; Richards,

1981; Chung, 1983).

2.4. Manipulation of shoot Kfowth by root treatments

The majority of research directed at controlling tree growth has focused on the above­

ground portions of the tree (Ferree et al., 1992). The recognition of the 'functional equilibrium'

between the root and shoot has opened up the possibility that manipulation of the root system

could be used to control growth and performance of above-ground portions. Yashhiroda (1960)

(as cited by Geisler and Ferree, 1984a) emphasised that root restriction, pruning and bending

play equal roles with root pruning in size control of bonsai trees. Reviews by Geisler and Ferree

(1984a) and Ferree et al. (1992) have reflected recent interest on the use of root pruning and

root restriction to control tree size.

2.4.1. Root prunin&:

Root pruning refers to the cutting of the root system allowing the remaining roots to

regenerate a bigger and denser system than before, if enough space is available (Geisler and

Ferree, 1984a). Dramatic improvements in mechanisation have enabled root pruning to be

accomplished mechanically although there is no way to control which roots are broken (Martin,

1989). This has been a primary factor for increasing commercial use of root pruning in recent

years in apple production in the eastern United States (Ferree et al., 1992).

15

2.4.1.1. Effects on plant erowth

With many different plant species, root pruning has been shown to reduce shoot growth

rates (Buttrose and Mullins, 1968; Richards and Rowe, 1977a; Randolph and Wiest, 1981;

Schupp and Ferree, 1990). Geisler and Ferree (1984b) found as the severity of root pruning in

young apple trees increased, total leaf area decreased. The number of leaves were reduced by

16% and 21 % and total leaf area reduced by 30% and 42% on trees with 28% and 59% root

pruning (proportion of total root dry weight) respectively. Similarly, pruning at 60 or 80 cm

from the trunk was very effective in reducing shoot growth of vigorous 'Melrose' apples, with

greater control of shoot growth achieved by root pruning closer to the trunk (Schupp and Ferree,

1988a).

Time of year that roots are pruned has been shown to influence shoot growth responses.

Schupp and Ferree (1987) recommended root pruning at dormant (March-northern hemisphere)

or full bloom growth stages in apple to control excessive shoot growth. Root pruning at June

drop or preharvest (August-northern hemisphere) had no influence on shoot growth. In contrast,

Ferree (1992) reported that there was little difference in shoot growth responses to root pruning

when performed from the late dormant period to June drop in a long-term field trial with

'Jonathan' apple trees. However, it was concluded that root pruning early (dormant-full bloom)

was preferable due to declines in cumulative yield when root pruning was delayed.

Generally, the growth rate of roots appears to remain unaffected or increases after root

pruning. In pea, McDavid et al. (1973) found the rate of dry weight increase of the root system

was similar between root pruned and control plants. Increases in the growth rate of roots have

been shown in peaches (Richards and Rowe, 1977a); grapevines (Buttrose and Mullins, 1968),

and holy (Randolph and Wiest, 1981) following root pruning. However, in the case of very

severe root pruning, growth rates of undisturbed roots may be temporarily retarded (Wilcox,

1955) (as cited by Geisler and Ferree, 1984a).

16

If enough space is available, the remaining roots following root pruning regenerate a

bigger and denser system than before (Geisler and Ferree, 1984a) because laterals regenerate

from each severed root (Sterling and Lane, 1975; Watson and Sydnor, 1987). Over time, one

or two of these roots eventually become dominant and collectively develop into the replacement

root system (Watson and Sydnor, 1987).

Generally, relatively warm temperatures, good soil aeration, absence of water stress and

relatively high light intensities are beneficial to root regeneration (Geisler and Ferree, 1984a).

Abod et al. (1979) (as cited by Geisler and Ferree, 1984a) observed that root regeneration in

Pinus species started two weeks after root pruning at optimal temperatures, but only after four

weeks at less favourable temperatures. Plant age and physiological status, and periodicity of root

growth also influence root regeneration. Fuchigami and Moeller (1978) observed that younger

plants usually have a higher capacity for root regeneration. Different species vary slightly in

their seasonal periodicity, but most woody plants have two periods of active root growth, one

in spring and one in autumn (Geisler and Ferree, 1984a). The influence of environmental and

physiological factors on plant responses to root pruning presumably contributes to the difficulty

in precisely establishing the optimum time for root pruning.

2.4.1.2. Effect on root:shoot ratio

As a portion of the root system is removed while leaving the shoot intact, it is not

surprising root: shoot ratios have been reported to be altered in favour of the shoot immediately

following root pruning (Richards and Rowe, 1977a; Randolph and Wiest, 1981). Subsequently

the plants reaction is to reestablish its 'functional equilibrium' which is evidenced by restoration

of root:shoot ratios following root pruning (Richards and Rowe, 1977a; Randolph and Wiest,

1981; Schupp and Ferree, 1990). Ferree (1989) reported that root pruning of young apple trees

increased the soluble and insoluble carbohydrate fractions in the roots providing further evidence

for a redistribution of assimilates in favour of the root system. The redistribution of dry weight

17

to the roots following root pruning appears to promote reductions in shoot growth. However,

Geisler and Ferree (1984a) suggested that increased root growth, stimulated by root pruning,

only leads to a reduction in shoot growth if growth factors are limiting. Ferree (1992) reported

that root pruning of 'Jonathan' apple trees achieved consistent reductions in shoot growth over

six years, except in a year with excessive rainfall where root pruning did not influence shoot

growth. The author concluded that the growth-controlling effect of root pruning in the field can

be negated by abundant soil moisture.

The time needed for root pruned plants to restore their root: shoot ratio varies greatly

between species. Twenty five days has been reported for peach seedlings (Richards and Rowe,

1977a); fifty two days for apple (Schupp and Ferree, 1990) and several months for 22-year old

trees of white pine (Stephens, 1964) (as cited by Geisler and Ferree, 1984a). The influence of

environmental and physiological factors on root regeneration; severity and timing of root pruning

also contribute to this variation.

. From greenhouse studies, it appears that the most likely cause of growth reduction due

to root pruning is due to a dramatic change in water relations (Ferree et aI., 1992). Significantly

reduced leaf water potentials in apples (Geisler and Ferree, 1984b; Schupp and Ferree, 1990)

and xylem water potentials in holy (Randolph and Wiest, 1981) have been reported immediately

following root pruning. Trees appear to adjust to lower water intake following root pruning by

closing stomates and reducing transpiration leading to a rapid recovery in leaf water potential

(Geisler and Ferree, 1984b; Schupp and Ferree, 1990) and reductions in net photosynthesis

(Geisler and Ferree, 1984b; Ferree, 1989).

Following root pruning there is a rapid regeneration of root growth, which would be

expected to be accompanied by a recovery in water uptake. Richards and Rowe (1977a) observed

that water uptake recovered 10 days after root pruning in peaches, and this was closely related

18

to the recovery of the root system. Transpiration and photosynthesis rates have also been shown

to recover with the regeneration of the root system, but at a slower rate than water potential

(Geisler and Ferree, 1984b).

Nutrient uptake may decline simply because the absorbing area is reduced following root

pruning. As the root system regenerates, uptake may increase accordingly (Geisler and Ferree,

1984a). Retarded supply of nutrients to the shoot has been implicated in root pruning responses

(eg. McDavid et al., 1973). In contrast, Schupp and Ferree (1990) discounted reduced nutrient

uptake as the cause of reduced growth following root pruning. Trees which were root pruned

twice through the season had significantly higher levels of nitrogen, phosphorus, calcium,

magnesium, iron and boron in the leaves compared with unpiuned controls. Richards and Rowe

(1977a) observed no initial nutrient deficiency with peach seedlings following root pruning.

A number of authors (Buttrose and Mullins, 1968; Richards and Rowe, 1977a; Geisler

and Ferree, 1984b) have suggested that retarded shoot growth in root pruned trees is caused by

a reduced supply of growth substances, especially cytokinins, from the root. It is generally

accepted that roots in particular their apices are sites for cytokinin, gibberellin and abscisic acid

synthesis. External applications of cytokinin have been shown to cause accumulation of

carbohydrates at the site of application (McDavid et al., 1973; Richards and Rowe, 1977a).

Thus the proportion of assimilates retained by the shoot may depend on the amount of cytokinin

supplied from the root (Geisler and Ferree, 1984a). A reduction in this supply may reduce the

sink capacity of the shoot and increase that of the root and may account for the redistribution

of dry matter to the root following root pruning.

Schupp and Ferree (1988b) found that xylem injections of cytokinin into apple trees could

not overcome the effect of root pruning. Carmi and Heuer (1981) concluded that growth

reductions due to root restriction could not be completely overcome by hormone treatment

possibly because several forms are needed for the normal growth of the shoot.

19

Geisler and Ferree (1984a) proposed the following synopsis to explain plant growth

responses to root pruning. "Immediately after root pruning, the plant has a reduced root:shoot

ratio and the supply of water, mineral nutrients, and hormones from the root to the shoot

declines. This causes a reduction in shoot growth. A greater percentage of the assimilates are

translocated to the root system and compensatory root growth takes place to restore the normal

root: shoot ratio. During the time of increased root growth, a reduction in the growth rates of

shoots can be observed. With the development of more root apices, more growth-promoting

substances are produced and translocated to the top. Shoot g~owth increases and the root:shoot

ratio characteristic for the plant is maintained."

2.4.1.3. Fruitin& and fruit quality

Documentation of fruiting responses to root pruning in fruit trees other than apple is

limited. "Historic reports suggested that root pruning induced flower initiation" (Ferree et al.,

1992). Hoad and Abbot (1984) (as cited by Geisler and Ferree, 1984a) assessed the effect of root

pruning of apple trees at different times of the year on flowering in the subsequent year. Early

root pruning (before June drop) caused fruit to abscise, induced prolific root growth and

increased blossom clusters fourfold compared with unpruned trees. Pruning later resulted in less

root regeneration particularly on trees with a good fruit load. The authors proposed that the

absence of active root meristems and the corresponding reduction in cytokinin synthesis results

in a balance of hormones in the spur that is detrimental to flower initiation. Cytokinins have

been reported to stimulate the development of inflorescences and the setting of fruit (Skene,

1975). In contrast, recent studies have reported no effect of root pruning on return bloom

(Schupp and Ferree, 1989) or fruit set (Schupp and Ferree, 1987; Schupp and Ferree, 1988a)

regardless of the time of year trees were root pruned.

20

Root pruning of large-fruited apple cultivars such as 'Melrose' and 'Golden Delicious'

has been found to have no effect on yield (Schupp and Ferree, 1987; Schupp and Ferree, 1988a;

Schupp and Ferree, 1989). Additionally, Schupp and Ferree (1987) reported root pruning

increased cumulative yield efficiency (cumulative yield per tree per cm2 trunk cross-sectional

area) in the apple cultivar 'Melrose'. However, Ferree (1992) found that the cumulative yield

was reduced by root pruning at full bloom or in mid-June (northern hemisphere) and cumulative

yield efficiency was reduced by root pruning in mid-June in the small-fruited apple cultivar

'Jonathan'. Many studies have reported reduced fruit size in apples following root pruning

(Schupp and Ferree, 1987; Schupp and Ferree, 1988a; Ferree, 1992). Schupp and Ferree

(1988a) suggest that decreased water potentials resulting from root pruning may have an adverse

effect on fruit size during the early development of the fruit.

Fruit quality characteristics such as colour, firmness and soluble solids have been

observed to increase and preharvest drop to decrease in response to root pruning (Schupp and

Ferree, 1987; Schupp and Ferree, 1988a; Schupp and Ferree, 1989; Ferree, 1992). Schupp and

Ferree (1988a) attributed increased fruit colour to improved light penetration in the bottom of

the canopy. Schupp and Ferree (1987) reported significantly higher calcium and lower potassium

and magnesium levels in apple fruit of trees root pruned during dormancy or at full bloom

compared with un pruned trees. However, this increase in fruit calcium was accompanied by

decreased fruit size. Large fruit tend to have lower calcium concentrations (Perring and Jackson,

1975; Ferguson and Triggs, 1990).

2.4.2. Root restriction

Root restriction confines root development to a volume of soil or growing medium thus

restricting the potential for new root growth. In comparison, root-pruned plants can regenerate

new roots into a undefmed volume of soil or growing medium. Therefore both root restriction

and root pruning inhibit the normal development of the root system, but are quite different in

21

the manner they achieve this. Root restriction has been achieved by growing plants in containers

or through partial irrigation to the soil. Additionally, root competition between crop plants

(Chalmers et al., 1981) and between the crop and orchard floor vegetation (Skroch and Shribbs,

1986) can restrict root growth. For the purposes of this review, no further discussion will be

covered here on the interaction and competition between plant species except to acknowledge

the potential for root restriction in this way.

However, it is important to recognise that differences in severity of root restriction may

~ve implications for the extent of responses observed (Henry, 1993). Many studies investigating

responses of root restriction have been conducted using non porous containers where there is

limited opportunity for new root growth (eg. Richards and Rowe, 1977a; AI-Sahaf, 1984; Krizek

et al., 1985; Dubik et al., 1989; NeSmith et al., 1992; Henry, 1993). In contrast, recent field

experiments on responses of deciduous fruit crops to root restriction have used fabric containers

to restrict root growth (Williamson and Coston, 1990; Myers, 1992; Williamson et al., 1992).

Under these circumstances, root growth is limited by the radial constraint of the container's wall

material but there is opportunity for new root growth in the form of fme roots around the

periphery of the porous container. The general responses of root restriction are reviewed below.

