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Pertanika J. Trop. Agric. Sci. 19(2/3): 103-110 (1996)ISSN: 0126-6128

© Penerbit Universiti Pertanian Malaysia

Effect of Irradiance on Growth, Physiological Processes and Yield ofMelon (Cucumis tnelo) Plants Grown in Hydroponics

MOHD RAZI ISMAIL1 and MOHD KAMIL YUSOF2

department of Agronomy and Horticulture,Department of Environmental Science

Universiti Pertanian Malaysia43400 Serdang, Selangor, Malaysia

Keywords: irradiance, melon cultivars, growth, stomatal conductance, photosynthetic rate, yield

ABSTRAKPengaruh radiasiyang berbeza ke alas tanaman tembikai wangi (Cucumis melo) cv. Birdie, Charity Ball danJade Dew yang ditanam di dalam hidroponik telah dikaji. Tanaman diberi rawatan min radiasi yang berbeza iaitu11.4, 8.2, 6.1 dan 3.0 MJ m han yang diperolehi dengan menggunakan teduhan. Hasil berat kering berhubungrapat dengan paras radiasi. Konduksi stomata dan kadar fotosintesis adalah tertinggi bila tanaman berada padaparas radiasi yang tertinggi. Tanaman yang ditanam di bawah radiasi 11A MJ m h'1 menghasilkan berat basahbuah dan kandungan pepejal terlarutyang tinggi. Semua kultivar gagal untuk menghasilkan buah pada radiasi 3.0MJ m~V.

ABSTRACTThe effect of different irradiance levels on melon fCucumis melo,) cv. Birdie, Charity Ball and Jade Dew grownin hydroponics was investigated. Plants were exposed to mean daily irradiance levels of 11 A, 8.2, 6.1 and 3.0 MJm day1 achieved by using different levels of shade. The dry matter yield appeared to be directly proportional to theirradiance level received by plants. Stomatal conductance and photosynthetic rate were kighest when the plants weregrown under the highest irradiance level. Plants grown under 11.4 MJ m2£J had the highest fruit fresh weight andtotal soluble solids. All cultivars failed to fruit when grown under irradiance of 3.0 MJ m2il\

INTRODUCTION

In Malaysia, the area of cultivation ofhorticultural crops under protected envir-onment expanded rapidly in the late 1980s.This development has been encouraged bymany factors such as the unpredictableweather conditions, the demand for qualityproduce and the introduction of soillessculture. As for open field cultivation, cropproductivity under protected environmentagriculture is dependent upon optimumenvironmental factors.

It is a common assumption that light isgenerally not limiting for the cultivation ofcrops in the tropics. This assumption is notalways true. Malaysia, for example, oftenexperiences periods of haze, which reduce

radiation interception by almost 30-40%and this is even more pronounced underrain shelters (Mohd Razi 1991, 1994).Apart from these changes, different designsof rain shelter result in 18-50% reduction inradiation interception (Yeoh 1991). Robin-son (1990) also reported that different typesof plastic used as roofing material causevariation in light interception.

Nearly all previously reported experi-ments showing benefits of increased irradi-ance have involved plants growing inglasshouses in temperate regions, wherelow levels of radiation are more criticalduring winter (Hurd and Thornley 1974;Gislerod et al. 1989; Cockshull et aL 1992).In glasshouses in the tropics, Mohd Razi

MOHD RAZI ISMAIL AND MOHD KAMIL YUSOF

and Ali (1994) found NFT-grown tomatoesfailed to fruit when plants received less than8.5 MJ m^d"1 despite a 5°C reduction intemperature in the plant canopy underglasshouse conditions in Malaysia.

Melon (Cucumis melo L.) of the reticula-tus type is a high value crop which can begrown successfully by hydroponics underrain shelters. Apart from a report byBouwkamp et al. (1978), little informationis available on the irradiance requirementfor the production of melon in the tropics,especially when water and nutrient supplyare not limiting factors in crop production,as is the case in hydroponics.

The present study was conducted toexamine the effects of different levels ofirradiance on growth, stomatal conduc-tance, photosynthesis rate and yield ofthree melon cultivars, and, based ongrowth and yield data, to determine theoptimal irradiance level for production ofmelon under protected environment in thetropics.

