3.3 Water potentials of plant cells and tissues3.3.1 Forces determining cellular water potentialThe water potential of a plant cell is determined by three kinds offorces which affect the free energy of the cellular water. (1) In plant cells, the cell wall exerts a hydrostatic pressure, theturgor pressure (wall pressure) on the protoplast; a cell within acompact tissue may also be under pressure from surrounding cells.Hydrostatic pressure in excess of atmospheric increases the freeenergy and raises water potential; thus the pressure potential Cp isa positive value.(2) Plant cells contain low-molecular-weight solutes, mainlyvacuolar in a vacuolated cell. These exert osmotic forces, whichdecrease the free energy and lower the water potential; the osmoticpotential Cp is therefore a negative value. (Osmotic pressure isnumerically equal to osmotic potential, but has a positive sign.)(3) Plant cells contain high-molecular-weight colloids, in thecytoplasm and the cell wall. Matric forces exerted by colloidsdecrease the free energy of water and lower the water potential;their effect is represented by the matric potential Cm. Surfacetension forces at air/water interfaces in cell wall capillary spacesalso contribute to Cm. 3.4 Water relations of whole plants and organsThe water relations of a whole plant, or even an organ such as a leaf,are much more complex than those of individual cells. The formulaegiven in the preceding section, relating water potential to pressureand osmotic (and matric) potentials are applicable only at the cellularlevel. There is no such thing as the Cp of a whole plant; the value willvary between different tissues. The overall C, too, usually variesbetween different parts of a plant. Most flowering plants are landplants and, in the terrestrial environment, theCof the atmosphere isnearly always much lower than that of plant tissues, often by tens ofMPa, and hence there is a great tendency for water loss from theplant. The large surface area necessitated by the photosyntheticmode of life provides a large surface for evaporation of water, transpiration.This loss must be made good from the soil, which isgenerally at a much higher C than the atmosphere or the plant.Hence for most of the time there is a flow of water through theplant, along a C gradient, as below:soil ____! root ____! stem ____! leaf ____! airThis flow is frequently termed the transpiration stream. Only in timesof a water-saturated atmosphere, or in times of extreme drought,may there be equilibrium, more or less, between plants and theenvironment, with gradients within the plant eliminated and watermovement nearly at a standstill. Of the water absorbed by the roots,only a very small fraction is retained by the plant in temperatehabitats. For maize, an annual, this fraction has been estimated atless than 1% of the water absorbed during its growing season. Duringone bright sunny day leaves may transpire several times their ownweight of water; a leaf of Senecio jacobaea growing on a sand dune cantranspire its own weight in water in 45 minutes. The water content ofaerial organs of a plant is generally lower in the daytime, when therate of transpiration is high, than during the night, when the transpirationrate is much lower (owing to the lower temperature andclosure of stomata) and the deficit is made good. In the roots, whichexperience much less temperature change over the diurnal cycle andwhich have no stomata, the water content fluctuates much less.3.4.1 Absorption of water by rootsThe root systems of plants are often very extensive. Roots of someplants extend much further underground than the shoot rises intothe air (Fig. 3.3). The roots of apple tree (Malus domestica) may go downto about 10 m, and even in herbaceous plants such depths can bereached, e.g. in alfalfa (Medicago sativa). The actual area of a rootsystem is formidable. It has been reported that a rye (Secale cereale)plant had a root surface area of over 600m2, of which two-thirds was root hair area, and the total length of the root system was over 11 000km, including 10 000km of root hairs. The total area of the shoot(including areas of cells bordering leaf air spaces) was only 28m2.Most of the water absorption takes place near the root tips, wherethere is a thin epidermis with root hairs (Fig. 3.4). Not only do the roothairs provide a large area, but they make intimate contact with thesoil, bending around soil particles and penetrating into tiny crevices.As the root tissues mature, the epidermis with its hairs is replaced bya more impermeable suberized periderm. For efficient water uptake,root growth must continuously regenerate the absorbing zonebehind each growing apex. Continuous growth is also necessary toinvade new areas of soil, for there is little lateral movement of waterin soil compared with downward drainage directly after water addition.Water will not move to the roots, so roots must grow to thewater. Positive hydrotropism (Chapter 12) may direct root growthtowards water. Roots can grow rapidly; a rate of 10 mm per day iscommon for grasses; maize (Zea mays) roots can extend by as much as5060mm per day. The average daily increase in length of the totalroot system of the rye plant discussed above (hairs excluded) wasalmost 5 km.Though the root hair zones provide the most efficient waterabsorbingsurfaces, uptake in older regions is still appreciable,particularly during conditions of water shortage and at times whenroot growth is slow, such as in winter. Points of emergence of lateralroots break the suberized layers and enable the entry of water.3.4.2 The route of water movement through the plantThe xylem as the water-transporting systemThe main channel for upward/long-distance movement of water inthe plant is