2.4.2.1. Effects on plant erowth

Reduction in shoot growth in a range of plant species has been shown where root volume

was restricted by non porous containers (AI-Sahaf, 1984; Dubik et al., 1989; Henry, 1993);

fabric containers (Myers, 1992; Williamson et al., 1992) or by drip irrigation (Proebsting et al.,

1977; Chalmers et al., 1981; Chalmers et aI., 1984; Proebsting et al., 1984). However, in the

case of irrigation experiments, it is difficult to separate the effects of water deficit and restricted

active root zone on tree performance (Proebsting et aI., 1977). Bravdo et ale (1992) using soil

matric potential sensors, reported a constant volume of irrigated soil could be maintained around

drip irrigated apple trees. Over a three year period, yield reductions associated with reduced tree

22

size were observed for trees irrigated to a lower soil matric potential. The authors concluded that

root restriction was the main contributor to growth reductions as 'crop density' or fruit size did

not differ greatly between treatments in the last two years of the experiment. Fruit size is known

to be sensitive to water stress.

Reductions in shoot growth reflect reductions in root growth caused by root restriction.

Root restriction has been shown to reduce the dry weight of roots and shoots (Richards and

Rowe, 1977a; Carmi and Shalhevet, 1983; Krizek et al., 1985; Ruff et al., 1987; NeSmith et

al., 1992; Henry, 1993). Additionally shoot growth in herbaceous and woody plants has been

shown to decrease with decreasing root zone volume (Carmi and Shalhevet, 1983; Robbins and

Pharr, 1988; NeSmith et al., 1992). Vigour in peach trees has been related to the volume of soil

available to roots (Cockcroft and Wallbrink, 1966). Similarly, Myers (1992) found that canopy

volume of peach and apple decreased linearly with decreasing container volume.

Richards and Rowe (1977a) reported that root restriction of hydroponically grown peach

seedlings reduced leaf number by 27 % and total leaf area by 51 % indicating that decreased leaf

size contributed substantially to reductions in total leaf area. Similarly, Dubik et ale (1989) found

leaf number declined by 25 % whereas total leaf area declined by 80 % in Euonymous

kiautschovica subjected to root restriction. These reductions in leaf size and number were

confined to lateral shoots. Other studies have also reported root restriction reduced lateral shoot

growth (AI-Sahaf, 1984; Robbins and Pharr, 1988; NeSmith et al., 1992).

Changes in root morphology have also been reported in response to root restriction.

Richards and Rowe (1977a) observed that root restriction of peach seedlings reduced the number

of root apices by 33 % compared to a root length reduction of 59 % indicating root restriction

induced a more densely branched root system. Other studies have also reported root restriction

generated denser root systems (Carmi and Shalhevet, 1983; AI-Sahaf, 1984; Ruff et al., 1987;

Ben-Porath and Baker, 1990; Peterson et al., 1991; Henry, 1993).

23

2.4.2.2. Effect on root:shoot ratio

The effect of root restriction on root: shoot ratios appears to be varied. Decreases in the

root:shoot ratios of tomatoes (AI-Sahaf, 1984; Peterson et al., 1991) and alder seedlings

(Tschaplinski and Blake, 1985) have been observed in response to root restriction. In contrast,

Ruff et al. (1987) reported root restriction of tomatoes increased the root:shoot ratio. Other

studies have reported that root restriction did not alter the root:shoot ratio (Richards and Rowe,

1977a; Krizek et al., 1985; Robbins and Pharr, 1988; Dubik et aI., 1990; NeSmith et al.,

1992). Although no alteration in the root:shoot ratio was observed in these studies, root

restriction may still induce a change in dry weight partitioning. Richards and Rowe (1977a)

found that root restriction reduced dry matter accumulation in all plant components of peach

seedlings except the butt. Similar observations were made by Hameed et al. (1987). Richards

(1981) also found root restriction did not alter the root:shoot ratio in tomatoes. However, upon

further analysis, it was found that when the loge of the root dry weight was plotted against loge

of the shoot dry weight, a line was generated whose slope decreased as pot size decreased,

indicating in dry weight terms, plants in large pots apportioned more growth into their roots than

those grown in small pots.

Henry (1993) in a study of the effects of container volume and media pore diameter on

grapevine growth reported an increase in the root:shoot ratio with smaller container volume. In

contrast, in a subsequent experiment it was found the root:shoot ratio decreased with smaller

container volume. The author concluded that in the fust experiment, the growing media was not

rigidly confmed allowing roots to grow unrestricted. In the subsequent experiment, the growing

media was rigidly confmed severely restricting root growth and this caused carbohydrate to be

redistributed to other plant organs. Henry (1993) suggested differences in severity of root

restriction may have implications for the extent of responses observed. Furthermore differences

in the stage at which root restriction is imposed and experimental duration may also contribute

24

to conflicting results. For example, in the trial conducted by Hameed et al. (1987), plants

established some leaf area prior to root restriction treatments been imposed which may have

influenced plant response.

Despite the demonstration of a close coordination of growth and activity between root

and shoot systems, the mechanism(s) by which root restriction controls growth has not yet been

elucidated.

Hameed et al. (1987) found that root restriction of tomato plants increased the hydraulic

resistance of the root system, induced more negative leaf water potentials, increased leaf

diffusive resistance, and reduced net assimilation rate in tomatoes. The authors concluded that

the principal effect of root restriction was to induce water stress through changes in root

morphology. It was proposed that this caused a reduction in photosynthesis through stomatal

closure and consequently reduced shoot and root growth and increased dry matter in the stem.

Tschaplinski and Blake (1985) also reported increased negative leaf water potentials in alder

seedlings in response to root restriction. In contrast, Carmi and Heuer (1981); Krizek et al.

(1985) and Dubik et al. (1990) concluded that reduction in plant growth in response to root

restriction was not due to inadvertent water stress.

Dubik et al. (1990) reported significantly lower concentrations of nitrogen, phosphorus,

calcium, magnesium, iron, aluminium and copper in the leaves of root restricted Euonymous

kiautschovica plants. The authors concluded that reduced nutrient uptake possibly due to

decreased numbers of root apices, decreased root length, and increased suberisation contributed

to observed growth reductions in root restricted plants. Similarly, Mutsaers (1983) and Ran et

al. (1992) implicated reduced nutrient uptake in growth reductions in response to root restriction.

AI-Sabaf (1984) also reported lower foliar concentrations of potassium, calcium, and magnesium

in root restricted tomato plants. However, rather than reductions in growth being a direct result

of a modification to nutrient uptake mechanisms, AI-Sabaf (1984) proposed reduced foliar

nutrient concentrations were a result of a reduction in the mean growth tate of root restricted

25

plants. Richards and Rowe (1977b) using the equation proposed by Thornley (1975) showed a

functional relationship existed between plant dry weight and nutrient uptake in peach seedlings.

This relationship held irrespective of whether plants were root pruned, root restricted or allowed

to grow normally. The authors concluded that the effect of root pruning or root restriction could

not be attributed to the absorption of nutrients by the root.

A number of authors (Richards and Rowe, 1977a; Carmi and Heuer, 1981; Carmi and

Shalhevet, 1983; Ruff et al., 1987) have suggested that retarded plant growth in response to root

restriction is caused by a reduced supply of growth substances from the root. As highlighted in

chapter 2, section 2.4.1.2, it is generally accepted that roots in particular their apices are sites

for cytokinin, gibberellin and abscisic acid synthesis.

Richards and Rowe (1977a) found that exogenous application of cytokinin partially

overcame growth reductions observed in root restricted peach seedlings. The authors concluded

that the limit set by roots on top growth involved an internal regulation by the root, in particular

the production and supply of growth substances. Carmi and Heuer (1981) found that a combined

treatment of cytokinin and gibberellin fully restored the growth of the stem and primary leaves

of root restricted bean plants. Application of cytokinin or gibberellin alone only partially restored

growth. Recently root produced abscisic acid has been implicated in reductions in plant growth

in response to increasing mechanical impedance (Masle, 1990; Tardieu et al., 1992). The role

growth inhibiting substances may have in growth responses to root restriction presently remain

speculative.

Robbins and Pharr (1988) proposed reduced sink demand induced by restricted root

growth in cucumbers may have been associated with reductions in plant growth. Lower rates of

photosynthesis, reduced starch accumulation, and reduced assimilate export in source leaves

were found for plants grown in restricted soil volumes. It was suggested that over time

accumulation of starch, due to reduced carbohydrate mobilisation could have provided a

26

feedback mechanism that reduced carbon metabolism for plants grown in a restricted root

volume. In contrast, Krizek et ale (1985) found root restriction of soybean did not significantly

effect photosynthetic rate or the starch fraction in any plant organ.

2.4.2.3. Fruitina: and fruit quality

Root restriction of fruiting plants has been reported to increase flower density (Richards,

1986; Ben-Porath and Baker, 1990; Williamson and Coston, 1990; Myers, 1992; Williamson

et al., 1992) and fruit set (Myers, 1992). Myers (1992) found that flower cluster and fruit

number per limb in apples increased linearly with decreasing container volume. Despite reports

of increased flowering, a number of studies have shown that reduction in plant size in response

to root restriction resulted in decreased yields (Carmi and Shalhevet, 1983; Richards, 1986; Ruff

et al., 1987). In contrast, root restriction has been reported to have no effect (Myers, 1992) or

to increase yield (Ben-Porath and Baker, 1990) even though plant growth was reduced.

Williamson and Coston (1990) with peach trees found that increased flowering compensated for

smaller tree size in root restricted trees resulting in no differences in yield for this system and

several other conventional planting systems.

Root restriction has been shown to increase the productive efficiency of fruiting plants.

NeSmith et ale (1990) reported that harvest indices (fruit dry weight divided by total plant dry

weight) in bell pepper were inversely proportional to container volume. Ben-Porath and Baker

(1990) also reported increasing harvest index in cotton with decreasing pot volume. Similarly

increases in yield efficiencies (yield per tree per cm2 trunk cross-sectional area) in peaches

(Williamson and Coston, 1990; Myers, 1992) and apple (Myers, 1992) have been observed in

response to root restriction. Increased fruitfulness has also been reported for trickle irrigated

fruit trees (Proebsting et al., 1977; Chalmers et al., 1981; Chalmers et al., 1984; Proebsting

et al., 1984). In these studies, the authors suggested that restricted root volume was a controlling

variable.

27

The physiological basis for increased productivity in fruiting species due to root

restriction has not been elucidated. Greater partitioning of assimilates in favour of reproductive

organs at the expense of vegetative organs (Cooper, 1972; Carmi and Shalhevet, 1983) and

storage organs (Ben-Porath and Baker, 1990) has been reported in response to root restriction.

Carmi and Shalhevet (1983) suggested an excess of assimilates were partitioned towards the

fruiting bolls in cotton as a result of reduced vegetative growth in response to root restriction.

The influence of root restriction on fruit size appears to be varied. Studies have reported

no effect of root restriction on average fruit weight in peaches (Myers, 1992) and tomatoes (AI­

Sahaf, 1984). Similarly Ben-Porath and Baker (1990) reported no effect of root restriction on

average boll weight in cotton. In contrast, Williamson and Coston (1990) reported a small

reduction of fruit diameter in peaches in response to root restriction. Richards (1986) found root

restriction reduced fruit weight of peaches in one year but not in the subsequent year.

The effect of root restriction on fruit quality has received scant attention. AI-Sahaf (1984)

reported that root restriction reduced the translocation of calcium to the shoot in tomatoes but

this was not reflected in the fruit. Root restriction did not effect fruit potassium, calcium, and

magnesium concentrations. However in a further experiment, AI-Sahaf (1984) reported that the

development and percentage of fruits affected by blossom end rot in a calcium deficient regime

was lower in tomato plants initially root restricted and subsequently released compared with

unconfmed plants. The development of blossom end rot in tomatoes has been associated with

calcium nutrition (AI-Sahaf, 1984; Wills et al., 1989). AI-Sahaf (1984) attributed this response

to a greater ability to remobilise stored calcium, particularly in the stem and to less competition

between the leaves and fruits for calcium in de-restricted plants. In contrast, Ruff et al. (1987)

observed no effect of root restriction on the severity of blossom end rot in tomato fruit.

Proebsting et al. (1977) reported that restricting the rooting volume of 'Delicious' apple

trees by trickle irrigation resulted in a higher soluble solids content in the fruit in comparison

with sprinkler irrigation. Similarly, Proebsting et al. (1984) found that apples from trickle-

28

irrigated trees had higher soluble solids, lower titratable acidity and that red colour in

'Delicious' and yellow colour in 'Golden Delicious' apples tended to be higher than those from

sprinkler-irrigated trees. When water deficits were induced in the sprinkler-irrigated trees, fruit

soluble solids increased suggesting that the effects on fruit quality from triclde-irrigated trees

resulted from water deficits (Proebsting et al., 1984).

2.4.2.4. Conclusion

Although the physiological basis for the effects of root restriction is still not clear, the

above studies indicate that restricting the root volume of fruit trees is a promising technique to

control vegetative vigour and improve orchard efficiency. Reid et al. (1993) in a study of root

demography in kiwifruit reported that for each year over a two year period, the cumulative

length of roots grown was equivalent to about 2.75 times the maximum net length of roots

visible. The authors concluded that the fine-root system has the capacity to be a large sink for

fIXed carbon and this capacity has been underestimated in the past. These fmdings emphasise the

potential influence root restriction may have on the tree's carbon balance and also suggest that

there is a need to distinguish between different root restriction treatments in respect to the

potential amount of root turnover.

Additional research is required to study the role of water relations, growth regulators and

nutrition in tree responses to root restriction in the field.

29

CHAPTER 3

The effects of root growth control using root restricting bags on the growth and fruitfulness

of apple (Malus x domestica Borkh. cv. 'Fuji').

