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The effects of interfruit competition on the size of sweet pepper (Capsicum annuum L.) fruits
Scientia Horticulturae, 52 (1992) 69-76
69
Elsevier Science Publishers B.V., Amsterdam
The effects of interfruit competition on the size of sweet pepper (Capsicum a n n u u m L. ) fruits A.M. Alia'~ and W.C. Kellyb aDepartment of Horticulture, Faculty of Agriculture, University of Khartoum, Shambat, Sudan bDepartment of Fruit and Vegetables Science, Cornell University, Ithaca, NY, USA (Accepted 27 April 1992)
ABSTRACT Ali, A.M. and Kelly, W.C., 1992. The effects of interfruit competition on the size of sweet pepper ( Capsicum annuum L. ) fruits. Scientia Hortic., 52: 69-76. Two cultivars of sweet pepper were used to study the effect of interfruit competition on the components of fruit size under glasshouse and field conditions. Variation in interfruit competition was artificially simulated by removal of flower buds, flowers and set fruits on the first three flowering nodes of the pepper plant. The inhibitory effect of old fruits on the increase in weight, length, diameter and pericarp thickness of the younger ones was significant only from flower bud inception through Weeks 2 and 4 after fruit set in the glasshouse and field, respectively. Thereafter, the effect was not significant. Histological investigations revealed less cell multiplication activity and formation of fewer cell tiers, in the ovary wall of flower buds and small fruits under competition stress, than those under no competition. These results suggest that maintenance of vigorous vegetative growth from flower bud formation throughout fruit development might ensure sufficient organic nutrient supply to alleviate the stress on growth processes in developing buds, thereby reducing the variations in fruit sizes on upper nodes of pepper plant. Keywords: Capsicum annuum L,; debudding; deflowering; defruiting; fruit competition; fruit growth. Abbreviations: DBU=debudding; DFL=deflowering; DFR=defruiting; DFS=defruiting at set; DF2W= defruiting 2 weeks from set; DF4W=defruiting 4 weeks from set.
INTRODUCTION
Competitive limitations on the growth rates of fruits begin as the increasing number of fruits mobilize nutrient supplies for their growth. The pattern of such mobilization is mainly determined by both the strength and proximity Correspondence to: A.M. Ali, Department of Horticulture, Faculty of Agriculture, University of Khartoum, Shambat, Sudan.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00
70
A.M. ALI AND W.C. KELLY
of the sink organs to the source (Szynkler, 1974; Walker and Ho, 1977; Cook and Evans, 1978). The physiological basis of the considerable demand for assimilates by the growing fruits was assumed, by some researchers, to be hormonal in nature (Nitsch, 1950; Nitsch and Nitsch, 1951; Marre and Murneek, 1963; Archibold et al., 1982 ). The inhibitory effect of growing fruits on the development of pistils before anthesis were observed in cantaloupes (Rosa, 1924) and in okra, (Perkins et al., 1952). However, these authors gave no explicit mention of the possible influence of interfruit competition on the early growth processes of cell division and multiplication, as an essential component of the ultimate fruit size. Therefore, the objective of this investigation was to evaluate the conspicuous variation in pepper fruit sizes per plant by examining the effect of different degrees and durations of simulated interfruit competition on the components of fruit size and early growth processes in the flower bud and small fruits. MATERIALS AND METHODS
A . - Seeds of sweet pepper cultivars 'New Ace' and 'Staddon's Select' were planted in polystyrene transplant trays (speedling trays). Seedlings were thinned to one plant per cell and were allowed to grow in a glasshouse set at 25/18 ° C, day/night until they reach the eighth leaf stage. Uniform plants were transplanted in 30 cm diameter plastic pots filled with Cornell peat-lite A (Boodley and Sheldrake, 1977 ). Pots were watered as necessary, and fertilized weekly with a 0.2% 15-16-17 ( N P K ) fertilizer solution. Treatments to vary interfruit competition started at the fruit set stage on the third flowering node. They were: (1) defruiting at fruit set (DFS); (2) defruiting 2 weeks after set (DF2W); ( 3 ) defruiting 4 weeks after set ( DF4W ); (5) no defruiting (control). Pots were arranged in a randomized complete block design with four blocks. Each block included 40 plants (two cultivars × four treatments X five plants per treatment ). Buds of almost the same age on the third flowering node were tagged and ten buds or fruits were sampled at the flower bud, anthesis, and 1 week after anthesis stages. Samples from each stage were preserved for microscopic examination of the pericarp thickness using Johanson's (1940) procedure. Data on fruit weight, length, diameter and pericarp thickness were recorded on harvested mature green fruits and were statistically analysed using the Statistical Analysis System (SAS Institute, 1979).