the xylem, the wood. When the tissues outside the xylemare peeled off over a short length of woody stem where the xylem iscentral, the conduction of water beyond the stripped region continuesunimpeded. The non-living cells of the xylem are filled witha watery sap, at least in young wood, and dyes and Indian ink can beseen to move in the xylem. Toxic solutions have been shown to passfrom roots to leaves, indicating that the route does not involve livingcells; heat-killed stems, too, can conduct water. Chilling does not stopwater movement as long as no freezing occurs. All this points to nonlivingcells of the xylem as the water-conducting cells. When the lumina of these cells are blocked with mercury or cocoa butter, watermovement is inhibited.The xylem is a complex tissue. In addition to water-conductingcells and lignified fibres which all are dead at maturity, it containsliving parenchyma cells and sometimes also living transfer cells (fortransfer cells, see Section 5.4.1). Functional xylem is accordingly not adead tissue, though it contains a large proportion of dead cells. Inflowering plants there are two kinds of conducting cells, the tracheidsand the vessel elements. They have lignified secondary wallswith the secondary thickening laid down in distinctive patternsleaving areas of primary wall as pits; these facilitate the passage ofwater from cell to cell (Fig. 3.5). Somewhat confusingly, the thinprimary wall across the pit is often called the pit membrane.The tracheids function as single cells, but vessel elements arejoined to make elongate vessels by the perforation or partial breakdownof end walls in files of cells; these end walls are then known asperforation plates (Fig. 3.5). Their possession is the distinguishingfeature of the vessel element. The diameters of vessel units rangefrom below 10 mm to several hundred mm, even 1000 mm in somelianas. Tracheid diameters overlap with those of the narrower vesselelements. Any one piece of xylem has conducting elements of variedwidth; this may be of functional importance (Section 3.4.4). Thelengths of vessels range from under 1cm to 10 m or more, and arevery variable even within the same plant. In some trees some continuousvessels run right from the crown to the roots, but most vessels areshorter than the height of the plant, and even in trees many vesselsmeasure only a few centimetres. The possession of vessels makinglong continuous channels for water movement is considered to beone of the advanced features of flowering plants; the earliest landplants had only tracheids, and while the evolution of vessels hasoccurred in several divisions of land plants they are lacking in theconifers. Tracheids offer much more resistance than vessels to watermovement.In woody perennials, new layers of xylem known as annual ringsare produced each year during the growing season. The older regionsof xylem eventually lose their water-conducting capability andbecome air-filled or blocked by ingrowths (tyloses) from adjacentliving parenchyma cells, or by gums, resins and tannins. The waterthen moves only through the young, outer xylem, the sapwood,which may comprise only the current years growth or include afew youngest annual rings. The inner non-conducting xylem isknown as the heartwood.Living parenchyma cells make up rays in secondary wood, runningradially through the xylem from the pith towards the cortex(Fig. 3.6) and lying also among the conducting cells. They storeorganic nutrients, and some botanists have assigned a role in watertransport to them (Section 3.4.4). Transfer cells are not always presentin xylem, but may occur next to conducting cells especially inleaf veins; they have highly involuted cell walls, which gives them a spaces; there are reports of these spaces in roots containing fluid. Thesymplast is the collective living part of the plant; nearly all the livingcells of the plant body are joined by plasmodesmata, submicroscopicprotoplasmic connections of diameters around 50 nm. In the symplasticroute, water has to cross a plasma membrane to enter thecytoplasm of an outer root cell; it would then move in the cells withinthe cytoplasm, around the vacuoles, and from cell to cell through theplasmodesmata without the necessity to cross more membranes, tillit exited into a xylem conducting cell. The transcellular route envisagesmovement straight through the vacuoles, crossing the tonoplastsof each cell; cell-to-cell movement could be via plasmodesmataor crossing the plasma membranes.The apoplast route has been supported on the grounds that it isthe path of least resistance, with minimal traversing of membranes.However, the radial walls of endodermal cells at the level of mostactive water absorption develop strips of wall thickening, theCasparian strips, chemically resembling the water-impermeablesuberin (Fig. 3.9). In older parts of the root, all endodermal wallsexcept the outer tangential ones become heavily thickened. It istherefore frequently suggested that at least at the endodermiswater must pass through living protoplasts, and that this layer regulateswater movement to the root xylem. There is some evidence tosupport this idea, but also data to the contrary, probably reflecting the extent of wall thickening in the material studied. Decreasedaquaporin content in plasma membranes of tobacco roots (Nicotianatabacum), achieved by antisense repression of an aquaporin mRNAsynthesis, has decreased greatly the roots hydraulic conductivity.This indicates that there is movement through the plasma membranesand certainly supports the symplast route. But the data donot exclude the transcellular route. This route has been criticized asbeing the path of greatest resistance in view of the large number ofmembranes traversed. However, the