3.1. Introduction

Achieving the proper balance between vegetative and reproductive growth is necessary

to enhance early orchard production and to maximise yields during the life of the orchard

(Williamson and Coston, 1990). To achieve these aims, the control of vigour has long been

viewed as a major consideration in the successful management of fruit trees. Many authors have

reported retardation of flowering and fruit growth due to excessive vegetative growth (Richards,

1986; Volz and Knight, 1986; Martin, 1989; Williamson and Coston, 1990). The detrimental

effect of vegetative. growth on fruiting has been associated with competition for substrates

(Quinlan and Preston, 1971; Quinlan, 1975; Forshey and Elfving, 1989) and the presence of

growth-promoting substances in growing shoots which suppress fruit bud formation (Mika,

1986). A further consequence of excessive vegetative vigour is the development over time of

large tree size. With a specific training system, early production increases with increasing tree

density (Wertheim et aI., 1986). However, reduced light penetration, internal shading, and poor

manageability (Boswell et al., 1975; Golomb, 1988) have been associated with the failure to

control tree size in planting systems incorporating high tree density.

The majority of research directed at controlling tree growth has focused on either genetic

means or on cultural practices aimed at the above-ground portions of the tree (Ferree et al.,

1992). The genetic approach has met with some success with the selection of dwarfing cultivars

(Scorza et al., 1984; Hansche et aI., 1986) and rootstocks to control fruit tree size. However,

the development of size controlling rootstocks for species other than apple and pear and the

selection of dwarfing cultivars has been limited (Martin, 1989; Ferree et al., 1992).

30

Cultural practices such as pruning, plant growth regulators, and girdling have also been used to

control tree size with varying degrees of success. Yet these practices can be costly and each have

their drawbacks.

It has become increasingly evident that a close coordination exists between root and shoot

growth and function. In vegetative plants grown in a given environment, there is usually a

precise linear relationship between the relative root and shoot growth rates (Richards, 1986).

Coordination of root and shoot growth is often expressed as the ratio of their sizes (Le.,

root:shoot ratio). Once, established in a given environment, plants appear to retain their

root:shoot ratio even following cultural treatments that may initially disrupt it (Richards, 1983).

Brouwer and Dewit (1969) referred to the ability of the plant to adjust and re-establish its

root: shoot ratio as its 'functional equilibrium'. The existence of a close coordination between

root and shoot growth opens up possibilities that above-ground growth can be altered by below­

ground manipulation.

One approach of below-ground manipulation to control tree size is root pruning.

Reductions in shoot growth rates following root pruning have been shown for apple (Schupp and

Ferree, 1987; Schupp and Ferree, 1988a; Ferree, 1989; Schupp and Ferree, 1990) and peach

(Richards and Rowe, 1977a). There are limited reports on use of root pruning to control growth

of plants other than apple (Ferree et al., 1992). The extent of reductions in shoot growth

depends on the frequency and severity of root pruning (Geisler and Ferree, 1984b; Schupp and

Ferree, 1988a; Schupp and Ferree, 1990) and on the phenological stage of the plant (Schupp and

Ferree, 1987). Ferree (1992) showed season-long reductions in shoot growth of apples can be

obtained from yearly repetitions of root pruning although in a year of abundant soil moisture,

the growth controlling effect of root pruning was negated.

Studies have reported no influence of root pruning on yields of large-fruited apple

cultivars (Schupp and Ferree, 1987; Schupp and Ferree, 1988a). However, Ferree (1992) found

that cumulative yield was reduced by root pruning in the small-fruited apple cultivar 'Jonathan'.

31

Additionally, numerous studies have reported reduced fruit size in apples in response to root

pruning (Schupp and Ferree, 1987; Schupp and Ferree, 1988a; Schupp and Ferree, 1990;

Ferree, 1992).

Root restriction is another approach of below-ground manipulation to control tree size.

Unlike root pruning, new root growth is limited as the root system is restricted to a confined

space. Vigour in peach trees has been related to the volume of soil available to roots (Cockcrofi

and Wallbrink, 1966). Reductions in shoot growth of fruit trees has been shown where root

volume was restricted by root containers (Richards, 1986; Williamson and Coston, 1990; Myers,

1992; Williamson et al., 1992) or by irrigation (proebsting et al., 1977; Chalmers et al., 1981;

Chalmers et al., 1984; Proebsting et al., 1984).

Accompanying reductions in tree vigour, increased fruitfulness has been reported for

trickle irrigated trees (Proebsting et al., 1977; Chalmers et al., 1981; Chalmers et al., 1984;

Proebsting et al., 1984). Additionally fruit size was unaffected. In these studies, restricted root

volume was implicated as a controlling variable. However, in many irrigation experiments, it

is difficult to separate the effects of water deficit and root restriction on tree performance

(Proebsting et al., 1977). Proebsting et al. (1984) concluded that effects on fruit quality

observed under trickle irrigation were primarily due to water deficit. In the field situation,

restriction of rooting volume by irrigation is only likely to succeed in regions of infrequent

precipitation and shallow soils where root spread can be controlled within the wetted zone.

Clearly many fruit growing regions do not fulfil these conditions.

Support for the implication of restricted root volume as a controlling variable in tree

responses to trickle irrigation has come from studies where root growth was restricted

physically. Where root growth was limited by root barriers, reductions in tree vigour have been

shown to be accompanied by increases in flower density in peach (Richards, 1986; Williamson

and Coston, 1990; Williamson et al., 1992) and apple (Myers, 1992). As a result of increases

in flower density, Williamson and Coston (1990) found no difference in fruit number or yields

32

per tree between root restriction and several other convential planting systems, despite the

smaller tree size of root restricted trees. In contrast, Richards (1986) showed that despite higher

flower production, final fruit yields were slightly lower for trees grown in small root volumes.

Nonetheless in these studies, the effect of restricted root volume translated into a increase in

yield efficiency (yield per tree/trunk cross-sectional area). The influence of root restriction on

fruit size is less clear. Myers (1992) found no difference in mean fruit weight between restricted

and unconfmed plants. In contrast, Williamson and Coston (1990) reported a small reduction in

fruit diameter in response to root restriction. Similarly, Richards (1986) found root restriction

reduced fruit weight in one year but not in the subsequent year.

Few studies have reported the effects of long-term root restriction in the field. Myers

(1992) reported that flower cluster and fruit number per limb in apples in the first year of

fruiting increased linearly with decreasing container size. The objective of this study was to

evaluate the effect of root restriction using root restricting bags on the growth and fruitfulness

of the apple cultivar 'Fuji' over a range of bag volumes.

33

3.2. Material and methods

3.2.1. Site

Trees were planted on a Wakanui silt loam at the Lincoln University Horticultural

Research Area in Canterbury, New Zealand (43°39'8, 11m above sea level). The climate at

Lincoln is temperate, with average daily mean maximum and minimum temperatures of 16.7°C

and 6.2°C respectively, and an average yearly rainfall of 666mm (Nagle et al. 1992).

3.2.2. Experimental desim

Unfeathered trees of 'Fuji' apple were planted in two rows at a spacing of 3.0 x 5.Om

in the spring of 1989. Trees were conventionally planted or planted in in-ground fabric

containers of 15, 34, 65 or 139 litre volume on MM106 or MM115 rootstocks. Container

dimensions (diameter (mm) x height (mm» were 250 x 275 (15 litre); 350 x 325 (34 litre); 450

x 375 (65 litre); 600 x 450 (139 litre). As the containers were planted with the top of the bag

50mm above ground level, the actual soil volumes of trees planted in bags were calculated to

be 10, 25, 48, and 102 litres respectively. These figures are those referred to in the subsequent

text. The trees were planted in a randomised complete block design, with trees with the largest

butt diameter 20cm above the graft union planted in block 1 and the smallest in block 6. The

blocks were arranged to account for possible future differences in tree growth caused by wetter

and colder soil and shading from a row of Cedrus deodara at the northern end of the trial site.

Each block consisted of ten trees (five bagging treatments on MMI06 or MMl15 rootstock).

The fabric bag used in this trial was the Root Control BagSM, (Root Control Inc.,

Oklahoma City, OK). Containers are made of 5.3 oz. UV-stabilised Duon~, a nonwoven

synthetic fabric (Phillips Fibers Corp.). Fabric containers are cylindrical, with one end closed

with clear plastic to form the bottom of the bag. Plate 3.1 illustrates a 10 litre volume root

control bag in situ in 1994, five years after planting.

34 In all treatments, trees were untrained, un pruned and allowed to develop natural crown

form and size. To prevent excessive weeping, the central leader was tied to a trellis wire where

needed. Fruit thinning was not carried out in any year to avoid obscuring any treatment effects.

The within-row and inter-row area was maintained weed free using a pre-emergence (Simazine)

and post emergence (Roundup®) herbicide spray programme. Irrigation water was applied

through drippers with all trees receiving the same frequency of irrigation and amount of water.

In other respects, normal orchard management practices were followed.

3.2.3. Measurement of tree erowth and fruitfulness

3.2.3.1. Measurement of vea=etative erowth and fruitine

At the end of each growing season, for all trees, two measurements of trunk diameter

were made at right angles to each other, 20 centimetres above the graft union and the average

diameter was used to calculate tree trunk cross-sectional area. For each tree, tree height and

measurements of branch spread from the tree centre on the north, south, east and west faces

were made at the end of the fifth growing season (April, 1995) to determine canopy volume.

Branch spread was measured at 5 to 7 heights dependent on tree size.

The trees did not fruit during the first season of growth. Yields were measured in the

second to fifth seasons of growth. Trees were strip picked on one occasion at the time of

commercial harvest. Harvested fruit was weighed and counted for each tree. During the third

and fourth seasons of growth, windfalls were also weighed and counted for each tree.

3.2.3.2. Productivity comparisons

Vegetative and fruit data were used for comparisons in productivity between treatments

(for description of terminology used see chapter 3, section 3.2.4). In 1994, all harvested fruit

were graded for size over a commercial grader to assess the effect of root restriction on size

35

distribution. An untimely, enforced recalibration of the grader's fruit size ranges resulted in one

trial replicate having to be omitted from subsequent data analysis. Frequency polygons of size

distributions (frequencies were scaled to one) were derived for each tree separately. Trees were

separated into their phase of bearing i.e., 'on' or 'off' in the 1994 season as a result of a

biennial bearing habit. Frequency polygons were then derived for bagged and unconfined

treatments by averaging trees in the same phase of bearing.

3.2.4. Description of terminolou used and statistical analysis

Comparisons between treatments in respect to crop density and yield efficiency within

years are based on those proposed by Lombard et ale (1988). Crop density (equation 1) refers

to the number of fruit per unit of tree dimension and is the product of both flower density and

fruit set.

Crop density = number of fruit

trunk cross-sectional area

(Uni ts: frui t no. / cm2 ) (1)

Yield efficiency (equation 2) incorporates crop density and fruit weight using the

complete tree as a unit (Lombard et al., 1988). Trunk cross-sectional area correlates directly

with tree fresh weight particularly on lightly pruned trees (Westwood and Roberts, 1970). In this

experiment, trees were unpruned.

Yield efficiency yield per tree Trunk cross-sectional area

(Uni ts: g/ cm2 ) (2)

36

Cumulative yield efficiency (EYE) (equation 3) integrates long term effects, such as the

cumulative yield for several years based on the final trunk cross-sectional area (Lombard et al.,

1988).

EYE = cumulative yield per tree Trunk cross-sectional area in last year

(uni ts: g/ cm2 ) (3)

Bienniality of trees was evaluated in terms of the intensity of deviation in yield and yield

efficiency in successive years (I) (equation 4) as proposed by Hoblyn et al. (1936).

(4)

where: n = number of years. aI' a2 , ••• , an, a(n-l) = yield in corresponding years. The calculation within the brackets is done ignoring signs of terms.

Statistical analysis was performed using the Genstat 5 statistical program. The data was

analysed by analysis of variance methods on untransformed data. Orthogonal contrasts were used

to determine differences between bagged and unconfmed treatments, and to determine the linear

and quadratic effects within the bagged treatments. Graphic presentation and accompanying

modelled relationships were performed using the graphics package Sigmaplot version 2.0.

37

Plate 3.1. 10 litre root control batM in situ in 1994.

38

3.3. Results

Unless stated otherwise, contrasts within bag treatments and/or between rootstocks are

not significant for all data presented.

3.3.1. Veeetative lUowth

At time of planting in 1989, all trees had similar trunk cross-sectional areas (TCSA)

(Figure 3.1). Root restriction resulted in a smaller TCSA in all four years of measurement (1991

to 1994) (P<O.Ol) when compared with the unconfmed treatment (Figure 3.1 and Table 3.1).

Within bag volumes, there was a significant (P<O.Ol) linear increase in TCSA with increasing

bag volume for all four years (Table 3.1). Trees grown on MM106 rootstock compared with

those grown on MMl15 had smaller TCSA in all four years (P<O.Ol) (Table 3.1). Although,

MMl15 had larger TCSA in all treatments compared with MM106, the difference (the absolute

difference or expressed as a percentage) was considerably greater in the unconfmed (Table 3.1

and appendix 1, Table Al.1). This interaction was significant (P<O.Ol) in 1992, 1993, and

1994.

In 1995, canopy volume and tree height measurements reflected the differences recorded

for TCSA in the previous years (Table 3.2). Canopy volume and tree height were reduced by

more than 80 % and 40 % respectively in any of the bag volumes compared with the unconfmed

treatment. In contrast to TCSA, within bag volumes, there were no differences in canopy volume

or tree height (Table 3.2). The relationship between TCSA and canopy volume was examined

further using regression techniques to relate TCSA in 1994 against canopy volume in 1995

(Figure 3.2). Canopy volume increased as TCSA increased. However, the slope of this

relationship differed significantly (P<O.05) between the bagged and unconfmed treatments.

Trees grown on MM106 rootstock were on average 25% smaller across all treatments

than those grown on MM 115 rootstock as indicated by canopy volume. Tree height did not differ

significantly between rootstocks (Table 3.2). As with TCSA, MMl15 had larger canopy volumes

39

in all treatments, but the difference (the absolute difference or expressed as a percentage) was

greater in the unconfined (Table 3.2). This was the only significant interaction (P< 0.01). Plates

3.2, 3.3, 3.4 illustrate the growth differences between the extremes of the bagged treatments,

10 litre and 102 litre respectively and the unconfmed treatment on MM106 rootstock in 1993.