MATERIALS AND METHODS

The effects of irradiance on three melon(Cucumis melo) cultivars grown in a Kyowadeep culture system (Lim and Wan 1984)were investigated at the Hydroponic Unit,Universiti Pertanian Malaysia. Uniform,three-week-old melon plants (cv. Birdie,Charity Ball and Jade Dew) were grownunder different shade regimes which gavevarying levels of irradiance. Various levelsof shade were achieved by placing anincreasing number of layers of plastic filmof ethylene vinyl acetate (EVA) copolymersover the plant canopy. EVA copolymersare transparent to visible light and allow allwavelengths essential for photosynthesis topass through (Robinson 1990). Meanirradiance received by plants under var-ious shade levels was 11.4, 8.2, 6.1 and 3.0MJ m^d"1 as recorded by solarimeters(Delta-T Device, Cambridge, UK). Air

temperature and relative humidity in theplant canopy were between 25-37°C and60-72%, respectively. The plants weresupplied with a nutrient solution contain-ing the ion concentrations given by Cooper(1979) with electrical conductivity main-tained between 2.4-2.6 mS cm"1. Plantswere arranged in a completely randomizeddesign in a split-plot arrangement whereirradiance and cultivar were assigned asmain plot and subplot, respectively. Eachplot contained 12 plants, which werereplicated 4 times.

At harvest, leaf length and breadthwere measured with a ruler and the leafarea determined using an automatic leafarea meter (Delta-T Cambridge, UK). Theshoot and root dry weights were deter-mined after drying at 80°C for 48 hours.Destructive sampling was performed at 0, 4and 9 weeks for determination of relativegrowth rate (RGR) and net assimilationrate (NAR). At each harvest, 4 plants wereharvested from each treatment and RGRand NAR were calculated using formulaegiven by Hunt (1982).

Measurements of the stomatal conduc-tance (gs) and net photosynthetic rate (Pn)for intact leaves were determined using aninfrared gas analyser IRGA (LCA-2 Por-table Photosynthesis System, ADC Hod-desdon, UK). The measurements weremade 4-5 h after sunrise on clear days onthe abaxial surface of young fully expandedleaves (3rd - 5th leaf from shoot apex). Allmeasurements were carried out in thedifferential mode at IRGA with Emax setat 1.0 and boundary layer resistance at 0.3mmol m"2 s"1 predetermined by placing thechamber on a mock leaf (of moist filterpaper).

Fruits were harvested from each plantat maturity when signs of cracks appearedat the basal part of the fruit. Fruit diameterwas measured at harvest using a Verniercaliper and their fresh weight was deter-

104 PERTANIKA J. TROP. AGRIC. SCI. VOL. 19 NO. 2/3, 1996

EFFECT OF IRRADIANCE ON GROWTH, PHYSIOLOGICAL PROCESSES AND YIELD OF MELON

TABLE 1Effect of irradiance and cultivar on leaf length, breadth, area and dry weight of leaf, root and

stem at day 56. Data are means of the main effect as interaction betweenirradiance x cultivar is not significant except for leaf area

Treatments

Irradiance

(MJ rrf2 day"1)

11.48.26.13.0

Cultivar

BirdieCharity BallJade Dew

Interaction

(P < 0.05)

IrradianceX

Cultivar

MeanLeafLength(cm)

16.24 a13.56 b12.92 b7.40 c

12.50 a12.37 a12.72 a

ns

MeanLeafWidth(cm)

21.24 a18.27 b16.84 c9.93 d

16.60 a16.51 a16.68 a

ns

LeafArea(cm2)

6190 a4774 b3959 c965 d

4644 a3659 b3613 b

**

Leaf

34.28 a27.26 b12.22 c5.63 d

21.86 a20.26 a17.42 b

ns

Mean DryWeight

Root

(g/plant)

7.66 a5.90 b3.58 c1.13 d

5.21 a4.55 a3.94 b

ns

Stem

17.40 a16.23 a12.24 b2.22 c

13.19 a12.03 a10.84 a

ns

Mean values in each column with the same letter are not significantly different at P < 0.05 according to DMRT. Forthe interaction effects; ** = significant at P<0.05.

mined. A fresh sample weighing 20g wasplaced in a weighed glass petri dish andoven dried at 80°C for 60 h, and total fruitdry matter was estimated. Data wereobtained on soluble solids content with ahand refractometer (Currence and Larsen1941) on all fruit harvested from eachplant.