density of aquaporins in tonoplastsis very high (up to 40% of total tonoplast protein), which maygive them a much higher permeability towards water than is shownby the plasma membranes, and crossing the vacuole may offer lessresistance than originally supposed. The three routes are notmutually exclusive and it is quite possible that all three contributeto water movement in proportions varying according to circumstances.When the rate of water movement is slow, most of themovement might be along the low-resistance apoplastic route, thehigher resistance pathways beginning to contribute when demandincreases. But the opposite has also been suggested, with a strongtranspiration pull (Section 3.4.3) increasing the apoplastic flow.In a herbaceous plant, where the vascular strands of the stem havenot been joined to a continuous vascular cylinder by secondarygrowth, any part of the root system normally supplies those partsof the shoot which are directly above it, these being the parts withwhich it is in direct vascular connection; lateral movement does notoccur or is very restricted. But if a part of the root system is deprivedof water, lateral movement is activated and the overlying aerial partsreceive a water supply from other root sectors. In trees with a continuous cylinder of wood, dye injection has shown that the pathof water movement frequently spirals round the stem, following ahelical arrangement of the conducting cells around the trunk.The route of water movement through stem and leaf tissue afterits exit from the xylem is also problematic. Fluorescent dyes introducedinto the xylem to act as tracers for water movement move fromthe conducting cells into cell walls or crystallize out in intercellularspaces in the shoot. Such observations have been interpreted asindicating an apoplastic route for water passing out of the xylem.However, careful analysis of the data points to the opposite view: thedye becomes concentrated in the apoplast precisely because thewater passes into the living cells near the xylem, leaving behindthe dye to which the plasma membranes are impermeable (Canny1990). It is therefore likely that water in leaves and stems first movessymplastically from the xylem, before it finally passes into cell wallsagain and evaporates.3.6 Water uptake and loss: control byenvironmental and plant factorsThe rates of water absorption and water loss, and consequently ofwater movement through the plant, are determined by an interactionbetween plant and environmental factors. The environmental factorscan be classified as soil (edaphic) and atmospheric. With regard to thesoil, important considerations are the amount and availability of soilwater, soil temperature and soil aeration. Above ground, the relevantfactors are atmospheric humidity, temperature, wind speed andlight. The plant factors are the area and water permeability ofthe absorbing surface in the roots; the area and water permeabilityof the evaporating surfaces of the shoot; the frequency of stomataand the degree of their opening. 8.2 Meristems and cell divisionMeristems are found throughout the plant. The apical meristemsgive rise to primary growth the shoot apical meristem (SAM) forming the above-ground structures (Section 9.2) and the root apicalmeristem (RAM) forming the below-ground structures (Section 9.6).The development of secondary meristems leads to branchingwhilst the cambial meristems, found outside of the apical regions,allow secondary growth leading to thickening, and hence strengthening,of structures. Not all of the meristems are active at any onetime and, indeed, the entire plant may enter a dormant state,whether as an embryo (Section 11.14) or as a mature plant, with there-initiation of cell division requiring specific environmental andinternal signals. Similarly, in a growing plant, growth hormonessuch as auxin, produced in the apical meristem, may generate signalsthat inhibit the development of the secondary meristems in theaxillary buds, leading to apical dominance (Section 7.2.2).Although growth normally occurs from the division of cellswithin meristems, many (but not all) differentiated plant cells retainthe ability to de-differentiate and regenerate an entire plant. Thisability is termed totipotency. Although this may be possible inanimals under exceptionally artificial circumstances (such as thoseused to clone Dolly the sheep from a differentiated udder epithelialcell), this ability is commonplace in plants. It is widely exploited inagriculture and horticulture to regenerate entire plants from cuttingsor single cells (Section 7.2.5). This is a fundamental requirement forthe genetic modification of many plants. Figure 8.1 shows an outlineof the different processes essential to plant development. All of thesemay be occurring at any time during a plants life cycle, even at anearly stage such as a developing seedling.Growth as a quantitativeProcess6.1 IntroductionGrowth is one of the most fundamental and conspicuous characteristicsof living organisms, being the consequence of increase in theamount of living protoplasm. Externally this is manifested by thegrowing system getting bigger, and growth is therefore often definedas an irreversible increase in the mass, weight or volume of a livingsystem. The size increase must be permanent; the swelling of a cell inwater is not growth, being easily reversed by returning the cell to asolution of lower C. It is, however, possible to consider as growthdevelopmental changes not immediately involving an increase insize. An amphibian embryo, or a Selaginella female gametophyte, fora long time utilizes the nutrient store with which it was released fromthe parent, to produce many new cells without any increase in overallsize, yet growing