3.3.2. Fruitine

At harvest in 1991, the yield produced per tree by any treatment was not significantly

different (Table 3.3). Root restriction significantly reduced the yield produced per tree (P< 0.01)

(Table 3.3) in 1992, 1993, and 1994. Over the duration of the trial, trees grown in containers

averaged approximately 40% of the cumulated yield per tree found for unconfined trees (Table

3.3).

Root restriction significantly reduced the number of wind-fallen fruit in 1992 (P<O.Ol)

and 1993 (P<0.05) (Appendix 1, Table A1.2). Number of wind-fallen fruit in unconfined trees

corresponded to 9 % and 8 % of the total tree's crop in 1992 and 1993 respectively. In

comparison, number of windfalls amounted to less than 5% of the total tree's crop averaged

across bagging treatments in 1992 and 1993.

Mean fruit weight was significantly larger in bagged compared with unconfmed trees in

1991 (P< 0.05) and 1993 (P< 0.05) (Table 3.4). Conversely, mean fruit weight was significantly

smaller in bagged trees compared with the unconfmed in 1992 (P<O.Ol) and 1994 (P<O.Ol)

(Table 3.4). In contrast, rootstock only effected mean fruit weight in 1994. In this year, fruit

from trees grown MMl15 rootstock compared with MM106, had a significantly higher mean

fruit weight. The affect of crop density was examined further using regression techniques to

relate mean fruit weight to crop density for each tree. In all years, except 1991, mean fruit

weight decreased linearly with increasing crop density across all treatments (Figures 3.3 and

3.4).

40

The yield efficiency of bagged and unconfmed trees varied between years. Bagged trees

were more efficient than unconfined trees in 1991 and 1992 (P<O.OI), while in contrast, in

1993, unconfined trees were more efficient than bagged trees (P<0.05) (Table 3.5). There was

no significant difference between bagged or unconfmed trees in 1994.

Similar trends to that of yield efficiency were observed for crop density. Bagged trees

carried significantly higher crop densities than unconfmed trees in the years 1991 (P< 0.05),

1992 (P<O.OI), and 1994 (P<0.05) (Table 3.5). In 1993, unconfined trees carried significantly

higher crop densities (P<O.05) than bagged trees (Table 3.5).

The alternating behaviour of the yield efficiencies and crop densities between the

unconfmed and root restriction treatments from year to year is indicative of a biennial bearing

habit. To investigate this, bienniality indices were calculated using the intensity of deviation in

yield and yield efficiency, in successive years and for all four years using the method proposed

by Hoblyn et al. (1936) (Chapter 3, section 3.2.4; equation 4). The intensity (I) will vary from

o to 1, where 0 denotes even cropping between years, and 1, no crop in the 'off' year (Hoblyn

et al. 1936).

All treatments displayed bienniality over successive year periods and overall for the four

years (Table 3.6). The bienniality indices calculated using yield efficiency figures did not differ

significantly between the unconfmed and root restriction treatments or between rootstocks in any

of the successive year periods or over the four year period (Table 3.6). The bienniality indices

calculated using yield figures differed significantly between the unconfmed and root restriction

treatments and between rootstocks in the 1991/1992 period only (Table 3.6).

3.3.3. Productivity comparisons

Crop density and yield efficiency have been used to evaluate productivity of trees of

various sizes in training, trellising and rootstock experiments (Lombard et al., 1983; Stang and

Werdman, 1986). In this study, evaluation of productivity between bagged and unconfined

41

treatments using crop density and yield efficiency between years was considered to be

inappropriate due to their alternating behaviour.

Lombard et al. (1983) showed cumulative yield efficiency as a valuable approach to

evaluate rootstock productivity. In this study, bagged trees produced a significantly higher

cumulative yield efficiency (P<0.01) (Table 3.7) over a four year period. Bagging increased

cumulative yield efficiency over the four years by between 215 to 1589 grams per cm2 of TCSA

at the 95 % confidence interval.

To investigate this relationship further, cumulated yield was plotted against TCSA in each

year (Figure 3.5). In this approach, fluctuations in yield are smoothed. The slope of treatment

lines were calculated from the cumulative yield per tree (1991 to 1994) minus yield per tree in

1991 over TCSA in 1994 minus TCSA in 1991 (Table 3.7). Analysis of variance showed that

bagged trees had a significantly steeper slope compared with unconfined trees (P<O.OI) (Table

3.7).

Root restriction affected fruit size distribution. Derived frequency polygons of fruit size

distribution for all treatments are shown for those trees in a 'on' phase (Figure 3.6) and in a

'off phase (Figure 3.7) in 1994. Root restriction caused the peak of the fruit size frequency

polygon to occur in a smaller fruit size grade in trees in a 'on' phase in 1994. This was reflected

in the frequency polygon means (Table 3.8). No such trend for bagging was evident for trees

in an 'off' phase. Means for frequency polygons were calculated by assuming fruit size

distribution was equally distributed throughout each size range (i.e., the mid value of each size

range was used for calculation of the mean). Those fruit less than 60 grams and larger than 210

grams were assumed to lie within a range of 39 grams (twice that of the standard size range)

corresponding to mid range values of 39.5 grams and 229.5 grams respectively.

Plate 3.2. Malus x domestica Borkh. cv 'Fuji' growing in 10 litre root control bagSM on MMI06 rootstock in 1993.

~ate 3.3. Malus x domestica Borkh. cv. 'Fuji' growing in 102 litre root control bagSM

on MMI06 rootstock in 1993.

Plate 3.4. Malus x domestica Borkh. cv 'Fuji' growing with unconfined root zone volume on MMI06 rootstock in 1993.

Figure 3.1.

~ .£. ca ~ ca

100

80

~ 60 o

13 m ~ (.)

.::t:. 40 c :::l .=

----D--- 10litre bag -6- 25litre bag -v- 48litre bag ~- 1 02 litre bag -0- No bag

1\

20

-----<>

~~---~~~~-=:::::::-~. =::~=-:: "!~~~::-~: ~ -=-~~=== o T

1989 1990 1991 1992 1993 1994 Year

The etTect of bag volume on tree trunk cross-sectional area in the years 1991, 1992, 1993, and 1994. (Error bars denote

two times the standard error of the mean, applicable to all treatments in any year). t

45

Table 3.1. The effect of bag volume and rootstock on tree trunk cross-sectional area

growth (cm2) in the years 1991, 1992, 1993, and 1994.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast between 'bag' vs nill

Linear contrast within 'bag'

Contrast between rootstocks

Contrast between nil vs 'bagged' rootstock

1991

10 litre 9.6

25 litre 9.3

48 litre 11.7

102 litre 12.4

Nil 18.7

MM106 11.2

MMl15 13.5

1.26

0.79

**

**

**

ns

1992

13.0

12.4

16.0

17.4

34.3

16.6

20.6

1.79

1.13

**

**

**

**

1993

18.0

16.6

23.0

24.0

54.9

25.0

29.6

2.82

1.78

**

**

**

*

1 Contrast significance at P<O.Ol and 0.05 denoted by **, * respectively; ns is not significant.

1994

22.1

19.6

27.3

30.1

83.2

32.8

40.0

3.57

2.26

**

**

**

**

46

Table 3.2. The effect of bag volume and rootstock on tree height and canopy volume in

1995.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast between 'bag' vs nill

Contrast between rootstocks

Contrast between nil vs 'bagged' rootstock

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

1 Contrast significant at P<O.Ol denoted by **; ns is not significant.

Tree height (metres)

2.28

2.25

2.45

2.38

4.10

2.63

2.75

0.173

0.109

** ns

ns

Canopy volume (metres3

)

3.00

2.74

4.56

3.85

28.40

7.27

9.75

1.488

0.941

** ** **

Figure 3.2.

50 r-- -CI 10 litre bag L:;. 25 litre bag v 48 litre bag ")

0 102 litre bag 0 No bag

40 ~ 0 MM106

• MM115 regression line

Lt) m m ..... 30 .5

.l)-

S CD

5 "0 > :>. g 20 co

c..:>

10 o

o o 20 40 60 80 100 120

Tree trunk cross-sectional area (cm2) in 1994

The effect of root restriction on the relationship between tree trunk cross-sectional area in 1994 and canopy volume in 1995.

Bag treatments y=O.1834x-l.005 r=59.8

Unconfined treatment y=OAO 12x -4.965 r=80.9 ~ -.....l

Table 3.3. The effect of bag volume and rootstock on yield per tree in the years 1991, 1992, 1993, 1994, and cumulated yield per tree.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast of 'bag' vs Dil2

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

1991

Yield (kg)

8.5

7.5

7.2

9.1

6.1

6.7

8.7

1.77

1.12

ns

ns

1 Cumulated yield (yield cumulated for the years 1991, 1992, 1993 and 1994).

1992

Yield (kg)

25.8

26.2

31.4

31.7

46.7

30.9

33.8

5.47

3.46

** ns

2 Contrast significant at P< .01 and 0.05 denoted by **, * respectively; ns is not significant.

1993

Yield (kg)

22.0

21.8

23.5

27.7

98.9

35.7

41.9

7.05

4.46

** ns

1994

Yield (kg)

34.9

31.7

44.2

43.5

121.1

54.5

52.1

11.05

6.99

** ns

Cumulated yield 1 (kg)

94.5

87.1

106.3

112.9

263.9

129.3

136.5

17.71

11.20

** ns

~ 00

49

Table 3.4. The effect of bag volume and rootstock on mean fruit weight (grams) in the

years 1991, 1992, 1993, and 1994.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast of 'bag' vs nill

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

1991

217

231

210

220

194

219

210

15.2

9.6

* ns

1992

123

120

123

131

154

131

130

9.3

5.8

** ns

1993

144

148

157

140

131

145

143

10.2

6.5

* ns

1 Contrast significant at P<O.Ol and 0.05 denoted by **, * respectively; ns is not significant.

1994

124

125

114

128

156

120

139

10.6

6.7

** **

50

1991 320

0 0 MM 106-1 Olitre

280 - t : • MM115-101itre 6. MM 1 06-251itre

I,6.r&-Vr#> • MM 115-251itre (i) 240 -

1&\ v MM 1 06-481itre E • MM115-481itre tU L.. 200 - ~ 0 MM106-1021itre .9 1: -/1. • • MM115-1021itre C) • • 0 0 MM106-no bag ·CD 160 -~

~.o 0 • MM 115-no bag :t:::::

.E 120 - -- Fitted line c 0 tU Q)

::2: 80 -

40 -

0 I I I I I I I

0 5 10 15 20 25 30 35 40

Crop density (fruit number/trunk cross-sectional area (cm2»

1992 320

280

(i) 240 E

10 tU L.. 200 .9 1: • C) ·CD 160 ~

:t:::::

.E 120 c tU Q)

::2: 80

40

0 0 5 10 15 20 25 30 35 40

Crop density (fruit number/trunk cross sectional area (cm2»

Figure 3.3. The effect of crop density on mean fruit weight in 1991 and 1992.

1991 y=220.38-1.73x P ns

1992 y=180.65-3.313x P<0.01

51

1993

320 0 MM 106-1 Olitre

280 • MM115-101itre b. MM 1 06-251itre

• MM 115-251itre en 240

" MM 1 06-481itre E

~ MM 115-481itre tU L..

200 MM 1 06-1 021itre .9 0" 0 E • • MM 115-1 02litre C)

0 MM106-no bag 'CD 160 == • MM 115-no bag = 2 120 -- Fitted line -c:: tU Q)

~ 80

40

0

0 5 10 15 20 25 30 35 40

Crop density (fruit number/trunk cross-sectional area (cm2))

1994

320

280

en 240 E tU • L.. 200 .9 -1: C)

'CD 160 == = 2 120 -c:: tU t::. Q)

~ 80

40

0 0 5 10 15 20 25 30 35 40

Crop density (fruit number/trunk cross-sectional area (cm2»

Figure 3.4. The effect of crop density on mean fruit weight in 1993 and 1994.

1993 y=176.33-3.082x P<O.01

1994 y=188.17-4.585x P<O.01

Table 3.5. The effect of bag volume and rootstock on yield efficiency and crop density in the years 1991, 1992, 1993, and 1994.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast between 'bag' vs nil3

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MM106

MMl15

1991

Yield efficiency!

958

822

706

776

344

673

770

189.3

119.7

** ns

1 Yield efficiency (yield per tree/trunk cross-sectional area) (g/cm2). 2 Crop density (fruit number/trunk cross-sectional area) (fruit/cm2).

Crop density2

4.9

3.6

3.2

3.6

1.8

3.2

3.7

1.01

0.64

* ns

1992

Yield Crop efficiency density

1937

2109

2016

1854

1406

1893

1836

232.2

146.9 .

** ns

16.9

18.2

16.7

15.1

9.3

15.6

14.9

2.13

1.35

** ns

3 Contrast significant at P<O.Ol and 0.05 denoted by **,* respectively; ns is not significant.

1993

Yield Crop efficiency density

1367

1359

1040

1265

1789

1351

1377

254.3

160.8

** ns

11.7

10.3

7.0

9.9

13.8

10.6

10.5

2.57

1.62

* ns

1994

Yield Crop efficiency density

1418

1608

1554

1507

1410

1568

1431

188.6

119.3

ns

ns

12.2

14.2

14.1

12.9

9.7

13.8

11.5

2.01

1.27

* ns

VI N

Table 3.6. The effect of bag volume and rootstock on bienniality indices calculated from yield and yield efficiency figures in 1991,1992,1993,1994.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast between 'bag' vs nil3

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

91/92

1rield 1rield index! efficiency

index2

0.52 0.44

0.55 0.45

0.62 0.51

0.60 0.50

0.7 0.59

0.65 0.55

0.56 0.45

0.073 0.083

0.046 0.052

** ns

* ns

92/93 93/94

1rield 1rield 1rield 1rield index efficiency index efficiency

index index

0.27 0.35 0.27 0.27

0.23 0.29 0.25 0.23

0.20 0.34 0.30 0.27

0.29 0.35 0.36 0.31

0.34 0.27 0.30 0.29

0.32 0.36 0.31 0.28

0.22 0.28 0.28 0.27

0.095 0.101 0.089 0.083

0.060 0.064 0.056 0.052

ns ns ns ns

ns ns ns ns

1 & 2 Calculated using Hoblyn's biennial index (chapter 3, section 3.2.4) where I is an evaluation of intensity of deviation in yield in successive years. 3 Contrast significant at P<O.Ol and 0.05 denoted by **, * respectively; ns is not significant.