Experiment
B . - Seedlings of the same cultivars were raised in the same way as in Experiment A until transplanting time in the field. Treatments were also identical. Soil type in the field was silty loam. In preparation of the field plots, 900 kg ha-1 of 15-15-15 ( N P K ) fertilizer were ploughed under. Trifluralin (Treflan) herbicide was incorporated at the rate of 0.56 kg ha -1 1 week be-
Experiment
INTERFRUIT COMPETITION ON THE SIZE OF SWEET PEPPER
71
fore planting. At transplanting time in the field, each seedling was given 250 ml of a starter fertilizer solution, 9-45-15 (NPK). Plots were arranged in a randomized complete block design with four blocks. Each block included eight rows (plots) of 15 plants spaced at 0.3 m × 0 . 9 m. Sampling, data recorded and analysis were carried out in the same way as in Experiment A except that no samples for microscopic examination were taken. This field experiment was essentially the same as Experiment B, but with the timing of treatments modified to vary competition at earlier stages of fruit development. Treatments were: ( 1 ) debudding of the first three flowering nodes, (DBU); (2) deflowering of the first three flowering nodes (DFL); ( 3 ) defruiting of the first three flowering nodes (DFR); (4) no debudding, deflowering or defruiting, (control). Samples for microscopic examination were taken at the bud and anthesis stages of fruit development. Data on fruit size components were recorded and analysed in the same way as in Experiments A and B.
Experiment
C. -
RESULTS
Pooled over the competition treatments, the main effect of the cultivar in the glasshouse (Experiment A) was significant for the fruit weight and pericarp thickness, but was not for the fruit length and diameter (Table 1 ). Microscopic observations showed that the pericarp of fruits from no competition treatments (Figs. 1, 3 and 5) had more cell tiers than that of fruits from competition treatments (Figs. 2, 4 and 6 ) at the three de-
Experiments
A a n d B.
-
TABLE1 Effect of defruiting the first and second flowering nodes on the fruit weight, length, diameter and pericarp thickness in 'New Ace' and 'Staddon's Select' cultivars grown in the glasshouse, (Experiment A) Treatments
Cultivars New Ace Staddon's Select SE Defruiting DFS DF2W DF4W Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (mm)
Pericarp thickness (mm)
143 159 7.1
99 95 3.5
73 74 2.1
4.4 5.5 0.61
165 149 152 136 6.1
99 96 95 95 2.9
76 72 75 72 1.9
5.2 5.0 4.9 4.7 0.28
72
A.M. ALl AND W.C. KELLY
Figs. 1-6. Longitudinal sections in the ovary wall at the bud, anthesis and one week after anthesis stages of New Ace cultivar grown in the greenhouse, showing the effect of interfruit competition on cell multiplication ( × 266 ). (P) pericarp; (gc) giant cell; (ovc) ovary cavity; (ma) meristematic activity; (oe) outer epidermis; (ie) inner epidermis; (ec) enlarging cells of mesocarp; (ovu) ovule; (st) stamen. Figs. 1, 3 and 5 are no competition treatment (DBU) at the bud, anthesis and 1 week after anthesis stage of fruit development respectively. Figs. 2, 4 and 6 are maximum competition treatment (control) at the bud, anthesis and 1 week after anthesis stages of fruit development respectively. Mean number of cell layers for DBU treatment at bud (Fig. 1), anthesis (Fig. 3) and 1 week after anthesis (Fig. 5 ) were 16 + 2.7, 21 _+2.1 and 23 + 1.8, respectively. For the control treatment at the same stages these numbers were 13 + 3.1 (Fig. 2 ), 16 + 2.9 (Fig. 4 ) and 20 + 2.2 (Fig. 6 ) respectively. ( + ) is the standard deviation.