RESULTS AND DISCUSSION

Table 1 shows the growth responses ofmelon cultivars to different levels ofirradiance. There was no significant inter-action (P>0.05) between irradiance andcultivar on the leaf length and width and

dry weight of leaf, stem and root. Leaflength and width were reduced signifi-cantly (P < 0.05) with irradiance below6.1 MJ m^d"1. Similarly, low irradianceresulted in a significant reduction(P<0.05) in leaf dry weight. This isconsistent with the fact that interceptedradiant energy determines the dry matterproduction in plant species (Lawlor 1992).Root dry weight was reduced to 22, 53 and88% in plants grown under 8.2, 6.1 and 3.0MJ m^d*1 respectively, relative to 11.4 MJm^d"1. The reduction in leaf growth withdecreased irradiance was reported toinhibit root growth and subsequently

PERTANIKA J. TROP. AGRIC. SCI. VOL. 19 NO. 2/3, 1996 105


7000,

6000'

c r̂ 5000

~ 4000

2 3000

g 2000

1000

06.1 8.2 11.4

Irradiance ^d"1)

t i Birdiet (Charity Bait• Jade Dew

Fig. 1: The effect of irradiance and cultivar on lead area

of melon plant. Means separation by DMR T (p< 0.05)

water uptake (Smith et aL 1984). Thereduction in plant growth with decreasingirradiance involves many physiological andbiochemical attributes which have beenreported elsewhere (Blackman and Wilson1951; Lawlor 1992). Between cultivars,Birdie produced greater root dry weight

than either Charity Ball or Jade Dew. Asignificant irradiance and cultivar interac-tion (P<0.01) was observed for leaf area.Cultivar Birdie produced a greater leaf areawhen grown under irradiance levels above6.1 MJ m'2 day"1 (Fig. 1).

In general, the RGR and NAR wereaffected by different irradiance levels(Table 2). In the first four weeks, RGRand NAR decreased proportionately withreduction in irradiance levels. Duringweeks 4 - 9, no significant difference inRGR and NAR between plants grownunder 11.4 and 8.2 MJ m^d"1 wasobserved. RGR and NAR were signifi-cantly reduced (P<0.05) with irradiancelevels below 8.2 MJ m^d"1. A similar trendof increased NAR and RGR with increasedirradiance had been reported for tomatoes(Hurd and Thornley 1974; Logendra et aL1990), and tomatoes, sweet pepper andcucumber (Bruggink and Heuvelink 1987).There was no significant interaction(P > 0.05) observed between irradianceand cultivar for NAR and RGR.

TABLE 2Effects of irradiance on relative growth rate and net assimilation rate of melon plants.Data on cultivar are not presented as no significants were observed within cultivars.

Interaction irradiance and cultivar are also not significant.

Interval/IrradianceTreatments

0-4 weeks

11.4MJ m'V8.2 :6.1 :3.0 :

4-9 weeks

11.4 MJnTV8.2 :6.1 :3.0 :

Relative Growth Rate(gg-1 week"1)

0.26 a0.20 b0.15 c0.06 d

0.30 a0.32 a0.24 b0.20 c

Net Assimilation Rate(gem"2 week"1 x 10'3)

1.6 a1.3 b1.1 c0.7 d

3.6 a3.7 a2.6 b1.4 c

Means separation by DMRT (P < 0.05), Mean values in each column with the same letter are notsignificantly different.



TABLE 3Effects of irradiance and cultivar on photosynthesis rate (Pn) and stomatal conductance (gs) measured atday 24 and 40 after treatments (DAT) on melon plants. Data presented as mean from main effect as the

interaction irradiance x cultivar is not significant.

Treatments

Irradiance

MJ m day

11.48.26.13.0

Cultivar


Interaction

(P<0.05Irradiance

X

Cultivar

24 DAT

Pn(|imol m'V1)

17.96 a16.37 a3.65 b0.73 c

10.36 a9.94 a8.81 a

ns

40 DAT

(mol m'V1)

0.63 a0.47 b0.29 c0.10 d

0.38 a0.39 a0.35 a

ns

Pn(uinol m'V1)

21.11 a15.94 a3.64 c

0.69 d

11.16 a10.31 a9.56 a

ns

gs

(mol m'V1)

0.67 a0.51 b0.27 c0.09 d

0.39 a0.39 a0.36 a

ns

Mean values in each column with the same letter are not significantly different at P<0.05 according to DMRT.ns = not significant.