91/92/93/94

1rield 1rield index efficiency

0.35

0.34

0.37

0.40

0.45

0.42

0.35

0.065

0.041

ns

ns

index

0.35

0.32

0.37

0.38

0.39

0.39

0.33

0.067

0.042

ns

ns

Vl W

54

Table 3.7. The effect of bag volume and rootstock on cumulative yield efficiency and the

slope of lines of cumulative yield plotted against trunk cross-sectional area.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast between 'bag' vs nil3

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

Cumulative yield efficiencyl

4076

4476

3932

4052

3232

3996

3911

430.4

272.2

** ns

Slope2

7201

8074

6433

6571

4156

6569

6405

858.1

542.7

** ns

1 Cumulative yield efficiency (cumulative yield per tree/trunk cross-sectional area in 1994; units: grams/cm2). 2 Slope «cumulative yield (1991 to 1994) per tree minus yield per tree in 1991)/(1994 trunk cross-sectional

area minus 1991 trunk cross-sectional area)). 3 Contrast significant at P<O.01 denoted by **; ns is not significant.

Figure 3.5.

300

250

200

~ ::2

CD .~

"'C 150 ~ :; E :l ()

100

50

o o

v • . /. .... pi. P T . J!/

I~/ I ..JJ' 1 I' !ll

14 -!'J"'/i /1/>-~~~

20 40 60 80

Trunk cross-sectional area (cm2)

-0 - MM 106-1 Olitre

--- MM115-101itre -60- MM 1 06-251itre -A-- MM 115-251itre "·XV··· MM 1 06-481itre ... ....... MM 115-481itre

-<>- MM 1 06-1 02litre --+- MM 115-1 02litre -0- MM 1 06-no bag

--- MM 115-no bag

100 120

The effect of bag volume and rootstock on the relationship between cumulated yield and tree trunk cross-sectional area

growth over the years 1991 to 1994.

U\ U\

0.45

A 0.40

0.35

0.30

~ 0 0.25 c (J) =s C"' (J) 0.20 ~

LL

0.15

0.10

0.05

0.00

0.45

B 0.40

0.35

0.30

~ 0.25 c (J) =s C"' (J) 0.20 ~

LL

0.15

0.10

0.05

0.00

Figure 3.6.

<59

···0··· 10 litre ~ 25 litre -v- 48 litre -0- 102 litre -0- No bag

60-89 90-109 110-129130-149150-169170-189190-209 >210

Size range (grams)

.~;--... /'/ "-.'

.' / .. ~ l/ )\

__ --7/ 7 ................ ' . \'. ~~ .' /~. . , ...•

f /... : '" \, .... . /Jr ., .....

f ./ / ~ \. ". f / / L" " .

...•... 10 litre

--..t.- 25 litre ~- 48 litre -+- 102 litre

----- No bag

. " / " ~." ".

f .,/ ~ " ~"....... . ....... . f /./ ~ \~, .•............. _-A

f··./ '. " ..... ---l~~ ... ····· •.. ~"

<59 60-89 90-109 110-129130-149150-169170-189190-209 >210

Size range (grams)

Frequency polygons of the fruit size distribution of trees in an 'on-

phase' in 1994 on A. MMI06 rootstock and B. MM115 rootstock.

56

0.45

0.40

0.35

0.30

~ 0.25 c Q) ::J 0-Q) 0.20 '-U.

0.15

0.10

0.05

0.00

0.45

0.40

0.35

0.30

~ 0.25 C Q) ::J 0-Q) 0.20 '-U.

0.15

0.10

0.05

Figure 3.7.

A

<59

...•... 10 litre -A- 25 litre -'t'- 48 litre -+- 102 litre

--- No bag

<59

7" JiiI: ....... : 0 '. /1.. .··, ....... z·· ·····'\··0 .' .. ' I j: 'A-__ '~':./

.' I 'V\ ' •

. d··· / / ~~\~ .. ...:. .. '>-:O--~ . I j

I . I . .' I /

o ..il ---

... {]... 10 litre -6.- 25 litre -v- 48 litre -{)- No bag

60-89 90-109 110-129130-149 150-169 170-189 190-209 >210

Size range (grams)

A B

)'.-.. ~\

.,/' \. .~ \ .-. i :.'

:j' .•..........•. .. .j .' j:" '.

' .. • '.

60-89 90-109 110-129130-149150-169170-189190-209 >210

Size range (grams)

Frequency polygons of the fruit size distribution of trees in an 'off-

phase' in 1994 on A. MM106 rootstock and B. MM115 rootstock.

57

58

Table 3.8. Number of trees by treatment used to calculate the frequency polygons of

figures 3.6 and 3.7, and the mean fruit size calculated from fruit size

distributions of the treatments.

Rootstock

MM106

MMl15

Bag

10 litre

25 litre

48 litre

102 litre

Nil

10 litre

25 litre

48 litre

102 litre

Nil

Trees in 'on' phase

Mean

132

109

119

118

146

149

129

122

127

155

Number of trees in

frequency polygon

3

3

4

4

3

4

4

3

3

3

Trees in 'off' phase

Mean

135

155

158

171

141

193

127

180

190

Number of trees in

frequency polygon

1

2

1

o 2

1

1

2

2

2

59

3.4. Discussion

3.4.1. Ve2etative a:rowth

The reduction in tree growth due to root restriction as indicated by TCSA, canopy

volume and tree height confirms the findings of previous studies (Richards and Rowe, 1977a;

Richards, 1986; Williamson and Coston, 1990; Myers, 1992; Costa et al., 1992; Williamson

et al., 1992). Myers (1992) reported canopy volumes of trees grown in containers of 0.02, 0.043

or 0.1 m3 volume, averaged 44% and 59% for peach and apple, respectively, compared with

control trees by the end of the third season. In the present study, canopy volumes of trees grown

in containers averaged 13 % in comparison with control trees by the end of the sixth season. The

larger reduction in growth than that reported by Myers (1992) may be a reflection of the longer

time frame. The growth rate of the unconfined trees continued to increase throughout the

experiment as indicated by TCSA. In contrast, trees grown in bags maintained a similar growth

rate throughout the experiment. This accounted for the differences in growth between the

unconfmed and root restricted treatments (Figure 3.1).

Throughout the experiment, TCSA decreased linearly as bag volume was reduced. This

agrees with the fmdings of Myers (1992) with peach and apple and Richards (1986) with peach

who showed that canopy volume decreased with decreasing root volume. Cockcroft and

Wallbrink (1966) also showed that peach tree vigour is related to the volume of soil available

to roots.

However, the growth differences in TCSA within bag volumes were not reflected in

canopy volumes (as determined by tree height and branch spread) in 1995. The relationship

between trunk size and tree size has been shown to vary with such factors as rootstock (Pearce,

1952), pruning and tree size (Westwood and Roberts, 1970), and soil conditions (Tukey and

Brase, 1938). In the present study, the results indicate that the relationship between trunk size

and tree size differed between the bagged and unconfined treatments (Figure 3.2). This is likely

60

to have contributed to the disparity between TCSA and canopy area volume measurements within

bag treatments. Increased allocation of dry matter into the stem relative to other plant organs has

been reported in response to root restriction (Richard and Rowe, 1977a; AI-Sabaf, 1984;

Hameed et al., 1987; Peterson et al., 1991). Nonetheless, canopy volumes in 1995 closely

followed the same trends as TCSA within bag sizes (Tables 3.1 and 3.2) indicating both

measurements are suitable for growth comparisons between treatments.

The mechanism(s) by which root restriction controls growth has yet to be elucidated.

Richards and Rowe (1977 a) studied the effects of root restriction on hydroponically grown peach

seedlings, where water and nutrients were non limiting and postulated that peach roots exert a

limit on shoot growth through the production and supply of growth substances. It is known that

different kinds of stress in the root zone may affect the biosynthesis and translocation of growth

substances from the roots to the shoot and thereby affect growth (Carmi and Heuer, 1981).

Williamson and Coston (1990) suggested, root restriction in the field would be more likely to

impose limitations on water and nutrient availability than in carefully controlled experimental

conditions. Ran et al. (1992) agreed and concluded that limiting the root growth of peach trees

grown in the field by restricting container volume resulted in the root systems not been able to

supply the potential demand for water and nitrogen by the trees. It seems likely that roots

regulate shoot growth through a complex interaction between root produced hormones, soil water

supply and nutrients (Davies et al., 1986).

Trees grown on MMl15 rootstock had larger TCSA and canopy volumes compared with

those grown on MM 106. This result is not unexpected as MM 115 has been reported to be a

more vigorous rootstock than MM106 (Tydeman, 1953; Preston, 1955). However, root

restriction appears to have reduced the invigorating effect of MMl15 as the difference in TCSA

or canopy volume (the absolute difference or expressed as a percentage) between the two

61

rootstocks was considerably greater in the unconfined than in the bagged treatments. Coker

(1958) and Rogers and Vyvyan (1934) showed that rootstocks retain their distinctive

characteristics despite the modifying influence of the soil.

3.4.2. Fruitin&

The lack of effect of root restriction on yield in the frrst cropping year, although tree

growth was reduced is similar to the findings of Williamson and Coston (1990) and Myers

(1992) for peaches. Reductions in tree size due to root restriction in the years 1992 to 1994 were

accompanied by reductions in yield per tree. Richards (1986) reported similar reductions in yield

in root restricted peaches, although yield reductions were attributed to decreased fruit size rather

than decreased fruit numbers in the first year of cropping. In the subsequent year, yield

reductions were attributable to decreased fruit numbers. In contrast, Williamson and Coston

(1990) reported that increased flowering compensated for smaller tree size in peach trees grown

in fabric-lined trenches resulting in similar yields between root restricted and conventional

planting systems.

Neither the yield per tree in any year or the cumulated yield per tree over the four year

period differed significantly within container sizes. Fruit number per tree, within bag volumes,

was also comparable (Appendix 1, Table Al.3) indicating that differences in tree size within

bagged treatments did not influence yields.

In this study, rootstock did not affect yield per tree in any year or cumulated yield per

tree over the four year period. Apples have been successfully grown on MMI06 (McKenzie,

1985) and MM115 rootstock (Preston, 1955).

62

How root restriction reduced the number of wind-fallen fruit (expressed as proportion of

tree's total crop) is unclear. The author does not know of any studies reporting the effects of

root restriction on fruit abscission. Schupp and Ferree (1988a) observed reduced fruit drop in

apples due to root pruning at full bloom. The authors suggested that root pruning may counteract

abscission without interfering with other ethylene-mediated ripening processes.

Treatment differences in mean fruit weight can be explained by their effect on crop

density. The negative relationship between mean fruit weight and crop density over all

treatments in the years 1992, 1993, and 1994 (Figures 3.3 and 3.4) confirms the findings of

Forshey and Elfving (1977) and Volz (1988). The similarity of this relationship between these

years underscores the close relationship between fruit size and crop density. Similarly, the

comparable crop densities across rootstocks may explain why rootstock significantly effected

fruit size in 1994 only. In 1994, trees grown on MM115 compared with MM106 had lower crop

densities, although this difference was not significant. This may have contributed to the higher

mean fruit weight observed for these trees in this year.

Increases in yield in bagged trees on a yearly basis were accompanied by increases in

crop density (Tables 3.3 and 3.5). This relationship was not observed in the unconfined trees.

The higher crop densities of the bagged trees compared with unconfined trees in 1991 and 1992

were reflected in higher yield efficiencies in these years. Similarly, the higher crop density in

the unconfined trees in 1993 resulted in a higher yield efficiency compared with bagged trees.

The number of fruit has been found previously to be closely related to yields (Forshey and

Elfving, 1977; Volz, 1988) and hence the association between crop density and yield efficiency

is not surprising. In contrast in 1994, higher crop densities in bagged trees were not reflected

in significantly higher yield efficiencies. This appears to be due to the inability of an increase

in crop density to compensate for reductions in mean fruit size in this year. Assaf et al. (1974)

and Richards (1986) showed that differences in fruit size can also be an important factor

contributing to differences in yields.

63

In the years that yield increased, mean fruit weight decreased, relative to the previous

year in root restricted trees (Tables 3.3 and 3.4). Assaf et ale (1974) found a similar inverse

relationship for yield and fruit size in apples under several irrigation regimes and suggested that

this indicated an 'overcharge' of fruiting. In unconfined trees, yield increases relative to the

previous year were not necessarily accompanied by decreases in mean fruit weight. It seems

likely that an increase in fruiting sites due to larger increases in tree size from year to year in

the unconfined trees may explain why such an inverse relationship was not always observed.

However, the alternating behaviour of crop density and yield efficiency between 1992

and 1994 of both the bagged and unconfined trees (Table 3.5) indicates both groups of trees may

have experienced an 'overcharge' of fruiting. Assaf et ale (1984) suggested that the ratio of yield

per tree to its TCSA increment affords a reliable measure for the degree of cropping. It follows

that the ratio of yield per tree to its TCSA is also indicative of the degree of cropping although

it is likely to be a less sensitive measure as it is influenced by tree growth in previous years.

Over-cropping is known to affect biennial bearing (Monselise and Goldschmidt, 1982).