v e l o p m e n t a l stages in spite o f the fact t h a t b u d s a n d fruits were o f the s a m e age. In the field, c u l t i v a r s p r o d u c e d fruits a l m o s t o f the s a m e length, b u t were significantly d i f f e r e n t in t h e i r d i a m e t e r , weight a n d p e r i c a r p thickness ( T a b l e
2). In b o t h e x p e r i m e n t s , fruit weight, d i a m e t e r a n d p e r i c a r p thickness were significantly l o w e r in the c o n t r o l t h a n in the d e f r u i t i n g t r e a t m e n t . F r u i t length was also r e d u c e d significantly as c o m p e t i t i o n effect i n t e n s i f i e d in the field, b u t r e d u c t i o n was n o t significant in the glasshouse. D e f r u i t i n g the first a n d s e c o n d f r u i t i n g n o d e s 2 weeks after set significantly i n c r e a s e d the fruit d i a m e t e r , weight a n d p e r i c a r p thickness in the field as c o m p a r e d with d e f r u i t i n g 4 weeks a f t e r set ( T a b l e 2). H o w e v e r , n o signifi-
INTERFRUITCOMPETITIONON THE SIZEOF SWEETPEPPER
73
TABLE2 Effect of defruiting the first and second flowering nodes on the fruit weight, length, diameter and pericarp thickness in the 'New Ace' and 'Staddon's Select' cultivars grown in the field, (Experiment B) Treatments
Cultivars New Ace Staddon's Select SE Defruiting DFS DF2W DF4W Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (mm)
Pericarp thickness (ram)
111 164 9.2
91 92 3.6
67 77 2.6
3.9 5.0 0.93
161 147 126 119 8.5
96 93 89 90 3.3
77 73 69 69 1.8
4.8 4.6 4.2 4.2 0.22
TABLE 3 Effect of inter-fruit competition on fruit weight, length, diameter and pericarp thickness in 'New Ace' and 'Staddon's Select' cultivars grown in the field, (Experiment C) Treatments
Cultivars New Ace Staddon's Select SE Competition DBU DFL DFR Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (ram)
Pericarp thickness (ram)
133 152 7.9
92 81 3.7
68 73 2.3
4.0 5.1 0.65
177 155 134 106 7.1
93 86 86 83 2.9
77 74 69 62 1.7
4.9 4.6 4.5 4.1 0.26
cant difference between the same treatments was observed in the glasshouse (Table 1 ).
Staddon's Select cultivar gave significantly higher fruit diameter, weight and pericarp thickness than 'New Ace'. On the other hand, fruit length was significantly higher in 'New Ace' than 'Staddon's Select' (Table 3 ). All parameters were inversely related to the intensity of competition. That is, when there was none (Debudding) or little (Deflowering) competi-
E x p e r i m e n t C. -
74
A.M.ALIANDW.C.KELLY
Fig. 7. Longitudinal sections in the ovary wall at the bud and anthesis stages of New Ace cultivar grown in the field, ( × 2 6 6 ) . (a) and (b) are bud stages from no (DBU) and maximum (control) competition treatments respectively. (c) and (d) are anthesis stages from no (DBU) and maximum (control) competition treatments respectively. Mean number of cell layers were 19 _+3.6 and 20 + 3.4 for DBU treatment at the bud (a) and anthesis (c) stages. For the control treatment at the bud (b) and anthesis (d) stages these numbers were 14+ 3.7 and 15+3.9 respectively. ( _+) standard deviation of layer counts.
