FRUIT GROWTH OF CUCUMBER IN RELATION TO ... - CiteSeerX

Scientia Horticulturae, 23 (1984} 21--33 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

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F R U I T GROWTH OF CUCUMBER IN RELATION TO ASSIMILATE SUPPLY AND SINK ACTIVITY

A.H.C.M. SCHAPENDONK and P. BROUWER

Centre for Agro biological Research, P.O. Box 14, 6700 AA Wageningen (The Ne the rlands) (Accepted for publication 19 October 1983)

ABSTRACT Schapendonk, A.H.C.M. and Brouwer, P., 1984. Fruit growth of cucumber in relation to assimilate supply and sink activity. Scientia Hortic., 23: 21--33. Assimilate distribution in Cucumis sativa, cultivar 'Farbio', was studied during 36 days after emergence of the inflorescense in the 5th axil. The interaction between vegetative growth and reproductive development was examined. Daily net photosynthesis and the related assimilate supply were calculated by a simulation model. The actual dry matter accretion of vegetative and reproductive organs was derived from non-destructive measurements of organ dimensions and subsequent convertion to dry matter by statistical relations. Integrated over the total period of investigation, dry matter accretion derived either from the calculated assimilate supply or from ac~;ual measurements agreed well. However, a significant discrepancy appeared in the dynamic resolution where apparent periods of assimilate surplus were followed by periods of apparent shortage of assimilates. In this respect, the regulation of temporary storage and subsequent remobilization of assimilates is studied in relation to source--sink interactions. A hypothesis is formulated that relates the dynamics of fruit growth to assimilate supply and the sink activity of individual fruits. Keywor,~s: competition for assimilates; cucumber; fruit growth; photosynthesis.

INTRODUCTION

Periodic fruit set, a p h e n o m e n o n related to assimilate distribution, has been well d o c u m e n t e d in the past (De Stigter, 1969; Liebig, 1978; Fartington and Pate, 1981). Source--sink interactions in general evoke the question as to which processes can be regarded as the ultimate cause of changes in assimilate distribution (Gifford and Evans, 1981). In the whole plant, the net assimilation must be in balance with the net consuraption, b u t the rate of export from mature leaves can be modified by leaf reserves (Ho, 1976). As pointed o u t by Ho (1979), rates of assimilation and rates of utilization of assimilates in the sink must be regarded simultaneously for a realistic assessment of the roles of source and sink in translocation. Such an approach may show the dynamics of assimilate distribution patterns, influenced by changes in source activity, sink activity or interactions between both phenomena. 0304-4238/84/$03.00

© 1984 Elsevier Science Publishers B.V.

22 This investigation also concerns interaction between individual sinks. In indeterminate flowering plants, an uncontrolled increase of the demand for assimilates would lead to a surplus of slowly growing fruits, only a few reaching full maturity. This can be overcome by self-regulation of the number of fruits that are growing simultaneously. For cucumber, the first fruits exert dominance over subsequent emerging fruits, which often fail to grow and finally abort (De Stigter, 1969). The mechanism of this process is n o t y e t resolved. A supply shortage of assimilates, hormonal regulation by competing fruits or a combination of these factors are evident possibilities. By independently manipulating the number of fruits growing and the assimilate supply, it is possible to assess the basic process underlying fruit abortion. MATERIAL AND METHODS Cucumis sativa 'Farbio' plants were grown in a growth cabinet for 2 weeks after germination on nutrient solution at 25°C (24 h) and a light intensity of 40 W m -2 (12 h). After this period, the plants were transplanted to greenhouse compartments in rows of 5 plants. Plant density was 2 m -2. Planting date was 27 May (Experiment 1) and 29 May (Experiment 2) 1980. The temperature was kept at a minimum set-point of 18°C (night)/20°C (day). Maximum temperature in daytime was 26°C. Irradiance inside the greenhouse was measured using a Kipp and Zonen thermopile. Estimations of photosynthetic active radiation (PAR) were made by multiplication of global radiation by a factor 0.5. Inflorescences were removed until the 5th axil. Plants were topped at node 22--24 and all lateral branches were removed. At the onset of flowering, a sample row was harvested to analyse dry matter distribution. During a period from flowering until the harvest of the last fruit, dimensions of individual leaves and fruits were measured 2--3 times a week. Fruit-shading experiments were done under glasshouse conditions where plants were grown in soft. Temperature set-point was 20°C (day)/18°C (night). Inflorescences were removed until the 10th axil. Fruits of alternating plants were enclosed with black cotton at the time of flowering. Length and circumference of fruits were measured twice a week on 15 plants with enclosed fruits and 15 control plants of which fruits were n o t enclosed. In another experiment, fruits of single plants were alternately enclosed with black cotton at the time of flowering. The effect of fruit load was investigated by variation of the number of fruits that were left on the plant.