All treatments exhibited degrees of bienniality as indicated by the intensity of deviations in yield

and yield efficiency values in successive years and over the four year period (Table 3.6). This

supports the view that the trees experienced an 'overcharge' of fruiting. The intensity of

deviations in yield from constant cropping were not strong. Singh (1948) reported bienniality

intensities in the range of 0.05 and 0.3 for two 'regular' apple varieties ('Ellison's Orange' and

'Ribston Pippin') and 1.0 for two 'biennial' ones ('Miller's Seedling' and 'Blenheim Orange').

Jones et ale (1991) reported that if 'Fuji' apples are overcropped they can become biennial.

Yearly variations in yield can be accounted for by climatic variations, as well as by

natural changes occurring in trees with the progress of time (Monselise and Goldschmidt, 1982).

The degree of bienniality exhibited between treatments was only significantly different as

indicated by the yield index for the 1991/1992 period (Table 3.6). This is likely to be associated

with an increase in bearing capacity after the initial year of fruiting, than with treatment

64

differences in biennial bearing habit. Supporting this view, the biennial bearing index calculated

using yield efficiency values did not differ significantly between treatments in the same period.

Williamson and Coston (1990) suggested that optimum crop loads for trees with restricted

root systems could be different from unrestricted trees. The present study confirms that there

is a need to investigate this issue further.

3.4.3. Tree productivity

Root restriction resulted in a higher cumulative yield efficiency over four years (Table

3.7) confIrming the fmdings of other studies of increased productivity for perennial tree species

where root volume was restricted by root barriers (Richards, 1986; Williamson and Coston,

1990; Myers, 1992; Williamson et al., 1992) or by irrigation (Proebsting et al., 1977; Chalmers

et aI., 1981; Chalmers et al., 1984; Proebsting et aI., 1984). The relationship between

cumulative yield and TCSA over four years (Figure 3.5) illustrates the increased productivity

of the bagged trees.

The physiological basis for increased productivity of root restricted perennial fruiting

species has not been elucidated. Greater partitioning of assimilates in favour of reproductive

organs at the expense of vegetative organs (Cooper, 1972; Carmi and Shalhevet, 1983) and

storage organs (Ben-Porath and Baker, 1990; Peterson et al., 1991) has been reported in

response to restriction of root growth in non porous containers. Carmi and Shalhevet (1983)

suggested an excess of assimilates were partitioned towards fruiting bolls in cotton due to

reductions in vegetative growth in response to root restriction. Henry (1993) concluded from a

study of the effects of container volume and media pore diameter on grapevine growth, that

redistribution of carbohydrate varied with the severity of root restriction. This may have

important implications for comparisons between experiments. Where root volume is restricted

using non porous containers there is limited opportunity for new root growth. In comparison,

65

root restriction achieved through the use of porous fabric containers such as in this experiment,

promotes new root growth in the form of fine roots around the periphery of the container and

may increase the capacity of roots to act as a sink for carbohydrates.

The effect of root restriction on fruit size distribution appears to be explained by the

negative relationship between fruit size and crop density rather than a restriction treatment effect

per se. In 1994, bagged trees carried a significantly higher crop density than unconfmed trees.

For trees in an 'on' phase of bearing this was reflected in the peak of the fruit size frequency

polygon occurring in a smaller fruit size grade for bagged compared with unconfmed trees. The

greater proportion of trees (across all treatments) in an 'on' phase of bearing (Table 3.8) also

would have been an important factor in the reduction of mean fruit weight observed for the

bagged in comparison to the unconfined trees in 1994. The effect of root restriction on fruit size

distribution in trees in a 'off' phase of bearing is less clear. Previous studies have reported that

thinning reduces the proportion of small fruit but effects on larger fruit are more variable

(Forshey and Elfving, 1977; Volz et al., 1988). This may contribute to the inconsistent pattern

in fruit size distribution in trees in an 'off' phase of bearing. Additionally, the weight of

individual fruits above 210 grams were not measured and this probably explains why some of

the treatments in a 'off' phase did not display a bell shaped curve.

This study suggests that orchard planting densities could be increased by a factor of six

times compared with standard spacing of unconfined trees due to the smaller tree size resulting

from root restriction. At this higher density, root restricted trees would have produced a

significantly higher yield per hectare of orchard. However, care should be taken when

extrapolating these results to commercial circumstances. In the trial, trees were not pruned or

thinned. The negative relationship between fruit size and crop density suggests a 'trade off'

exists between obtaining desirable fruit size and tree yield.

66

To conclude, root restriction successfully controlled tree growth and increased yield

efficiency. This approach may have particular promise in fruit crops such as cherries and

peaches where suitable size-controlling rootstocks are currently not available. Other practical

methods of restricting root growth in the field such as irrigation warrant further investigation.

The physiological basis by which root restriction controls tree growth and increases reproductive

growth is still not clear. Additional research is required to study the role of water relations,

growth regulators and nutrition in tree responses to root restriction in the field. This study

emphasises the need to determine optimum crop loads for root restricted trees to obtain desirable

fruit size and reduce yearly yield fluctuations in cultivars predisposed to biennial bearing before

the potential of root restriction for commercial use can be further evaluated.

CHAPTER 4.

The effects of root growth control using root restricting bags on fruit calcium

concentrations and contents of apple (Malus x domestica Borkh. cv. 'Fuji').

4.1. Introduction

67

Restriction of root growth by growing plants in confmed volumes of soil or growing

medium results in a reduction in top growth (Myers, 1992; Bravdo et al., 1992; Williamson et

aI., 1992). However, few studies have reported the effects of root restriction on fruit quality in

perennial fruiting trees. Commonly, manipulations that successfully restrict vegetative growth

at the critical periods for fruit set, growth, and maturation enhance fruit set, increase fruit size,

advance fruit maturation and may improve fruit quality (Martin, 1989).

Enhancement of fruit colour in deficit irrigated apple trees has been suggested to be

associated with reduced shoot growth (Mills, 1993). Proebsting et al. (1977) reported that

restricting the rooting volume of 'Delicious' apple trees by trickle irrigation resulted in a higher

soluble solids content in the fruit in comparison to sprinkler irrigation. However, in the case of

irrigation experiments, it is difficult to separate the effects of water deficit and restricted active

root zone on tree performance (Proebsting et al., 1977). Proebsting et al. (1984) showed that

apples from trickle-irrigated trees had higher soluble solids, lower titratable acidity and that red

colour in 'Delicious' and yellow colour in 'Golden Delicious' apples tended to be higher than

those from sprinkler-irrigated trees. When water deficits were induced in the sprinkler-irrigated

trees, fruit solid solubles increased suggesting that the effects on fruit quality from triclde­

irrigated trees result from water deficits (Proebsting et al., 1984).

AI-Sabaf (1984) reported that potassium, calcium, and magnesium concentrations in

tomato fruit were similar between plants restricted to 0.05 and 0.1 litre volume containers and

unconfined plants (4.5 litre containers) grown hydroponically. However, root restriction did

68

reduce calcium translocation to the shoots which resulted in a higher accumulation of calcium

in the roots and lower calcium concentrations in the leaves. Dubik et ale (1990) and Richards

and Rowe (1977a) also reported significantly lower concentrations of calcium in the leaves of

Euonymous kiautschovica plants and peach seedlings respectively, when grown within a

restricted root zone. Aloni (1986) concluded that root restriction of Chinese cabbage by limiting

root volume impairs calcium uptake and translocation into young leaves.

In a further experiment, AI-Sahaf (1984) reported that the severity of the effect of a

calcium-deficiency treatment (as indicated by general appearance and percent of fruits with

blossom end rot) was very much lower in tomato plants initially root restricted and subsequently

released compared with unconfmed plants. This was attributed to less competition between the

leaves and fruits for calcium and a greater ability of de-restricted plants to remobilise stored

calcium, particularly in the stem. Ruff et ale (1987) reported no differences in the severity of

blossom end rot in tomatoes under root restricted or unconfmed conditions. In this case, calcium

was unlikely to be deficient as nutrients were delivered through an automatic watering system.

Although the physiological basis for the effects of root restriction has yet to be

determined, the above studies indicate root restriction may have important effects on fruit

quality.

Mineral concentrations, particularly of calcium, are important determinants of apple fruit

quality during storage (Ferguson and Watkins, 1989). In addition to bitter pit, calcium has been

associated with numerous other physiological disorders in apple fruits such as lenticel blotch and

breakdown, corkspot, cracking, and water core (Wills et al., 1989). Despite the importance of

the final mineral status of a fruit, there is a lack of information on the mechanisms determining

fruit nutrient accumulation (Ferguson and Watkins, 1989).

Large fruit generally have lower calcium concentrations and are more susceptible to bitter

pit (Perring and Jackson, 1975; Ferguson and Triggs, 1990). However, for any given fruit size,

69

significant variation in fruit mineral content and hence concentration can still occur (perring and

Jackson, 1975). Contributing to this variation, movement of minerals into fruit can be affected

by crop density independently of fruit size (Ferguson and Watkins, 1992). Vigorous shoot

growth and higher shoot:fruit ratios in light-cropping trees (Sharples, 1968) may contribute to

this influence of crop density. Calcium translocation to the fruit and hence fruit calcium contents

also appear to be influenced by seed number (Bramlage et al., 1990) and therefore pollination

may also contribute to variations in calcium concentrations in fruit of the same size.

Within-tree variation in fruit calcium status is also associated with the effects of fruit

position (Jackson et al., 1971). Shading reduces fruit size thereby directly increasing mineral

concentration (Jackson et al., 1977). Ferguson and Triggs (1990) reported lower calcium

concentrations in apple fruit from the upper parts of the tree. This has often been associated with

larger fruit size (Jackson et aI., 1971) whereas, Ferguson and Triggs (1990) found that fruit of

the same size showed the same pattern. Recently, Volz et ale (1994) reported that fruit position

on the outside of the canopy can also account for substantial variation in fruit mineral

composition. Fruit calcium and magnesium concentrations and contents were found to be higher

for terminal borne fruit compared to those borne laterally or on fruit spurs. A localised leaf

effect on calcium inflows into the fruit may contribute to such results because leaves influence

the pattern of spur water deficit and thus the volume of xylem sap exchanged, between fruit and

tree (Lang and Volz, 1993). However, Volz et ale (1994) found that terminal fruit with total

spur leaf areas and fruit size similar to those on two-year spurs had higher calcium content and

suggested an additional 'positional' factor would also seem important.

This study reports on the influence of root restriction of apple trees using root restricting

bags on fruit calcium concentrations and contents at the time of commercial harvest. New root

growth is not completely restricted with the fabric bags used in this trial as the bags material is

porous, and allows new root growth in the form of fine roots around the periphery of the

container.

70

4.2. Materials and methods

Fruit from the root restriction trial of Malus x domestica Borkh. cv. 'Fuji' at the Lincoln

University Research Orchard were sampled in 1993 and 1994 for fruit calcium analysis. For a

description of the experimental site and design refer to chapter 3.

4.2.1. 1993 Experimental samplina: and analysis

At harvest on the 27th April, two fruit from each tree in a size range of 115 to 130

grams were randomly sampled from all fruit that occupied the outer half of the branches and

were able to be picked from the ground (up to 2m height). Fruit was then stored at 2°C for

seven weeks.

Fruit calcium concentrations were determined from washed fruit which were weighed

and then cut longitudinally twice to produce two opposite, seedless, wedge-shaped segments

which formed the fruit sample and amounted to approximately 25 percent of the whole fruit

mass. Each fruit sample (two segments combined) was immediately weighed. This method

follows that described by Perring and Wilkinson (1965) and Turner et ale (1977). Each tree in

the experimental trial was represented by two fruit samples and each treatment, for example, the

10 litre bag treatment on MM106 rootstock was represented by 12 fruit samples (two fruit

samples/tree replicated six times).

Following weighing, fruit samples were dried at 60°C in a force-draught oven for seven

days to ensure complete dehydration. Fruit samples were then removed and stored in air tight

containers and placed in a desiccator containing silica crystals to prevent reabsorption of water.

Subsequently, the fruit samples were ground using a coffee grinder to produce a fine powder

which was transferred to air tight containers and returned to the desiccator.

71

For analysis, about 0.8 grams of dried tissue (each sample weight was recorded so as to

convert back to a fresh weight basis) was weighed into digestion flasks. Wet digestion (5ml conc

HN03, held overnight, followed by Iml 60% HCI04) was then carried out and the digestates

were made up to 20ml with distilled water and transferred to vials and stored in a refrigerator.

Upon analysis, 2mls of KCI was added to the vials and calcium concentration was then measured

by atomic absorption spectrophotometry. Fruit calcium contents in 1993 were subsequently

estimated from this data.

4.2.2. 1994 Experimental samplina: and analysis

To obtain a better estimate of the influence of fruit size on calcium concentration the

sampling procedure was modified in the second year. All fruit were graded for fruit size after

harvest in april 1994. After grading, a random sample of five fruit in each of four size

categories from half of the trees in the field trial were selected for calcium analysis. The four

fruit size categories corresponded to fruit weight ranges of 90-109, 130-149, 150-169 and 190-

209 grams. Thus the trees selected for calcium analysis represented the 10 treatments in the

experiment design replicated by three of the experimental blocks (Blocks 3,4 and 5). A total of

twenty fruit were sampled per tree over four size categories. Fruit was then stored at 2°C for

six weeks.

Fruit tissue sampling which analyses the cortical plugs of tissue from just inside the skin

as described by Turner et ale (1977) was used for calcium analysis in 1994. An equatorial slice

(approximately lcm thick) was taken from each of the 5 fruit in the sample, and two plugs of

cortical tissue were then taken with a cork borer (diameter IOmm) from the slice inside the skin.

The 10 plugs were then combined and weighed.

Following fruit sample weighing, the same procedures described previously for calcium

analysis of fruit in the 1993 season were used for determination of fruit calcium concentrations

in the 1994 season. Fruit calcium contents in 1994 were subsequently estimated from this data.