tion from fruits on the first three nodes, those on the fourth node tended to be blocky; as the competition effect increased (defruiting, control), fruits became shorter and narrower (Table 3 ). Microscopic observations showed that the carpel wall of the bud stage was thicker and cells were more meristematic when no competition was exerted at the bud stage (Fig. 7 ( a ) ) as compared with the thin pericarp of small and less meristematic cells (Fig. 7 ( b ) ) of the m a x i m u m competition. The same effect was maintained through anthesis stage (Fig. 7 (c) vs. ( d ) ) . DISCUSSION
Outlining the nature of the interfruit competition, Nitsch ( 1965 ) reported that competitive limitations on the growth rates of fruits began as the increasing numbers of fruits mobilize nutrient supplies for their growth. Translocation of such nutrients was found to be a function of the capacity and proximity of the sink organs to the source (Szynkler, 1974; McArther et al., 1975; Walker and Ho, 1977; Cook and Evans, 1978 ). Results of our 3 experiments have, likewise, demonstrated the limitations excerted by the lower fruiting nodes on fruits of the third and fourth nodes and the negative consequences on their size. That is, at the time when the lower fruits were actively mobilizing assimilates and nutrients for their growth, those on the upper nodes were at a less competitive flower bud stage. This observation agrees with Hale and Weaver's ( 1962 ) findings which showed that grape flowers were a weak sink,
INTERFRUIT COMPETITION ON THE SIZE OF SWEET PEPPER
75
while berries were a stronger one. However, Walker and Ho, ( 1977 ) demonstrated that tomato fruits at 20% of their maximum size imported carbohydrates at a rate twice that of larger fruits. These two studies imply that sink strength and capacity may also be a function of its metabolic activity rather than its size. The glasshouse experiment (Experiment A ) indicated that the competition effect was severe until 2 weeks after fruit set, thereafter, the effect was not significant. In the identical experiment grown in the field (Experiment B), competition effect lasted up to 4 weeks, after which there was no significant difference when compared with the control where competition was maintained throughout. Shift in sink strength and capacity in response to changes in metabolic activity of sink components seems to have occurred earlier in the glasshouse than in the field. This could be attributed to the more favourable growing conditions in the glasshouse which would enhance the growth rate of one sink component and consequently shortens the duration of the shift in metabolic activity to other components. On the other hand, likely limitations in mineral nutrients and soil moisture as well as the fluctuations in temperature in the field, could have slowed the growth rate of sink components thereby delaying the metabolic shift and consequently extending the duration of competition. To produce high yield and good quality fruits, Kirti and Nettles ( 1961 ) reported that debudding the first five nodes of pepper plants was more effective than deflowering or defruiting, which is in line with the findings of Experiment C in the field. Intuitively they illustrated the importance of competition alleviation very early in the development of the fruits, i.e. when buds were being formed by cell multiplication which is responsible for determining the number of growth units of the fruits. Microscopic observations on the ovary wall did show that competition clearly affected development (Figs. 17 ). It resulted in fewer and less meristematic cell layers than treatments where competition was less. The stress of competition might be attributed to deprivation of the necessary growth factors for cell division in the buds because most of the assimilates would be diverted to the growth of the metabolically more active sinks in older fruits. Consequently, the rate of cell multiplication would be lower and result in smaller buds. Competitive ability acquired after fruit set, would build on the smaller initials established at the time when older fruits were dominating the supply of nutrients. CONCLUSION
The present study revealed that the critical period in fruit development was from flower bud inception to 2 and 4 weeks after set in the optimum (glasshouse) and suboptimum (field) growing environments respectively. One aspect of competition limitations on fruit growth could be a check on the early
76
A.M.ALIANDW.C.KELLY
process of cell multiplication that lays down the growth units for the ultimate fruit size. Thus maintenance of vigorous vegetative growth from flower initiation throughout harvesting might ensure sufficient organic nutrients to alleviate competition stress and lead to less variation of fruit size on the upper nodes of pepper plant.