MATHEMATICALANALYSES An evaluation of dry matter production and partitioning often requires elaborate growth analysis of numerous plants harvested at different timeintervals. In this study, the cumbersome aspect of such an approach is part-

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ly o v e r c o m e by a c o m b i n a t i o n o f in vivo m e a s u r e m e n t s o f plant organ d i m e n s i o n s and a related statistical analysis o f dry m a t t e r c o n t e n t . G r o w t h o f leaves w a s c a l c u l a t e d f r o m t h e w i d t h and t h e length o f individual leaves. P o l y n o m a l regression revealed that S = 1 . 0 7 6 2 p - - 0 . 0 0 9 2 9 p2 _ 0 . 0 0 0 4 6 p3 _ 0 . 6 2 5 9 4 p = length × w i d t h S = l e a f surface

( m 2 10 -:) ( m s 10 -2)

(1)

n = 197 r2 = 0 . 9 0

Multip:Lication o f t h e c a l c u l a t e d increase o f leaf surface w i t h the reciprocal o f the specific leaf area ( S L A ) gives t h e dry m a t t e r increase o f t h e leaves. D a t a o n S L A were derived f r o m m e a s u r e m e n t s o f L o r e n z ( 1 9 8 0 ) . F o r the presew~ p u r p o s e , S L A was e s t i m a t e d f r o m a m u l t i p l e regression analysis as a f u n c t i o n o f i n c i d e n t daffy irradiation at t h e t o p o f t h e plants and temperatu:re (Fig. 1). S L A = 3 . 3 6 4 T + 27741-1 - - 3 4 . 5 3

(2)

T = t e m p e r a t u r e (°C) I = daffy s u m o f P A R (J c m -2 day -1)

n = 15 r2 = 0 . 9 7

SLA (m~kg -~) 150

130

110

9O

70

50

30

10 00

i

50

i

100

I

150

i

200

1

250

I

300

I

350

I

400

I I 450 500 J. cm-2day -1

Fig. 1. Multiple regression analysis of specific leaf area as a function o f daily integrated radiation (PAR) at the top of the plant at different temperatures: solid circles, 24°C; asteriks, 21°C; open squares, 18°C; open circles, 12°C. (Experimental data from Lorenz, 1980.)

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The stoichiometry for dry matter distribution over the vegetative plant organs was found to be almost constant (Table I). R o o t growth stopped soon after the onset of fruit production, confirming the results of De Stigter (1969). Because plants were decapitated soon after flowering and all shoots were removed, leaf and stem growth decreased after 12--17 days. TABLE I Dry matter distribution in cucumber plants. Distribution of dry matter between vegetative plant organs. Numbers in parentheses give the percentage of dry matter of the whole plant. Day 1 is the time of flowering in the 5th axil. On Day 36, all plants were harvested and analysed. Experiments 1 and 2 are different rows but under identical environmental conditions Sampling date

Root (g)

Stem (g)

Leaves (g)

Leaf surface (dm 2)