in the sense that living protoplasm is increasingat the expense of stored nutrients. Again, if dry mass is measured, aflowering plant seedling loses dry mass while utilizing reserves andgrowing.Growth is an exceedingly complex process. Every reaction associatedwith the synthesis and maintenance of living protoplasm isassociated with it, which makes it complicated enough at thecellular level. At the organismal level, it means the coordinatedmultiplication, size increase and specialization of millions of cells,all arranged in precise positions. Growth processes are also synchronizedwith seasonal changes, plants responding to appropriateenvironmental stimuli to achieve this synchronization. Thischapter is an introduction to growth and development of floweringplants, discussing the overall process, methods of measurement,elementary quantitative analysis of growth patterns, andgrowth rhythms. This is followed in the next chapters by a moredetailed discussion of events at the cellular level and then byan analysis in so far as it is feasible of the controlling andintegrating factors, hormonal, genetic and environmental, whichlead to the visible patterns of development at tissue and higherorganizational levels.6.2 The measurement of plant growthGrowth can be measured in a variety of ways. Since increase in theamount of protoplasm, the fundamental growth process, is difficultto measure directly, generally some quantity is measured which ismore or less proportional to the increase in protoplasm. In higherplants or their organs the four most commonly employed measures(parameters) are:(1) Fresh weight. This measurement is technically easy. However,a plant organ must be detached from the plant to be weighed, killingthe organ in the process, and an entire plant which has been removedfrom its growth environment can rarely be replaced undisturbed.Thus growth measurements by weighing almost always necessitatethe taking of successive samples from a series of plants.(2) Dry weight. This is sometimes considered more meaningfulthan fresh weight, because the increase in fresh weight may belargely the result of water uptake, and fluctuations in fresh weightmay occur as a result of chance fluctuations in a plants water content.In a germinating seedling, living on seed reserves, the dryweight decreases until the seedling builds up an adequate photosyntheticcapacity, and in such a case the fresh weight increase is abetter indication of growth.(3) Linear dimensions. For an organ growing predominantly inone direction, such as a root tip or a pollen tube, length is a suitablemeasure. Indeed linear elongation can be used to assess the growth ofan entire shoot. An increase in width (diameter) may be relevant inother cases, e.g. an expanding fruit or a thickening axis.(4) Area. This can be used to assess growth in a system extendingmainly in two dimensions, such as an expanding leaf.Measurements of length, width and area have the attraction that theycan be carried out on the same plant or organ over a period of timewithout destroying it. Growth in length can be measured with highlysophisticated auxanometers which detect or magnify size incrementsfar too small to be measured directly, and auxanometers record thedata automatically. Auxanometers can be sensitive enough to recordgrowth increments of less than 1 mmand hence can detect changes ingrowth rate within minutes of applying, say, a hormone to an organwith an overall growth rate of no more than 12mmh1. At the sametime the apparatus can be robust enough to be used in the field. Thedrawback of auxanometers is that they involve some kind of attachmentof the plant to the apparatus, although the force therebyexerted on the plant is generally considered to be too small to affectthe growth rate.Refined optical methods for measurement of length, width and areaare available. Beginning with simple time-lapse photography, wheregrowth is estimated from successive photographs with a camera electronically controlled to take exposures at set time intervals, thetechniques have progressed to the use of digital cameras interfacedwith computers to analyse the images. Great sensitivity can beattained both spatially and temporally, detecting small incrementsover short time periods (Schmundt et al. 1998). Photography involvesminimal interference with the growing system although somephysical fastening of the plant (organ) to a support may be necessary,for vibrations might be recorded as growth movements by theanalyser. The system is illuminated with near-infrared radiation,since much of the visible spectrum has the potential to affect plantgrowth rates.For assessing the growth of plant cell cultures, the methods appliedare those used for estimating the growth of microorganisms. Volumeof a cell culture can be determined by centrifuging the cells into agraduated centrifuge tube; for single cells, the volume can be calculatedfrom measurements made under the microscope, often utilizingconfocal microscopy. Increase in the turbidity of a cell suspension isproportional to the increase in cell number and this can also beobtained by direct counting in a special counting chamber on a slide,a haemocytometer. Automatic cell counters are available. Other criteriafor estimating the mass or number of cells in culture are analyses ofprotein nitrogen or rate of respiration.The units in which growth rate is expressed are as diverse as themethods of measurement and depend also on the overall size ofthe system under study. The length increase of a pollen tube mightbe measured in mmmin 1; the length (height) growth of a tree mightbe expressed in m year1.