In this study, stomatal conductanceand photosynthesis rate were reducedsignificantly (P < 0.01) with decrease inirradiance (Table 2). Turcotte and Gosse-lin (1989) reported a similar result forglasshouse cucumber. Low dry weightvalues in the various plant parts indicatedthat less carbon was fixed in the leaves thatcould be translocated to other parts of theplant including fruits. For tomatoes, Hoand Hewitt (1986) showed that photo-synthesis rate is mainly affected by irradi-ance and CO2 concentration and that theexport rate of assimilates from a leaf duringthe light period is proportional to theconcurrent photosynthesis rate. Further-more, leaf reserves are very low in plantsgrown in low light and the rate of export

i nfrom such leaves can be reducedunfavourable light conditions. This is inagreement with our study on melon whereplants grown under low irradiance showeda significant decrease in fresh and dry fruitweights (Table 4). With decreased irradi-ance from 8.2 to 6.1 MJ m^d"1, yield wasreduced by 20-60% relative to the plantsgrown under 14 MJ m~2d"\ The propor-tional yield and intercepted radiant energyhave already been established in tomatoes.Cockshull et al. (1992) showed that 2 kg m~2

fruits were produced for every 100 MJ m"2

of solar radiation received by the crop.Their study also showed that average fruitsize was reduced with decrease in inter-cepted irradiance, which was also observedin this study. Within cultivars, cv. Birdie



TABLE 4Fruit diameter, fresh weight and dry matter and total soluble solids as influenced by irradiance and

cultivar. Data presented are the mean from the main effect as interaction irradiance x cultivaris not significant

Treatments

IrradianceMJ m^day'1

11.48.26.13.0

Cultivar


InteractionIrradiance

X

Cultivar

FruitDiameter

(cm)

10.52 a9.38 a7.13 b

10.05 a9.20 a8.78 b

ns

Fruit FreshWeight

(g/plant)

0.90 a0.73 b0.39 c

0.68 a0.63 a0.56 b

ns

Fruit DryWeight

(g/plant)

45.13 a36.41 b21.88 c

30.27 a26.10 b22.20 b

ns

Total SolubleSolids

(% Brix)

10.40 a7.16 b5.21 c

_

8.03 a7.32 a7.41 a

ns

Mean values in each columm with the same latter are not significantly different at P<0.05 according to DMRT.

and Charity Ball produced greater fruitfresh weight than Jade Dew. No significantinteraction (P>0.05) was found betweencultivar and irradiance levels.

All melon cultivars failed to fruit at thelowest irradiance level. The disturbance inthe photosynthetic activities might haveinhibited assimilate partitioning whichsubsequently resulted in a failure inreproductive processes. The benefit of highirradiance to the reproductive processes hasbeen reported for a wide range of crops(tomatoes: Boivin et al. 1987; Cockshull etaL 1992; strawberry: Ceulemans et al. 1986;rose: Zieslin and Mor 1990).

Total soluble solids (TSS) is a goodmeasure of sweetness of melon. The relativedegree of irradiance reduction was wellreflected in decreased TSS. Table 4 showsTSS was reduced by approximately 3 and5% with a reduction in irradiance inter-

ception from 11.4 to 8.2 and 6.1 MJ m^d"1,respectively. Winsor and Adams (1976)showed a similar trend of increased TSSwith high irradiance in tomatoes. Ourresults, however, disagree with those ofBouwkamp et al. (1978) who found solublesolids content decreased with increasedlight intensity in most of the meloncultivars they studied. This discrepancymay be due to the amount of interceptedirradiance. In their study, soluble solidsdecreased when irradiance increased fromapproximately 19 to 25 MJ m^d"1 for 6days prior to harvesting. This high lightintensity may cause fruits to accumulateheat and attain temperatures exceeding airtemperature; this subsequently results inhigher respiration rates, thus loweringsoluble solid content. Throughout theduration of the experiment, the maximumirradiance recorded in the present study



was only approximately 16.2 MJ m^d"1.We suggest that when plants are grownunder unlimited water and nutrient supply,environmental factors that inhibit photo-synthesis rate and limit the distribution ofassimilate to various plant parts includingthe fruit play a significant role in yield andquality.

CONCLUSION

The response of melon to the amount ofirradiance varies. Irradiance lower than 8.2MJ m^d"1 reduced dry weight accumula-tion and yield. None of the cultivars wastolerant of the lowest irradiance level (3.0MJ m^cT1). The reduction in net photo-synthesis may have contributed to reduc-tion in yield. This result has practicalapplications in showing the need to max-imize light transmission under protectedenvironment.

ACKNOWLEDGEMENTS

The authors are grateful for HydroponicIRPA (50307) grant which financed thisproject. We wish to thank Mr. Roslan Parjofor technical assistance.

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(Received 5 October 1994)(Accepted 21 November 1996)


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