Experiment 1 Day 1 Day 36

2.8 (9.2) 8.8 (8.5)

8.5 (27.9) 30.9 (29.9)

19.1 (62.9) 63.6 (61.6)

77.0 153.0

Experiment 2 Day 1 Day 36

2.0 (8.8) 10.0 (8.5)

6.0 (26.9) 33.2(28.4)

14.6 (64.3) 74.1 (63.1)

57.0 185.0

Growth of cucumber fruits was determined by measurements of length and circumference in analogy with the m e t h o d applied for leaf dimensions. Polynomal regression analysis of the calculated volume and the actual fresh weight revealed that G = 0.928 V -- 0.0492 V2 + 0.0031 V3 + 0.02117 V = length × ~ × (0.5 diameter): G = fruit fresh weight

(m 3 X 10 -3) (g × 103)

(3) n = 158 r: = 0.98

The dry matter c o n t e n t of fruits was about 3.2%. Thus, the time-course of total plant growth can be calculated from in vivo measurements (Fig. 2). Daily dry matter production can be estimated from a photosynthetic model, similar to the one proposed by Acock et al. (1978). ~C l n (

Pc= K

~IKlo+(1--m) 7"C ) ~lKIoexp(--KL)+(1--m)1C

(4)

The leaf light-use efficiency a l is dependent on the binding capacity of CO: on the ribulose-biphosphate--carboxylase--oxygenase enzyme in a competitive interaction with oxygen. According to Peisker and Apel (1981), this relationship can be described as

25 gm

g m 2day-1

~ a y I1

25I

a

25

b 20

_

20

---

15

-q

.J

15

:

Jl 4

8

°/o a b o r t i o n 100

12

16

20

24

2B

32

4

36

doy

8

12

20

16

24

28

°to abortion 100 d

•

32

36

doy

g m-2day"1

4 ~/

50

',

/

3

"x

50

..~

,

s

12

1%'2o

2'~

2'8

3'2

36

dgy

7

I~";~ "

12

16

....,--X' 20

24

2;

3'2

3'6

'

d(]y

Fig. 2. Calculated daily assimilate offer (continuous lines) and actual dry matter increase (broken lines) of cucumber plants. The shaded area represents the actual daily increase of vegel;ative dry matter (a and b). (c and d) Abortion percentage of fruits as a function of the flowering date (continuous lines). The broken lines represent the dry matter increase of the fruits numbered chronologically, i.e. Number I represents the average daily dry matter increase of all fruits growing in the lowest axils.

( c - r, ) (~1

(~

Pc 7"

= ~\C = = = =

(5)

+ 2F,

q u a n t u m e f f i c i e n c y at a h i g h CO2 c o n c e n t r a t i o n ].eaf l i g h t - u s e e f f i c i e n c y igoss photosynthesis :[eaf c o n d u c t a n c e t o CO2 t r a n s f e r

( 0 . 1 3 ( m g CO2 j-l)) ( m g CO2 j - l ) ( m g CO~ m -2 s -1) ( m s -1)

26 I0 m K C L r,

= light intensity at top of canopy = transmission coefficient of leaf = canopy extinction coefficient = CO2 concentration = leaf area index = CO2 compensation point

(W m -2 (PAR))

(0.15) (o.52) (mg CO2 m -3) (72 mg CO2 m -3)

Variations in r have been suggested to be proportional to the averaged light flux density (S) at the leaf surface during a certain period prior to the present situation. The relationship proposed by Acock et al. (1978) is

aS r

(6)

-

(1 + b S) a = 8.5 10 -s m 3J-1 b = 2 . 1 1 0 - 2 J - ' m 2s

Over the total leaf area, I will decrease with depth of canopy because the light flux density incident on a leaf in the canopy is approximated by

DoK

S - - -

exp(-- K L)

(1 - - m )

(7)