REFERENCES Archibold, D.D., Dennis, F.G. and Flore, J.A., 1982. Accumulation of C~4-1abelled material from foliar-applied C~a-sucrose by tomato ovaries during fruit set and initial development. J. Am. Soc. Hortic. Sci., 107: 19-23. Boodley, J.W. and Sheldrake, R., 1977. Cornell Peat-lite mixes for commercial plant growing. Cornell Bulletin, No. 43. Cook, M.G. and Evans, L.T., 1978. Effect of relative size and distance of competing sinks on the distribution of photosynthetic assimilates in wheat. Aust. J. Plant Physiol., 5: 493-509. Hale, C.R. and Weaver, R.J., 1962. The effect of developmental stage on direction of translocation of photosynthate in Vitis vinifera. Hilgardia, 33:89-131. Johanson, D.A., 1940. Plant Microtechnique. McGraw-Hill, New York, 523 pp. Kirti, S. and Nettles, V.F., 1961. Effect of defloration, defruiting, nitrogen and calcium on the growth and fruiting responses of bell peppers, Capsicum annuum L. Proc. Fla. State Hortic. Soc., 74: 204-209. Marre, E. and Murneek, A.E., 1963. Carbohydrate metabolism in the tomato fruit as affected by pollination, fertilization and application of growth regulators. Plant Physiol., 28: 255266. McArther, J.A., Hesketh, J.D. and Baker, D.N., 1975. Cotton. In: L.T. Evans (Editor), Crop Physiology. Cambridge University Press, London, pp. 297-325. Nitsch, J.P., 1950. Growth and morphogenesis of strawberry as related to auxin. Am. J. Bot., 37:211-215. Nitsch, J.P. and Nitsch, C., 1951. Growth factors in the tomato fruit. In: R.M. Klin (Editor), Plant Growth Regulation. Iowa State University Press, Iowa, pp. 687-705. Nitsch, J.P., 1965. Physiology of flower and fruit development. Encyco. Plant Physiol., 15:15371647. Perkins, D.Y., Miller, J.C. and Dallyn, S.L., 1952. Influence of pod maturity on the vegetative and reproductive behaviour of okra. Proc. Am. Soc. Hortic. Sci., 50:311-314. Rosa, J.T., 1924. Fruiting habit and pollination of cantaloupe. Proc. Am. Soc. Hortic. Sci., 21: 51-57. Statistical Analysis Systems, 1979. SAS User's Guide; Statistics, 1979 Edition. Cary, NC, 494 PP. Szynkler, K., 1974. The effect of removing of supply or sink organs on the distribution of assimilates in two varieties of garden pea, Pisum sativum L. Acta Agric. Scand., 24: 7-12. Walker, A.J. and Ho, L.C., 1977. Carbon translocation in the tomato: Carbon import and fruit growth. Ann. Bot., 41: 813-824.