6.3 Growth, development and differentiationGrowth is always accompanied by a change in form and in physiologicalactivity, by differentiation. The identical cells produced bycell division in a meristem (region of cell division) enlarge, i.e. grow,and at the same time become different from the meristematic cells,and fromeach other, forming for instance pith parenchyma, or xylemvessels, or companion cells. In their mature formthese cells are of verydifferent structure and function. Nevertheless, certain growth processesare common to all plant cells and, during the initial stages ofa cells development, these common processes predominate. In themeristem, the newly formed cells first grow by plasmatic growth, asynthesis of protoplasm including multiplication of organelles. Incells destined to remain meristematic, a doubling of cell mass isfollowed by division. In cells destined to undergo further growth,there follows a phase of expansion (elongation) growth, characterizedby a rapid volume increase, extensive water uptake accompanied byvacuolation, and by cell wall synthesis. Then, as expansion slowsdown, divergences in cell development become dominant. Whilebasic protoplasmic components such as proteins and nucleic acidsincrease in quantity in all cells during growth, the proportionalincreases in particular proteins differ so that cells of varied structureand metabolismare formed. Cell wall growth occurs in all cells, but asdifferentiation proceeds cell-type-specific differences in the chemistryand structural arrangement of the cell wall components becomeapparent. These processes are considered in more detail in subsequentchapters. The completion of differentiation leads to the formation ofthe mature tissue cells. Finally, mature cells ultimately age and die.Such a sequence of changes constitutes the overall sequence ofdevelopment:development growth differentiationAlthough the growth and differentiation stages overlap in time atleast partly, it is possible to separate growth from differentiationconceptually and experimentally. Thus it is possible to suppressdifferentiation by inhibitory chemicals, while permitting growth tocontinue. Further, these two aspects of development seem in manydeveloping systems to be mutually competitive: conditions whichfavour rapid growth often suppress differentiation, and vice versa.For instance, plants whose growth is retarded by a deficient watersupply may show an enhanced degree of cellular differentiation.In the light of the above discussion it is possible to advance adefinition of growth as follows:Growth is a synthesis of protoplasm, usually accompanied by a change in form andan increase in mass of the growing system. The total mass increase may be manytimes that of the increase in the mass of the protoplasmic components proper.Levels of growthGrowth can be studied at several levels of structure. In order ofincreasing complexity, one can consider the physiology of growthat levels ofcell _____> tissue _____> organ _____> organismThe growth of a system above the cell level is brought about by acombination of cell multiplication and cell growth. The extent to whichthese two processes contribute to the growth of a tissue or organdepends on the particular system under study, and on its developmentalstage: it is common to find during the growth of a tissue ororgan a stage of cell division followed by cell expansion (Section 8.4).6.4 Localization of growth in space and timeFlowering plants continue growth throughout their life history byvirtue of persistent localized growth regions. Cells are formed by celldivision in meristems in these growth regions, and size increase and organ formation result from the activities of these meristems; theassociated cell expansion, differentiation and maturation take placein close proximity to the meristems. Whilst still in the seedling stagea flowering plant already contains cells at all stages of development meristematic, expanding, differentiating, mature and dead. The ageof a plant and the age of its cells are consequently two quite differentthings. Trees live for hundreds and even thousands of years; ages ofancient bristlecone pines (Pinus longaeva), growing at high altitudes inCalifornia, have been estimated to exceed 5000 years. But few livingcells in a tree are more than a few years old. Leaves typically survivefor one growth season and even the so-called evergreen leaves areshed after a few years, or a few decades at the most. Tree pith cellsmay live to 100 years. This continuous replacement of the livingtissues of a perennial is probably one of the secrets of tree longevity.Another characteristic of plant growth is its indeterminacy: the totalsize and the size and number of organs are not precisely fixed. Theplasticity that plants can exhibit in response to environmental conditionsis quite remarkable. A common garden weed, the groundsel(Senecio vulgaris), growing in a well-fertilized vegetable bed will growto a bushy plant some 30 cm high and bear over 50 flower heads.A groundsel established in a crack between paving stones producesa tiny plantlet, a few centimetres tall with a single flower head,scarcely recognizable as the same species. Continued growth enablesa plant to respond to environmental stimuli with growth reactions tothe end of its life. Individual organs leaves, fruits, floral parts on theother hand typically have determinate growth which ceases when themature size is reached, and their shapes, too, are fixed.Localization of growth in particular areas and along particulardirections achieves the shaping of plant organs, the process of morphogenesis,which is the subject of later chapters.For flowering plants, which are immobile, growth takes the placeof movement in response to certain environmental stimuli, particularlydirectional ones. A motile unicellular alga might swimtowards a source of light (phototaxis); a flowering plant shoot willgrow towards the light (phototropism). Such responses are known asgrowth movements and are the subject of Chapter 12.6.5 Conditions necessary for growthTo grow, a plant tissuemust be in a potentially growing state. Amaturetissue is no longer capable of growth except in response to specialstimuli such as wounding or the application of growth hormones; suchstimuli restore it to a potentially growing state. The potentially growingstate is an internal, physiological condition that must be satisfied.Additionally, environmental conditions and other factors within theplant must be favourable. Some of the requirements for plant growthhave already been mentioned in preceding chapters. The environment must provide a supply of water, O2 and inorganic nutrients. Plants cangrow only when they are turgid and in nature, as stated, water supplyoften limits growth. Most flowering plants require O2 for growth,although some aquatic plants can pass through the stages of seedgermination and early seedling growth under more-or-less anaerobicconditions (see Chapter 2, Section 2.10.1). For some types of growth aspecific environmental stimulus may be required. The dividing andgrowing cellsmust also be nurtured bymoremature parts of the plant,which supply organic nutrients and growth hormones or theirprecursors.The temperature range compatible with plant growth varies fromspecies to species (Table 6.1). Within this range there is an optimumtemperature whose value will depend also on factors such as previousgrowth conditions and irradiance levels. Plants native to warmhabitats require higher temperatures for growth than those of coolerregions. The optimum growth temperature for winter wheat (Triticumaestivum), a cereal of temperate climate, is 2025 8C; for maize (Zeamays), a cereal from a warm climate, it is 3035 8C.The effect of temperature on plant growth is complex. An alternationof lower temperature by night and higher temperature by dayis frequently better for growth than any one constant temperature.The optimum growth temperature varies not only between speciesbut between organs of a plant, and changes with age. What emergesas the optimum temperature also depends on whether one measuresgrowth over a short time interval or over a prolonged period. Thetemperaturelimitsforgrowtharegenerallynarrowerthanthetemperaturelimits for individual physiological processes, or for survival.Respirationforinstancecontinuesattemperaturesabovewhichgrowthis inhibited (Fig. 6.1). Growth requires a harmonious interaction of allphysiologicalprocesses. Individual chemical reactions inthe celldonotall havequite the samevalues ofQ10 (temperature coefficient,measureofchangeofrateper10 8Ctemperaturedifference).Hencethebalanceofreactions needed for growth can be disturbed at temperatures notinhibitory to individual reactions or reaction series.6.6.1 Comparing growth ratesQuantitative comparisons between the growth rates of living systemscan be made from two viewpoints. One can measure and compare their absolute growth rates, i.e. the total growth per unit time; ortheir relative growth rates (RGR), the growth of each per unit timeexpressed on a common basis, e.g. per unit mass. To estimate plantyield the absolute amount of growth may be appropriate, but forcomparing the growth activities of two different systems values ofRGR are more meaningful. If two leaves with respective leaf areas of5 cm2 and 50 cm2 both expand by a further 2 cm2 in a day, the absolutegrowth rates are the same, but the smaller leaf has a ten times higherRGR and generally would be considered as growing faster.The relative rates of linear elongation growth of a number offlowering plant organs, and two fungi, are compared in Table 6.2.The table is compiled fromresults obtained under varied experimentalconditions, but the differences in growth rates are far greater than canbe accounted for by differences in external conditions. Under noconditions will the growing zone of a Vicia root double its length perminute, as does that of the fungus Botrytis cinerea. Pollen tubes andstaminal filaments show rates equalling or approaching those offungal growth, but these rates are short-lived; staminal filamentsexpand over a period of minutes, whereas the fungal growth is maintainedsteadily over long periods.Bacteria have the highest growth rates of all terrestrial livingorganisms; bacterial cells can double their mass and divide to formdaughter cells in 2030 minutes. The duration of a cell cycle in plantsis much longer than this; in pea root tip meristems, mitosis takes onehour at 30 8C and the interval between divisions is much longer (seeChapter 8). The high growth rates of microorganisms reflect their generally high rates of physiological activity and are believed to berelated to their small cell size, which allows rapid diffusion of metabolitesand gases into and out of the cells.Table 6.2 presents the RGR, growth per unit length of growingzone. The absolute growth rates depend on these values and on theactual lengths of the growing zones. In the hyphae of Botrytis thegrowing zone is only 0.018 mm long, and even with a 200% increasein length per minute the total extension growth made in 24 hours,the daily absolute growth rate, is about 5 cm. In the bamboo shoot,with a RGR of only 1.27% per minute, the growing zone is 5 cm longand hence the daily absolute growth rate is 90 cm.In the above examples, lengthhas beenused as the growthmeasure.Other bases can be used for calculating relative growth rates, e.g. massincreaseperunitmass is very widely utilized.Once the basis of calculatingtheRGRhasbeenstated,thegrowthmeasureunitmaybeomittedinquoting the RGR numerically, since it cancels out in the ratio: The extent of