Do = averaged light flux density (W m -2) incident at the top of the canopy filtered by a digital filter with a time-constant of 6 days. Substituting eqns. (5), (6) and (7) into eqn. (4) and integrating over the entire leaf area of the canopy results in

aC Pc = ~-~ln

(

bcqIoDoK+(1--m)(c~,lo+aDoC} b a, I0 Do K exp(--KL) + (1 -- m) {(~, I0 + a Do C}

) (8)

The conversion of assimilated CO2 into structural dry weight involves metabolic activity to supply the energy that is necessary for various related biochemical processes. In addition, energy is required for transport of assimilates to the sinks. For cucumber plants, it was calculated that these processes lead to a conversion efficiency of 0.68, i.e. 0.68 g CO2 is built in structural dry weight for each g CO2 that is initially assimilated (Schapendonk and Gaastra, 1984). In addition, metabolic processes are involved in the energy supply for maintenance of plant integrity and protein turnover. From maintenance respiration measurements with cucumber fruits, and their protein content, the total plant maintenance respiration as function of temperature could be estimated on the basis of the protein content. At a temperature of 18°C, the maintenance coefficient (b) was 2.08 10 .7 g CO2 s-' per g structural vegetative dry weight. Thus, net CO2 uptake can be calculated according to

Pn = 0.68 (Pc -- b W)

(9)

27 b = maintenance coefficient (2.08 i 0 -7 g CO2 s -1) W = dry weight of the crop (g m -2) Conversion to daily dry weight increase can be derived from the carbon content of the different organs. RESULTS Growth analyses (Fig. 2a, b; broken lines) and simulated dry matter productions (Fig. 2a, b; solid lines) show marked discrepancies. Integrated over the total period, however, the simulated dry weight increase showed a good resemb!.ance to the dry weight increase calculated from non-destructive measurements; 440 g m -2 and 505 g m -2 measured in Experiment 1 and Experiment 2, respectively, against 449 g m -2 and 506 g m -2 simulated. Obviously, storage of assimilates at times of photosynthate surplus covers the shortage during the period where the demand exceeds the supply. This phenoraenon, however, cannot be detected by growth analyses because the estimations made are integrated results over 2--3-day time-intervals, n o t including changing storage capacity or an altered source--sink interrelationship. The initial surplus of assimilates in the stage of first inflorescence igadually vanishes as an increasing amount is invested in the fruits and finally, after Day 9 (Fig. 2a, . ) and Day 12 (Fig. 2b, ,), a supply shortage has to be made up by storage material. The balance between demand and supply is greatly affected by periodic fruit growth. Rapid growth of the fruits in the lowest axils (Experiment 1) subseq~aently leads to a high abortion percentage higher up the main stem (Fig. 2c). A relatively moderate growth of the first fruits, as in Experiment 2, causes a shift of the abortion peak to a later stage and a decrease of the maximum abortion frequency (Fig. 2d). The maximum abortion frequency of inflorescences concurred with a steadily increasing shortage of assimilates; fruits being most sensitive during a period of 8 days after flowering. This is validated by the highest abortion rates of inflorescences being initiated after c~ and 12 days in Experiment 1 and Experiment 2, respectively, after an almost continuous shortage for 8 days. With respect to the total dry weight accretion, y o u n g fruits would need only a minor percentage of the total supply to survive. In fact, there is an increa~.e of the growth rate of the remaining fruits, whereas young fruits abort. To investigate whether fruits abort due to assimilate shortage solely, or to a combination of assimilate shortage and dominance of competing fruits, cucumber plants were defoliated, leaving a single leaf at different axils (5--12). One fruit was left (Axil 5) and all other inflorescences were removed. The single fruits grew slowly b u t did n o t abort. Fruit growth rate was irLdependent of the position of the feeding leaf with respect to the fruit position. Decapitated plants with 2 leaves and 3 inflorescences showed a marked