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Elsevier Science Publishers B.V., Amsterdam
The effects of interfruit competition on the size of sweet pepper (Capsicum a n n u u m L. ) fruits A.M. Alia'~ and W.C. Kellyb aDepartment of Horticulture, Faculty of Agriculture, University of Khartoum, Shambat, Sudan bDepartment of Fruit and Vegetables Science, Cornell University, Ithaca, NY, USA (Accepted 27 April 1992)
ABSTRACT Ali, A.M. and Kelly, W.C., 1992. The effects of interfruit competition on the size of sweet pepper ( Capsicum annuum L. ) fruits. Scientia Hortic., 52: 69-76. Two cultivars of sweet pepper were used to study the effect of interfruit competition on the components of fruit size under glasshouse and field conditions. Variation in interfruit competition was artificially simulated by removal of flower buds, flowers and set fruits on the first three flowering nodes of the pepper plant. The inhibitory effect of old fruits on the increase in weight, length, diameter and pericarp thickness of the younger ones was significant only from flower bud inception through Weeks 2 and 4 after fruit set in the glasshouse and field, respectively. Thereafter, the effect was not significant. Histological investigations revealed less cell multiplication activity and formation of fewer cell tiers, in the ovary wall of flower buds and small fruits under competition stress, than those under no competition. These results suggest that maintenance of vigorous vegetative growth from flower bud formation throughout fruit development might ensure sufficient organic nutrient supply to alleviate the stress on growth processes in developing buds, thereby reducing the variations in fruit sizes on upper nodes of pepper plant. Keywords: Capsicum annuum L,; debudding; deflowering; defruiting; fruit competition; fruit growth. Abbreviations: DBU=debudding; DFL=deflowering; DFR=defruiting; DFS=defruiting at set; DF2W= defruiting 2 weeks from set; DF4W=defruiting 4 weeks from set.
INTRODUCTION
Competitive limitations on the growth rates of fruits begin as the increasing number of fruits mobilize nutrient supplies for their growth. The pattern of such mobilization is mainly determined by both the strength and proximity Correspondence to: A.M. Ali, Department of Horticulture, Faculty of Agriculture, University of Khartoum, Shambat, Sudan.
© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00
70
A.M. ALI AND W.C. KELLY
of the sink organs to the source (Szynkler, 1974; Walker and Ho, 1977; Cook and Evans, 1978). The physiological basis of the considerable demand for assimilates by the growing fruits was assumed, by some researchers, to be hormonal in nature (Nitsch, 1950; Nitsch and Nitsch, 1951; Marre and Murneek, 1963; Archibold et al., 1982 ). The inhibitory effect of growing fruits on the development of pistils before anthesis were observed in cantaloupes (Rosa, 1924) and in okra, (Perkins et al., 1952). However, these authors gave no explicit mention of the possible influence of interfruit competition on the early growth processes of cell division and multiplication, as an essential component of the ultimate fruit size. Therefore, the objective of this investigation was to evaluate the conspicuous variation in pepper fruit sizes per plant by examining the effect of different degrees and durations of simulated interfruit competition on the components of fruit size and early growth processes in the flower bud and small fruits. MATERIALS AND METHODS
A . - Seeds of sweet pepper cultivars 'New Ace' and 'Staddon's Select' were planted in polystyrene transplant trays (speedling trays). Seedlings were thinned to one plant per cell and were allowed to grow in a glasshouse set at 25/18 ° C, day/night until they reach the eighth leaf stage. Uniform plants were transplanted in 30 cm diameter plastic pots filled with Cornell peat-lite A (Boodley and Sheldrake, 1977 ). Pots were watered as necessary, and fertilized weekly with a 0.2% 15-16-17 ( N P K ) fertilizer solution. Treatments to vary interfruit competition started at the fruit set stage on the third flowering node. They were: (1) defruiting at fruit set (DFS); (2) defruiting 2 weeks after set (DF2W); ( 3 ) defruiting 4 weeks after set ( DF4W ); (5) no defruiting (control). Pots were arranged in a randomized complete block design with four blocks. Each block included 40 plants (two cultivars × four treatments X five plants per treatment ). Buds of almost the same age on the third flowering node were tagged and ten buds or fruits were sampled at the flower bud, anthesis, and 1 week after anthesis stages. Samples from each stage were preserved for microscopic examination of the pericarp thickness using Johanson's (1940) procedure. Data on fruit weight, length, diameter and pericarp thickness were recorded on harvested mature green fruits and were statistically analysed using the Statistical Analysis System (SAS Institute, 1979).