total growth achieved during a given time periodvaries enormously between plant species. A marrow plant (Cucurbitasp.) or a hop (Humulus lupulus) grows from seed to a length of 12 m ina summer; an oak seedling may grow 12 cm in the same time, andsince mass varies as [length]3, the differences in mass are greater still.Marrow and hop are plants with large growing zones and producemuch soft, thin-walled tissue; a large proportion of the plant mass isgreen photosynthetic tissue. The oak, a perennial, has a small growingzone and is woody; it can be regarded as showing a higher degreeof differentiation, and growth and differentiation are to some degreemutually antagonistic. Generally, herbaceous species attain higherRGR at comparable developmental stages and under comparableexternal conditions (Table 6.3). Herbaceous plants of open, disturbedhabitats, including species regarded as weeds, have the highest RGR.Such differences are genetically programmed. Even individuals of thesame species can show genetically determined variations in growthrates. This has been found for example in a study of the RGR in 48Table 6.3 Relative growth rates (RGR), as dry weight increase per day inherbaceous and woody plants measured over 14 days (herbaceous) or 21 days(woody). Young but photosynthetic seedlings were studied, grown from 43herbaceous and 16 woody species derived from a wide range of naturalhabitats. Data from Hunt & Cornelissen (1997).6.6.3 Growth rhythmsDiurnal rhythmsIf measurements of growth are made at short time intervals, sayhourly, then rhythmic changes in growth rate become apparent.Growth has a diurnal rhythm, with maxima and minima occurringat definite times of day. This rhythm conforms to the general patternof endogenous diurnal rhythms controlled by the biological clockmechanism (in conjunction with the regular diurnal alternation oflight and darkness) and exhibited by all types of living organisms. Aplant grown from seed in darkness and under completely constantconditions does not show a diurnal growth rhythm; but once arhythm has been initiated a single period of illumination maysuffice for this the rhythm persists with an approximately24-hour periodicity in the darkness for several days. Plants maygrow better with an alternation of light and darkness than in continuouslight, the alternating regime harmonizing with their naturalrhythmicity. The alternation of light : dark must fit an approximately 24-hour cycle. The growth of tomato plants (Lycopersicon esculentum)is inhibited if the plants are subjected to light : dark cycles of 6 : 6 or24 : 24 hours; the endogenous rhythm of the plants apparentlycannot adjust to cycles so far removed from the natural. A diurnalgrowth cycle may show more than one maximum in 24 hours.Short-term rhythmsGrowth rhythms not related to the 24-hour period are also known.Growing shoot tips bend so as to rotate in a circle as viewed fromabove. This movement is known as circumnutation or nutationand is most pronounced in climbers, in which it aids the plant to findand to twine round a support. Tendrils also nutate till they contact asupport. Nutation involves a wave of cell extension growth movinground the axis tip; at any instant growth is most intense in onelimited region of the tip and the wave of activity completes a cycleround the tip in anything from 1.25 to 24 hours according to speciesand environmental conditions. Changes in cell turgor, however, alsocontribute to the bending. The nutational rhythm is not dependenton any rhythm in the external conditions, though the speed of rotationis affected by environmental factors such as temperature. Thedirection of nutation is fixed and in most species is anticlockwise asviewed from the top.Annual and other long-term rhythmsThe grand period of growth represents the entire life history ofephemerals and annuals; in perennials it corresponds to the courseof growth during one growing season. In most temperate-zone perennialsgrowth has a regular annual rhythm, with shoot growthcompleted within a short period in the spring and early summer;some trees for example complete 90% of a years growth in a 30-dayperiod, starting 714 days after the commencement of growth. Thefirst days represent the slow phase of the grand period (Fig. 6.4).Such an early cessation of growth during prime climatic conditionsmust be the result of an internal control mechanism. Sometimes asecond flush of growth occurs later in the season. Root growth continueslonger into the summer: plants must tap fresh areas of the soilfor water and minerals even when the shoots are not growing.The annual growth cycle of plants living in a climate with distinctseasonal changes is synchronized with the climatic cycle so that thegrowth period coincides with the favourable season, environmentalsignals such as changes in daylength and temperature serving assynchronizing agents (Chapter 11). Nevertheless the endogenousrhythm also plays a part. Trees of the same species and age, growingside by side, may show slight differences in their times of e.g. leaf fall,which are consistent from year to year, with the same individualsbeing the earliest and the latest in each season.In tropical habitats, growth of a plant community as a wholecontinues all the year round with almost equal intensity, but in individual plants periods of high and low growth activity alternate,each lasting some months. Perennials seem to have an innate tendencyfor an alternation of growth and rest periods. In tropicalclimates where the environment does not exercise a synchronizinginfluence, each plant sometimes even a branch of a plant growsaccording to its own internal rhythm.The first plant growth hormone to be discovered was auxin thename is derived from the Greek auxein