28

dominance of the first inflorescence over the others, even if the delay between flowering was 1 day. The abortion rate of the first-formed fruits was zero; in contrast, all subsequent fruits were aborted. Effect o f fruit shading. -- Selective shading of fruits in alternating axils

with black cotton decreased their growth rate. The interaction between shading of fruits and overall availability of assimilates is expressed by a progressive growth inhibition of shaded fruits when the number of fruits increases (Table II). With 10 fruits, 2 out of 5 shaded fruits aborted. Fruit TABLE II Effect of shading on fruit growth. Harvest weight (g fresh weight) and abortion percentage (in parentheses) of fruits growing on a cucumber plant with 19 leaves. Fruits were covered alternately. The total number of fruits was chosen in the range between 2 and 10. Fruits were harvested 22 days after flowering of the first inflorescence Number of fruits

Shaded

Unshaded

2 4 6 8 10

828 (0) 991 (0) 1085 (0) 1359(25) 1128(40)

944(0) 1204(0) 1182(0) 1885(0) 2058(0)

g/plant 50 /o

Olo 100

o/O "°

/

40

s

30

x x

~ 20

10

o

/

o °~°~°~ ° ~

/°

././' Z/

10 20 3o 4~3 50 60

•

•

•

x,

50

x

•

x

x x

x

x

7'o

clay

I

10

2'0

20

day

Fig. 3. (a) Cucumulative dry matter increase of shaded (solid circles) and unshaded (open circles) fruits. (b) Abortion percentage of shaded (crosses) and unshaded (solid circles) fruits. Day 1 is the time of emergence of the first inflorescence.

29

abortion was n o t caused by local effects of temperature or an increased concentration of abscission-stimulating factors, as shown by a control experiment with all fruits enclosed. In that case, abortion frequency is n o t increased compared with fruits exposed to normal daylight. However, fruit yield was diminished significantly (Fig. 3). Hypothetically, photosynthesis of the fruit shell might cause a fruit to become partly autotrophic. The photosynthetic contribution of the fruit shell, relative to that of one of the leaves, can be expressed as the surface ratio of fruits and leaves. This ratio, however, never exceeded 7% and even appeared to be lower than 2% during :half of the growth period. DISCUSSION

The mechanism by which plant resources are partitioned between shoot, r o o t arLd fruits has led to speculation, but no unequivocal conclusions are available at present (Gifford and Evans, 1981). It has been suggested that growth substances released either from sinks or sources redirect assimilates to a developing sink (De Stigter, 1969; Hurd et al., 1979). Once a sink has been established, the high metabolic activity may maintain a downhill solute gradient in the phloem between source and sink. Such a mechanism seems realistic, given the possibility of the sink keeping soluble carbohydrate low (Walker and Thornley, 1977). The amount of photosynthetic substrate available for distribution is determined by environmental conditions, leaf surface and plant architecture. However, feedback control of photosynthesis by sink: activity has been reported (Hall, 1977). In addition, remobilisation of reserves provides a temporary mechanism to bridge the gap between assimilate demand and assimilate supply (Fig. 2). This is of great interest, especially at the onset of fruiting when assimilates become diverted by competition from the developing fruit. At that time, vegetative growth, apart from the r o o t system, proceeds at the same rate as during the period before fruit setting. Therefore, storage material has to be remobilised, which indicatA~s that in the short term, the activity of the source system is of less importance than the processes that underlie sink activity. The relative adequacy of phyllotaxis for assimilate distribution to individual fruits is only temporary. A source leaf definitely exports its assimilates to a sink which has the lowest resistance to mass flow, but removal of the sink will lead to an altered path along which export proceeds to a subsequent sink. Large variations of assimilate demand during the generative phase are mainly caused by interactions of competing fruits. Fruit abortion occurs if fruit setting is followed by a period of supply shortage of more than 8 days (Fig. 2a, b). However, assimilate shortage was n o t a controlling factor for fruits that were n o t competing. Single fruits deliberately supplied with few assimilates by defoliation reacted with an inhibited growth rate, but did n o t abort. This shows that the signal for abortion is given elsewhere.