72

4.2.3. Statistical analysis

Statistical analysis was performed using the Genstat statistical program, and analysis of

variance was carried out on untransformed data. Graphic presentation and accompanying

modelled relationships were performed using the graphics package Sigmaplot version 2.0.

73

4.3. Results

Bagged trees produced fruit with higher calcium concentrations than those produced on

unconfmed trees in 1993 and 1994 (Table 4.1). There were no significant differences in fruit

calcium concentrations within bag treatments and/or between rootstocks in either year (Table

4.1). In the 1994 season, in which a range of fruit sizes were sampled, fruit calcium

concentrations increased with decreasing fruit size (Table 4.1).

Substantial variation in fruit mineral concentration occurs within a tree (V olz et al.,

1994) of which part can be explained by variation in fruit size (Perring and Jackson, 1975). In

the 1993 season, the effect of root restriction does not appear to be due to fruit size, as the fruit

sizes of all samples were similar (data not shown). Similarly in the 1994 season, fruit calcium

concentrations were higher in bagged trees compared with unconfmed trees, but the bag by fruit

size interaction was not significant.

Bagged trees also produced fruit with higher calcium contents (calculated from fruit

calcium concentration and fruit weight) than those produced on unconfmed trees in 1993 and

1994 (Table 4.2). There were no significant differences in fruit calcium contents within bag

treatments and/or between rootstocks in either year (Table 4.2). The effect of root restriction

on fruit calcium content also appears to be independent of fruit size, as fruit sizes of all samples

were similar in 1993 (data not shown) and in 1994, the bag by fruit size interaction was not

significant. In 1994, fruit calcium content increased with increasing fruit size (Table 4.2).

Fruit calcium concentrations have been shown to be influenced by crop density

independently of fmal fruit size (Ferguson and Watkins, 1992). To further investigate this

possibility, the relationship between fruit calcium concentration and crop density was examined.

There was considerable variation in fruit calcium concentrations in 1993 which did not appear

to be directly influenced by crop density (Figure 4.1). As a consequence no attempt was made

to estimate such a relationship.

74

In the 1994 season, over all of the fruit size categories, fruit calcium concentrations

increased as crop density increased (P< 0.10) (Figure 4.2). Additions of fruit size and its

interaction with crop density to the nonlinear model were not significant. However, the low r value for the model suggests that other factors contribute to the observed variation in fruit

calcium concentrations. Confirming this view, in all but the 150 to 169 gram fruit size range,

analysis of fruit calcium concentrations using crop density as a covariate were found to be non

significant. The linear relationship between crop density and fruit calcium concentration was not

significant.

75

Table 4.1. The effect of bag volume, rootstock, and fruit size on apple fruit calcium

concentration in the years 1993 and 1994.

Bag

Rootstock

Fruit size (grams)

SED (bag)

(rootstock)

(fruit size)

Contrast between 'bag' vs nil2

Contrast between rootstocks

Contrast between fruit sizes

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

90-109

130-149

150-169

190-209

1993

Calcium concentration (mg/l00g)l

2.8

2.3

2.2

2.1

1.7

2.3

2.1

0.32

0.20

**

ns

1 Calcium concentration expressed as mg per 100 grams fruit fresh weight.

1994

Calcium concentration

(mg/l00g)

1.6

1.8

1.5

1.7

1.1

1.6

1.5

1.7

1.6

1.5

1.4

0.12

0.08

0.08

**

ns

*

2 Contrast significant at P<0.01 and 0.05 denoted by **,* respectively; ns is not significant.

76

Table 4.2. The effect of bag volume, rootstock, and fruit size on apple fruit calcium

content in the years 1993 and 1994.

Bag

Rootstock

Fruit size (grams)

SED (bag)

(rootstock)

(fruit size)

Contrast between 'bag' vs nil2

Contrast between rootstocks

Contrast between fruit sizes

10 litre

25 litre

48 litre

102 litre

Nil

MMI06

MM115

90-109

130-149

150-169

190-209

1993

Calcium content (mg) 1

3.5

2.9

2.7

2.6

2.1

2.9

2.7

0.39

0.25

** ns

1994

Calcium content (mg)

2.4

2.5

2.2

2.4

1.6

2.3

2.1

1.7

2.2

2.4

2.5

0.16

0.10

0.12

** ns

** 1 Calcium content calculated as calcium concentration multiplied by fruit weight (1993) and mean weight of

fruit in sample (1994). 2 Contrast significant at P<O.Ol denoted by **; ns is not significant.

6

A

5j 0

• 0

• 0

§>4

... • 0 0

,... Cb # • g • ...

0 0

c: 1 0

~ , (:)

C 3 o • A 15 0 • Q) ... 0 A 0 c: • • A 0 • • 0 • 0

E ... ...... ... 0 Ov ... <> ;:, • .O·A

0

·0 2 • -a .... ......... . .... 0 0 ••• • ~ 0 0 Of:::. ... • A 00

~ ... ..... 0. V 0 0 <0 0 No bag

0 0 • • 0 10litre bag ..... • • 1 -1 ~ ·0 0

A 25litre bag - 0 v 48litre bag

• • 0 1 02 litre bag

• 0 0 Open symbols - trees in on year

• Closed symbols - trees in off year

0 0 5 10 15 20 25 30 35

Crop density (fruit number/trunk cross-sectional area (cm2»

Figure 4.1. The effect of crop density on apple fruit calcium concentration in 1993.

40

.....)

.....)

3

'Ci 0 0 2 ...-0, E -c: 0

~ E Cl) (.) c: 8 E ::l ·0 1 as C,)

0

Figure 4.2.

• V 6,

6, ~

• 00 6,0

6,

~ 6,

6, 6, 0 ~ • 6, sg 6,

0

• 0 • • • • • • 0

ovo 6, vO 6, 6,

00 0 0

° v o 0 0 0 °8

• • • 0

• • • 0 No bag I 0 10litre bag

v 6, 25litre bag

v v 48litre bag 0 1 02 litre bag

° Open symbols - trees in on year

• Closed symbols - trees in off year Fitted line

0 5 10 15 20 25 30

Crop density (fruit number/trunk cross-sectional area(cm2»

The effect of crop density on apple fruit calcium concentration in 1994.

y=2.086 - 1.102 x 0.9376x r=20.9 P<0.10

35

-..J 00

79

4.4. Discussion

Few studies have reported the effects of root restriction on fruit quality or fruit mineral

concentrations in perennial fruiting trees. In the present study, root restriction resulted in higher

fruit calcium concentrations and contents. This effect appears to be independent of fruit size.

However, significant variation in fruit mineral content can still occur at any given fruit size

(Perring and Jackson, 1975).

The trees carried a range of crop densities in both years which was accentuated by a

biennial bearing habit (Chapter 3, Table 3.6). This may have influenced fruit calcium

concentrations. Ferguson and Watkins (1992) found that light-cropping apple trees compared

with trees with heavy crop loads produced fruit with lower calcium concentrations. This effect

was independent of fruit size. Characteristics of light cropping trees such as vigorous shoot

growth and higher shoot:fruit ratios (Sharples, 1968) are likely to affect mineral movement into

developing fruit.

In the present study, the relationship between fruit calcium concentration and crop density

did not satisfactorily explain the variation in fruit mineral concentration between the bagged and

unconfmed treatments (Figures 4.1 and 4.2). It is possible other factors such as fruit position

(Ferguson and Triggs, 1990; Volz et al., 1994) and seed number (Bramlage et al., 1990)

partially obscured the influence of crop density and may have contributed to the higher calcium

concentrations and contents found in fruit from bagged trees. In 1993, fruit which occupied the

outer half of the branches and were able to be picked from the ground were randomly sampled

for calcium analysis. Nonetheless, Volz et al. (1994) reported fruit position on the outside of

the canopy can also contribute to large variations in fruit mineral composition. As a result, fruit

positional effects cannot be dismissed.

Schumacher et al. (1980) reported that transport of calcium to the developing fruit was

inhibited by intensive shoot growth which was confirmed by assessing the incidence of bitter pit

in the crops of trees of medium and high vigour. In the present study, vegetative growth was

80

influenced by root restriction. The growth rate of the unconfined trees continued to increase

throughout the experiment. In contrast, bagged trees maintained a similar growth rate throughout

the experiment (Chapter 3, Figure 3.1). Additionally, calcium sprays were applied during each

growing season and differences in spray penetrability and thoroughness of spray application may

also have contributed to the variation in fruit calcium concentrations and contents.

Alternatively, a possible explanation for increased fruit calcium concentrations and

contents may be because of an increase in the proportion of white roots relative to the whole

root system under root restriction conditions. Calcium has been shown to have poor mobility in

symplastic pathways of roots (Ferguson and Clarkson, 1976) and as a result it has often been

assumed that the absorption of calcium occurs entirely through the younger regions of roots.

Calcium uptake in apples has been reported to be similar for woody and white roots (Atkinson

and Wilson, 1980). However, the contribution to nutrient uptake made by the different root

types will not only depend on the inherent rates of absorption but on their abundance and contact

with the surrounding soil (Atkinson and Wilson, 1979; Atkinson and Wilson, 1980).

In root control bags, root growth is limited by the radial constraint of the bag's wall

material and as a consequence fresh new roots emerge within the bag and pass through the wall

creating a large number of rme roots around the periphery of the bag. This situation differs to

that reported by AI-Sahaf (1984) and Aloni (1986) who concluded that root restriction impairs

calcium uptake and translocation, as roots were restricted within the confines of non-porous

containers with limited opportunity for new root growth. In a further experiment with tomatoes,

AI-Sahaf (1984) reported that releasing plants from root restriction compared with unconfined

plants, reduced the severity of the effect of a calcium-deficiency treatment (as indicated by

general appearance and percent of fruits with blossom end rot). The author suggested that less

competition between the leaves and fruits for calcium and a greater ability of de-restricted plants

to remobilise stored calcium, particularly in the stem may have accounted for this result. The

release of root restricted plants may be more analogous to the root environment encouraged by

81

root control bags. To date, no root excavations have been conducted on the 'Fuji' apple root

restriction trial at the Lincoln Universities Research Orchard to substantiate such a possibility.

Another possible explanation is that root restricted trees experience higher daytime water

deficits. Lang (1990) showed that the diurnal pattern of fruit shrinkage and expansion in apples

occurs as a result of reversals of xylem sap flow between fruit and tree. When xylem sap cycles

each day between fruit and tree, much less calcium will leave the fruit in the outflowing sap than

will enter it in the inflowing sap due to lower calcium concentrations in the fruit xylem sap than

in the spur wood xylem sap (Lang and Volz, 1993). Thus any factor which increases the volume

of xylem sap leaving the fruit during the day will also increase the volume flowing into the fruit

at night and so increase fruit calcium (Lang and Volz, 1993).

Hameed et ale (1987) proposed the principal effect of confIDing roots to a restricted root

zone is to induce drought stress in the shoots through increased hydraulic resistance within the

root system. Other studies (Krizek et al., 1985; Dubik et al., 1990) have suggested that the

impairment of growth caused by soil moisture stress and root restriction involve different

physiological mechanisms. Nonetheless, Krizek et ale (1985) reported that restriction of root

zone volume may cause symptoms of water stress under conditions of high transpiration possibly

due to a decreased ability of water absorption by the roots.

Furthermore, it is possible that a change in the allocation of dry weight in favour of the

shoot in the smaller bagged trees could contribute to higher daytime water deficits in root

restricted trees. Increased shoot:root ratio's in response to root restriction have been reported

in previous studies (AI-Sahaf, 1984; Tschaplinkski and Blake, 1985; Peterson et al., 1991).

However, others have reported no change in the shoot:root ratio in response to root restriction

(Richards and Rowe, 1977a; Richards, 1981; Krizek et al., 1985; Dubik et al., 1990). Despite

no change in the shoot:root ratio on a fresh weight basis, Richards (1981) reported that in dry

weight terms, plants grown in larger pots apportioned more growth into their roots than those

in small pots. The influence of fme root growth on a possible redistribution of carbohydrate

82

within the plant where root volume is restricted using fabric porous containers compared to non

porous containers is not clear.

The decrease in fruit calcium concentrations with increasing fruit size confirms previous

studies (Perring and Jackson, 1975; Ferguson and Triggs, 1990). As the rate of fruit expansion

is greater than the rate of mineral uptake (Ferguson and Watkins, 1989) fruit growth will lead

to an increasing dilution of the mineral content. Calcium accumulation was greater for larger

fruit which confirms the findings of Volz et ale (1994). The marked difference in calcium

concentrations between years may be a reflection of the two sampling methods used. Apple core

and skin have higher calcium levels than the cortex (Perring and Wilkinson, 1965). The higher

fruit calcium concentrations and contents found in 1993 may have been caused by the inclusion

of core and skin in segment analysis used in this year compared to the cortical plug analysis used

in 1994.

Rootstocks failed to produce significant differences in fruit calcium concentrations or

contents. These results are similar to those reported by Terblanche et ale (1980). However, in

some cases, bitter pit incidence has been related to differences in fruit calcium content caused

by rootstock effects (Ferguson and Watkins, 1989). Ferguson and Watkins (1989) suggest that

it is the distribution and growth characteristics of the root system that are the important factors

in nutrient supply by different rootstocks. In the present study, it seems likely that rootstock

differences in root distribution and growth characteristics were largely dominated by the root

restriction treatment. Root restriction appears to have reduced the invigorating effect of MM115

as the difference in trunk cross-sectional area or canopy volume (the absolute difference or

expressed as a percentage) between the two rootstocks was considerably greater in the

unconfined than in the bagged treatments (refer to Chapter 3).

Many physiological effects of root restriction have not been determined for fruit trees

(Ferree, 1992). Lower foliar mineral concentrations have been reported in response to root

restriction (AI-Sabaf, 1984; Dubik et al., 1990). However, it is yet to be determined whether

83

reductions in nutrient uptake cause slower shoot growth or reflect a lower shoot demand. AI-

Sahaf (1984) reported potassium, calcium, and magnesium concentrations in tomato fruit were

similar between root restricted and unconfmed plants in hydroponic glasshouse conditions.