meaning to increase. At lowconcentrations (10_12 to 10_3 M) auxin promotes the elongation ofcertain plant organs, principally as a result of cell expansion. Figure 7.1shows the development of a typical grass seedling and the structureof the coleoptile which protects the first true leaf. In many grasses thecoleoptile increases in length from under 10 mm to over 80 mm asthe cells within it expand. If the tip of the coleoptile is excised,growth is much reduced, but this can be restored if it is replaced.A key breakthrough came when it was found that this restoration ofgrowth would occur even if a small agar block was placed betweenthe tip and the lower part of the coleoptile. This demonstrated thata diffusible substance (auxin) was passing from the tip, through theblock, and then stimulating growth lower in the coleoptile. If aseries of excised tips is placed on the block, the diffusible substanceaccumulates within it and the block alone can then stimulategrowth. Placing the block (or tip) asymmetrically on the coleoptilecauses directional growth and auxin-stimulated growth is a centralfeature of many directional growth responses or tropisms (seeChapter 12). system provided a quantitative and sensitive bioassay for auxin whichwas used for many years although it has now been superseded bymodern analytical techniques. If agar blocks are placed asymmetricallyon decapitated oat (Avena sativa) coleoptiles, the degree ofcurvature under standardized conditions is related to the auxin concentration.Similarly if coleoptile or hypocotyl segments are floatedon different concentrations of auxin solution, the growth rate islogarithmically related to the auxin concentration. Such a systemillustrates well the essential features of a plant growth hormone auxin is produced in the tip of the coleoptile and transported downthe coleoptile stimulating a response, cell elongation. The growthresponse which is observed depends upon both the concentration ofthe hormone and the target tissue. Figure 7.3 illustrates the growthresponses of roots and shoots to different concentrations of auxin.Note that the concentration ranges which stimulate growth are quitedifferent, and in both roots and shoots either promotion or inhibitionof growth can result from exposure to auxin depending upon theconcentration.The structure of IAA is relatively simple and it is the most commonlyfound natural auxin although many other related compoundswith auxin-like actions can be found within plants. A general featureof these compounds is that they have a carboxylic acid group linkedto an aromatic ring. Many synthetic auxins have been produced suchas naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid(2,4-D) (Fig. 7.2), and these are widely used in agriculture. As well asthese simple, free auxins plants also contain large amounts of conjugatedauxins in which the auxin is covalently bound to anothermolecule (ranging from simple sugars to proteins). These conjugatedauxins are inactive and are transported via the phloem to other partsof the plant where they may be deconjugated, and hence activated.However, the growth responses of coleoptiles described above resultfrom the basipetal (basewards) transport of free auxin from cell tocell (discussed in more detail in Chapter 12).As well as stimulating growth, auxins have many other effectson plant development. For example, auxin plays a key role in apicaldominance. As seen in later chapters, auxin is also importantin regulating the formation of lateral roots (Section 9.6.3), the developmentof the vascular system (Section 9.5.3), parthenocarpy(Section 11.11.3) and in senescence. This diversity of function ledK. V. Thimann to state The trouble with auxin is that its actions areso numerous and apparently unrelated (in Palme & Galweiler 1999).Although its actions are certainly numerous, it is clear that its functionscan be understood only when interactions with other plantgrowth hormones are considered. This interaction between plantgrowth hormones is a theme which will continually arise, and it isfundamental to the way in which plants respond to interacting, andat times conflicting, environmental signals.Molar concentration of IAA1011109 107 105 103 101Inhibition PromotionRootsStemsAuxin and apical dominanceIt has been recognized for many years that auxin, produced in theshoot apex, can repress the development of axillary buds in a processknown as apical dominance. If the shoot apex is removed, thisinhibition is released and the axillary buds develop as side shoots.In many species, application of auxin to the decapitated stump cansubstitute for the presence of the shoot apex, preventing axillary budgrowth. However, auxin does not inhibit axillary bud developmentdirectly it is not transported into the buds and direct applicationdoes not inhibit their growth. It has been proposed that the plantgrowth hormone cytokinin (Section 7.2.4) plays a role in the relief ofapical dominance, as application of cytokinin to axillary buds willstimulate their development and cytokinin export from the rootincreases following shoot apex decapitation. However, recent graftingexperiments have indicated that other mobile signals are alsoinvolved (Beveridge et al. 2003, Sorefan et al. 2003). Mutants of pea andArabidopsis have been identified which exhibit increased shootbranching. If the shoots of these plants are grafted onto an unmutatedrootstock, normal branching patterns are restored. It is proposedthat auxin travels down the stem and generates a mobile,branch-inhibiting signal which is then transported upwards (presumablyin the xylem) inhibiting axillary bud development. The mobilesignal has yet to be identified, but current evidence points towards acompound derived from carotenoids.