30 In this respect, the dominance of older fruits is of great significance, regulating fruit abortion of younger fruits. This mechanism might initially be triggered by a stress reaction on assimilate depletion. The period during which older fruits exhibit dominance is limited. Figure 2b,d shows a normal development of inflorescences at Day 16. By that time, the fruits from the first axils were not yet harvested, but the high rate of abortion preceding Day 16 diminished the number of younger dominant fruits. Despite the evident lack of assimilates, the fruits did not abort. This shows that the fruits in the lowest axils lost their dominance after 12 days, which coincides with a period of decreased growth rate. The mechanism by which older fruits exert dominance remains obscure. Selective shading of fruits revealed an increased sensitivity to abortive signals and a decreased sink activity. The decreased sink activity in shaded fruits is determined intrinsically. If all developing fruits on a plant are shaded, total fruit production decreases because fruits remain relatively small. Abortion rates, however, are not affected. Therefore, the absolute sink activity must have been decreased. In competition with unshaded fruits, however, there was a tendency for shaded fruits to lose more fruits by abscission. These results confirm observations of Hole and Scott (1981), who noticed an increased abscission of flowers as a result of shading fruits on pea plants. They concluded that fruit photosynthesis might be responsible for this. For cucumber, it might be that fruit photosynthesis is of some importance in the early stage (3 days), but if the demand increases, the supply of assimilates will lag behind. It is difficult to reconcile why the photosynthetic contribution of less than 7% of the total plant photosynthesis would prevent abortion assuming equal photosynthetic activities in fruits and leaves per surface area. Furthermore, fruit abortion rates are n o t changed if all fruits of a plant are shaded. Though speculative for the m o m e n t , these observations tend to the conclusion that the sink function of y o u n g fruits may be partly regulated by a local growth factor synthesised in the light. The presence of this factor increases the sink function of the fruits and concomittantly protects young fruits from abcission signals produced by older fruits during a period of 9--10 days. The strength of the abscission factor depends on the a m o u n t of assimilates available to the (dominant) fruit. APPENDIX One of the major problems encountered in simulation models of crops production is the basic mechanism by which fruit growth is controlled. In addition to the overall harvest index, that appears to be almost constant under various environmental conditions, the dynamic properties of fruit growth and fruit abortion are of interest. From the grower's point of view, fruit quality and earliness of production are as important as total fruit production rate.

31

A better insight into the mechanism of fruit competition may lead to improw~d pruning techniques and growing methods, that ensure a better light distribution within the canopy to prevent excessive shading of developing fruits. In this section, the viability of our theory on inter-fruit competition is tested with a simple simulation model. Figure 4a shows a simulation of fruit growth during the period under investigation. Assimilates were partitioned according to sink function of individual fruits (Fig. 4b) given by (g) 20 18

a

16

12

O0

5

10

-1~-

t=O

15

20

r

o

25

]

_

3Q

35

--40 dqy

i

lO

Fig. 4. (a). S i m u l a t i o n o f fruitgrowth. The curves present dry w e i g h t increase o f individual fruits. (b) The sink f u n c t i o n o f individual fruits. The m o d e l assumes an x-value o f - - 1 2 at the time o f f l o w e r i n g (t=O).

32

W(i) = 4 e x p ( - 0 . 6 t)/(1 + e x p ( - 0 . 6 t)) 2 t = time from flowering Maximum sink activity was assumed to be reached after 12 days (Schapendonk and Challa, 1980). The relative sink function of a fruit is given by W(i)

S(i) =

W (total) W represents the total fruit sink activity. Fruit growth is given by

Fg(i)

= S(i)