However, roots were restricted within the confines of non-porous containers with limited

opportunity for new root growth whereas the fabric bags used in the present study allows new

root growth around the periphery the container and this may have contributed to the higher fruit

calcium concentrations and contents observed for bagged compared with unconfined trees. The

author does not know of any other studies reporting the effects of root restriction on apple fruit

mineral concentrations under field conditions. The results of this study suggest that apple fr,uit

from root restricted trees possess higher calcium concentrations and contents and thus may be

less susceptible to the development of calcium related disorders such as bitter pit. Further

investigation is needed to determine the importance of crop density, root tum-over, water

relations and fruit position on the final fruit mineral status under root restriction conditions.

84

CHAPTERS

Conclusions.

Root growth control using root restricting bags reduced tree size. Reductions in tree

growth due to root restriction were reflected in decreased trunk cross-sectional area (TCSA) ,

canopy volume and tree height. This reduction in tree size was substantial as evidenced by the

87% decrease in canopy volume (averaged across all bag volumes) by the end of the sixth

growing season. Throughout the experimental period, TCSA decreased linearly as bag volume

was reduced. However, growth differences in TCSA within bag sizes were not reflected in

canopy volumes.

MMI06 rootstock reduced tree size compared with MM115. This was shown by

decreased TCSA and canopy volume. The magnitude of the growth reduction was evidenced by

a 25 % decrease in canopy volume. However, root restriction reduced the invigorating effect of

MM115 as the difference in TCSA or canopy volume (the absolute difference or expressed as

a percentage) between the two rootstocks was considerably greater in the unconfmed than in the

bagged treatments.

The smaller trees resulting from root restriction had lower yields when compared with

the unconfmed treatment. However, the cumulative yields per unit of TCSA were higher where

roots were restricted, indicating these trees had a higher productive efficiency. Biennial bearing

masked this effect in some years, and there were no differences in yield within bag volumes or

between rootstocks. The effect of root restriction across all bag volumes was to increase

cumulative yield efficiency by between 215 to 1589 grams per cm2 of TCSA at the 95%

confidence interval.

Treatment differences in mean fruit weight and fruit size distribution can be explained

by their effect on crop density.

85

The alternating behaviour of crop density, yield efficiency, and mean fruit weight

indicates that both root restricted and unconfined trees experienced an 'overcharge' of fruiting

in some years. Bienniality indices calculated using yield and yield efficiency values confirmed

this. There were no differences between treatments in the propensity to biennial bearing. Further

work is required to establish whether optimum crop loads differ between root restricted and

unconfmed trees.

Fruit calcium concentrations, across all treatments increased with decreasing fruit size,

thus confirming previous studies. Root restriction increased apple fruit calcium concentrations

and contents. This effect was independent of fruit size and was not adequately explained by

differences in crop density between treatments.

86

CHAPTER 6

Agronomic implications and further research.

6.1. Aa:ronomic implications

The control of excessive vegetative vigour is a major consideration in achieving

maximum yields throughout the life of the orchard. Reducing shoot competition improves flower

initiation and fruit set. A further benefit of reducing excessive vegetative growth, is enhanced

light penetration into the tree which gives better fruit quality, especially in terms of fruit colour

and fruit size (Wilton, 1993). In New Zealand, mild climatic conditions and moderate rainfall

ensures that vigour control is an essential management practice especially where orchards have

been planted on deep fertile soils.

The results of research presented in this thesis suggest that root restriction can

substantially reduce vegetative vigour. Root restriction may also be used in conjunction with the

appropriate rootstock. The selection of rootstock which has a level of vigour that is compatible

with tree spacing is important to maintain trees to their allotted space. With apple, the successful

use of root restriction would expand the range of vigour control provided by currently available

rootstocks. This should have immense value for selecting rootstocks to match the orchard site

and provides the orchardist with further options for vigour control where issues such as pest

resistance for example, woolly apple aphid in New Zealand narrows the range of feasible

rootstocks. In other fruit crops, such as stonefruit, where the range of proven size-controlling

rootstocks is more limited, root restriction has the potential to become a key component in the

management of tree vigour.

The smaller trees resulting from root restriction may result in lower yields per tree as

observed in this trial. However, this yield decrease was partly negated by increases in tree

productivity in terms of increased yield efficiency which supports the findings of previous studies

87

(Williamson and Coston, 1990; Myers, 1992). Furthermore, in New Zealand, orchards have

been predominantly planted at semi-intensive densities which have proven to be very efficient

if well managed. Recently higher density plantings have attracted increasing evaluation (Wilton,

1993) as orchardists seek to become more efficient and produce more fruit per area of orchard.

The results of this trial suggest that orchard planting densities compared with current practices

could be increased by factor of six times as a consequence of smaller tree size resulting from

root restriction. At this higher density, root restricted trees would have produced a significantly

higher yield per hectare of orchard compared with standard spacing of unconfined trees.

However, care should be taken when extrapolating results presented here to commercial

circumstances as trees were not pruned or thinned. The integrated use of root restriction and tree

management needs further investigation.

In the field, restriction of the rooting volume may be achieved in several ways. Shallow

soils or soils that are impenetrable to roots can restrict root growth. This highlights the

opportunity to restrict rooting volume through site selection. Root growth may also be restricted

through the use of porous fabric containers as was the case in this study or alternatively through

fabric lined trenches as demonstrated by Williamson et al. (1992). Drip irrigating a constant

volume of soil has also been suggested to restrict rooting volume (Bravdo et al., 1992).

Successful restriction of root growth through the use of drip irrigation is only likely to succeed

in regions of infrequent precipitation and shallow soils where root spread is not extensive. The

use of these techniques in combination is likely to lead to enhanced control of root growth and

greater adaptability to a wider range of growing conditions.

A major hurdle under field conditions may be the limitation imposed by restricting the

root volume on water and nutrient availability. With restricted root volume, the buffering

capacity against events such as drought is reduced, increasing the risk of tree damage and crop

loss (Henry, 1993). Exacting water and fertilisation management will be required to prevent

water stress or nutrient deficiencies developing in root restricted trees in the field.

88

Mineral concentrations are important determinants of apple fruit quality during storage.

Calcium has been associated with numerous physiological disorders in apple fruits, including

bitter pit, lenticel blotch and breakdown, corkspot, cracking and water core (Wills et al., 1989).

The results of this study suggest that fruit from root restricted trees possess higher calcium

concentrations and contents and thus may be less susceptible to the development of such

disorders.

6.2. Further research

The physiological basis by which root restriction controls tree growth and increases

productivity is not clear. Additional research is required to study the role of water relations,

growth regulators and nutrition in tree responses to root restriction in the field. The nature and

severity of root restriction may have important implications for plant responses. Where root

volume is restricted using non porous containers there is limited opportunity for new root

growth. In contrast, root restriction achieved through the use of porous fabric containers such

as in this experiment, promotes new root growth in the form of fine roots around the periphery

of the container and may increase the capacity of the root system to act as a sink for

carbohydrates. As discussed in chapter 4, this may also have implications for other plant

processes such as nutrient (calcium) uptake.

Williamson and Coston (1990) suggested that optimum crop loads for trees with restricted

root systems could be different from unrestricted trees. The negative relationship between fruit

size and crop density indicates a 'trade off' exists between obtaining desirable fruit size and tree

yield. The results presented in this thesis confirms there is a need to determine optimum crop

loads to obtain desirable fruit size for root restricted trees before productivity comparisons

between root restriction and conventional planting systems can be further evaluated.

Additionally, determination of optimum crop loads should account for the level of cropping

required to reduce yeady yield fluctuations in cultivars predisposed towards biennial bearing.

89

The effect of root restriction on fruit quality remains unanswered. Further research is

required to elucidate the mechanism(s) by which root restriction increased fruit calcium

concentrations and contents. The results of this study suggests that such research would be best

directed by determining the importance of crop load, root tum-over, water relations and fruit

position on fmal fruit mineral status under root restriction conditions.

Additionally, enhancement of fruit colour in deficit irrigated apple trees has been

suggested to be associated with reduced shoot growth (Mills, 1993). Similar effects on fruit

colour may result from root restriction. Visual observations in the 1992 season, suggested a

difference in fruit colour at harvest between the restricted and unconfmed trees in this trial. In

the 1993 season, fruit from one block were graded for colour by eye using a New Zealand

Apple and Pear Board export colour grading chart (1993) as a reference. Plate A2.1 (Appendix

2) illustrates the colour grades, depicted by a representative apple in each colour category. Root

restricted trees appeared to have a higher percentage of the trees crop in the more colour full

grades (Appendix 2, Figures A2.1 and A2.2). Due to insufficient replication, no statistical

analysis was performed. However these results suggest that further research on the effects of

root restriction on fruit quality is warranted.

90

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113

APPENDICES

Appendix 1 Additional data for chapter 3

Table A1.1. The effect of bag volume and rootstock on trunk cross-sectional area growth (cm2) in the years 1991, 1992, 1993, and 1994.

1991 1992 1993 1994

MM106 MMl15 MM106 MMl15 MM106 MMl15 MM106 MMl15

Bag 10 litre 8.0 11.2 11.1 14.9 15.0 21.1 18.4 25.7

25 litre 8.9 9.8 11.8 12.9 16.0 17.2 18.7 20.5

48 litre 11.1 12.3 15.0 17.1 22.1 23.9 26.6 28.1

102 litre 11.4 13.5 16.8 18.0 23.5 24.5 30.1 30.0

Nil 16.7 20.6 28.5 40.0 48.5 61.4 70.5 95.9

SED (Bag) 1.26 1.79 2.82 3.57

(rootstock) 0.79 1.13 1.78 2.26

(bag rootstock) 1.78 2.53 3.99 5.06

Contrast between 'bag' vs nill ** ** ** **

Linear contrast within 'bag' ** ** ** **

Contrast between rootstocks ** ** ** **

Contrast between nil vs ns ** * ** 'bagged' rootstock

..... 1 Contrast significance at P<O.Ol and 0.05 denoted by **, * respectively; ns is not significant. .....

~

115

Table Al.2. The effect of bag volume and rootstock on the number of wind-faUen fruit

per tree (expressed as a percentage of the tree's total crop) in the years 1992

and 1993.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast of 'bag' vs nill

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MM106

MMl15

1992

6.9

4.3

3.1

4.7

9.1

6.2

5.0

2.13

1.35

** ns

1 Contrast significant at P< .01 and 0.05 denoted by **, * respectively; ns is not significant

1993

4.0

5.4

3.0

5.1

8.0

5.6

4.6

2.25

1.42

* ns

Table Al.3. The effect of bag volume and rootstock on fruit number per tree in the years 1991, 1992, 1993, and 1994.

Bag

Rootstock

SED (bag)

(rootstock)

Contrast of 'bag' vs nill

Contrast between rootstocks

10 litre

25 litre

48 litre

102 litre

Nil

MM106

MMl15

1991

42

33

34

43

32

31

43

9.6

6.1

ns

ns

1992

224

226

255

257

308

244

263

42.0

26.6

* ns

1 Contrast significant at P< .01 and 0.05 denoted by **, * respectively; ns is not significant

1993

176

164

160

214

770

272

322

61.2

38.7

** ns

1994

303

284

396

369

764

450

396

92.1

58.2

** ns

.......

....... 0\

117

Appendix 2 Effects of root restriction on fruit colour

Plate A2.1. Representative apples depicting colour grading scale.

118

10 litre bag 25 litre bag

50 50 Q) Q)

"0 "0 e e CJ) 40 CJ) 40 :; :; 0 0

"8 30

"8 30 .5 .5 - -:g :g

'0 20 '0 20 Q) Q) CJ) g> S c::::

10 'E

10 Q)

~ e Q) Q)

0.. 0..

0 0

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Colour grade Colour grade

48 litre bag 102 litre bag

50 50 Q) Q)

~ "C e

CJ) 40 CJ) 40 :; ~

::J 0 0

"8 30

"8 30 .5 .5 - -:g :g

'0 20 '0 20 Q) Q) CJ) CJ) as as 'E 10

"E 10 ~ ~

Q) Q) 0.. 0..

0 0

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Colour grade Colour grade

Unconfined

50 Q) "0 e CJ) 40 :; 0 "0 0

30 .5

:g '0 20 Q)

g> "E

10 Q)

e Q) 0..

0

1 2 3 4 5 6 7 8 9 10 11 12

Colour grade

Figure A2.1. The etTect of bag volume on the number of fruit (expressed as a

percentage of the trees total crop) falling into colour grades on MMI06

rootstock.

119

10 litre bag 25 litre bag

50 50 CJ) CJ)

"0 "0

e e C) 40 C) 40 :s :s 0 0 "0 (5 0 0

30 .£ 30 .£ - -:g :g 15 20 15 20

CJ) CJ) C) C)

ctI ctI C C

10 ~ 10 CJ)

e CJ) CJ)

0.. 0..

0 0

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Colour grade Colour grade

48 litre bag 102 litre bag

50 50 CJ) CJ)

"0 "0 e e C) 40 C) 40 :s :s 0 0

"8 "8 30 .£ 30 .£

~ ~ 15 20 15 20

CJ) CJ) C) C) ctI ctI 'E 'E

10 ~ 10 CJ)

e CJ) CJ)

0.. 0..

0 0

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

Colour grade Colour grade

Unconfined

50 CJ) "0 e C) 40 :s 0

"8 30 .£

~ 15 20 Q) C) ctI C 10 CJ)

e Q) 0..

0

1 2 3 4 5 6 7 8 9 10 11 12

Colour grade

Figure A2.2. The etTect of bag volume on the number of fruit (expressed as a

percentage of the trees total crop) falling into colour grades on MMl15

rootstock.