•

Pn

The actual calculated fruit growth is compared with the potential growth rate of individual fruit by g(i) = W(i) • 2 g(i) = potential fruit growth 2 (g) is the maximum daily dry weight accretion of a single fruit. If the growth rate of fruits was inhibited due to assimilate shortage (i.e. Fg(i) < g(i)), the sink function of the subsequent fruit was decreased by 90% in favour of the dominant fruit. This results in a simulated pattern of fruit abortion that is quite c o m m o n for indeterminate flowering plants (Fig. 4). Thus, dynamic simulation of fruit growth may be a useful tool to predict the onset of fruit production and the subsequent time-course of harvest. However, it should be noticed that the accuracy of the results will be biased by genetic variability. ACKNOWLEDGEMENTS

The authors are grateful to Dr. H.M. Dekhuijzen and Dr. H. Veen for critically reading the manuscript.

REFERENCES Acock, B., Charles-Edwards, D.A., Fitter, D.J., Hand, D.W., Ludwig, L.J., Warren Wilson, J. and Withers, A.C., 1978. The contribution of leaves from different levels within a tomato crop to canopy net photosynthesis: An experimental examination of two canopy models. J. Exp. Bot., 29: 815--827. Farrington, P. and Pate, J.S., 1981. Fruit set in Lupinus angustifolius cv. Unicrop. I. Phenology and growth during flowering and early fruiting. Aust. J. Plant Physiol., 8: 293--305. Gifford, R.M. and Evans, L.T., 1981. Photosynthesis, carbon partitioning, and yield. Annu. Rev. Physiol., 32: 485--509. Hall, A.J., 1977. Assimilate source--sink relationships in Capsicum annuum L. I. The dynamics of growth in fruiting and deflorated plants. Aust. J. Plant Physiol., 4: 623-636.

33 Ho, L.C., 1976. The relation between the rates of carbon transport and of photosynthesis in tomato leaves. J. Exp. Bot., 27: 87--97. Ho, L.C., 1979. Regulation of assimilate translocation between leaves and fruits in the tomal~o. Ann. Bot., 43: 437--448. Hole, C.A. and Scott, P.A., 1981. The effect of fruit shading on yield in Pisum sativum L. Ann. Bot., 48: 827--835. Hurd, R.G., Gay, A.P. and Mountifield, A.C., 1979. The effect of partial flower removal on the relation between root, shoot and fruit growth in the indeterminate tomato. Ann. Appl. Biol., 93: 77--89. Liebig, H.P., 1978. EinfKisse endogener und exogener Faktoren auf die Ertragsbildung yon Salatgurken (Cucumis sativas, L.) unter besonderer Ber~icksichtigung yon Ertragsrhythmik, Bestandesdichte und Schnittmassnahmen. Thesis: Fakult~t fiir Gartenbau und Landeskultur der Technischen Universit~t Hannover. Lorenz, H.P., 1980. Modelluntersuchungen zur Klimareaktion yon Wachstumskomponenten am Beispiel Salatgurkenpflanzen (Cucumis sativus L.) -- Ein Beitrag zur Teml~,eraturfiihrung in Gew~ichsh~'usern. Thesis: Fakult~t ffir Gartenbau und Landeskultur der Technischen Universit~'t Hannover. Peisker, M. and Apel, P., 1981. Influence of oxygen on photosynthesis and photorespiration in leaves of Triticum aestivum L. 4. Oxygen dependence of apparent quantum yield of CO 2 uptake. Photosynthetica, 15: 435--441. Schapendonk, A.H.C.M. and Challa, M., 1980. Assimilate requirements for growth and maintenance of the cucumber fruit. Acta Hortic., 118: 73--82. Schapendonk, A.H.C.M. and Gaastra, P., 1984. A simulation study on CO 2 concentration in protected cultivation. Scientia Hortic., 23: in press. Stigter, H.C.M., de, 1969. Growth relations between individual fruits, and between fruits and roots in cucumber. J. Exp. Bot., 27: 87--97. Walker, A.J. and Thornley, J.H.M., 1977. The tomato fruit: import, growth, respiration and carbon metabolism at different fruit sizes and temperature. J. Exp. Bot., 41: 977--985.