Experiment
B . - Seedlings of the same cultivars were raised in the same way as in Experiment A until transplanting time in the field. Treatments were also identical. Soil type in the field was silty loam. In preparation of the field plots, 900 kg ha-1 of 15-15-15 ( N P K ) fertilizer were ploughed under. Trifluralin (Treflan) herbicide was incorporated at the rate of 0.56 kg ha -1 1 week be-
Experiment
INTERFRUIT COMPETITION ON THE SIZE OF SWEET PEPPER
71
fore planting. At transplanting time in the field, each seedling was given 250 ml of a starter fertilizer solution, 9-45-15 (NPK). Plots were arranged in a randomized complete block design with four blocks. Each block included eight rows (plots) of 15 plants spaced at 0.3 m × 0 . 9 m. Sampling, data recorded and analysis were carried out in the same way as in Experiment A except that no samples for microscopic examination were taken. This field experiment was essentially the same as Experiment B, but with the timing of treatments modified to vary competition at earlier stages of fruit development. Treatments were: ( 1 ) debudding of the first three flowering nodes, (DBU); (2) deflowering of the first three flowering nodes (DFL); ( 3 ) defruiting of the first three flowering nodes (DFR); (4) no debudding, deflowering or defruiting, (control). Samples for microscopic examination were taken at the bud and anthesis stages of fruit development. Data on fruit size components were recorded and analysed in the same way as in Experiments A and B.
Experiment
C. -
RESULTS
Pooled over the competition treatments, the main effect of the cultivar in the glasshouse (Experiment A) was significant for the fruit weight and pericarp thickness, but was not for the fruit length and diameter (Table 1 ). Microscopic observations showed that the pericarp of fruits from no competition treatments (Figs. 1, 3 and 5) had more cell tiers than that of fruits from competition treatments (Figs. 2, 4 and 6 ) at the three de-
Experiments
A a n d B.
-
TABLE1 Effect of defruiting the first and second flowering nodes on the fruit weight, length, diameter and pericarp thickness in 'New Ace' and 'Staddon's Select' cultivars grown in the glasshouse, (Experiment A) Treatments
Cultivars New Ace Staddon's Select SE Defruiting DFS DF2W DF4W Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (mm)
Pericarp thickness (mm)
143 159 7.1
99 95 3.5
73 74 2.1
4.4 5.5 0.61
165 149 152 136 6.1
99 96 95 95 2.9
76 72 75 72 1.9
5.2 5.0 4.9 4.7 0.28
72
A.M. ALl AND W.C. KELLY
Figs. 1-6. Longitudinal sections in the ovary wall at the bud, anthesis and one week after anthesis stages of New Ace cultivar grown in the greenhouse, showing the effect of interfruit competition on cell multiplication ( × 266 ). (P) pericarp; (gc) giant cell; (ovc) ovary cavity; (ma) meristematic activity; (oe) outer epidermis; (ie) inner epidermis; (ec) enlarging cells of mesocarp; (ovu) ovule; (st) stamen. Figs. 1, 3 and 5 are no competition treatment (DBU) at the bud, anthesis and 1 week after anthesis stage of fruit development respectively. Figs. 2, 4 and 6 are maximum competition treatment (control) at the bud, anthesis and 1 week after anthesis stages of fruit development respectively. Mean number of cell layers for DBU treatment at bud (Fig. 1), anthesis (Fig. 3) and 1 week after anthesis (Fig. 5 ) were 16 + 2.7, 21 _+2.1 and 23 + 1.8, respectively. For the control treatment at the same stages these numbers were 13 + 3.1 (Fig. 2 ), 16 + 2.9 (Fig. 4 ) and 20 + 2.2 (Fig. 6 ) respectively. ( + ) is the standard deviation.
v e l o p m e n t a l stages in spite o f the fact t h a t b u d s a n d fruits were o f the s a m e age. In the field, c u l t i v a r s p r o d u c e d fruits a l m o s t o f the s a m e length, b u t were significantly d i f f e r e n t in t h e i r d i a m e t e r , weight a n d p e r i c a r p thickness ( T a b l e
2). In b o t h e x p e r i m e n t s , fruit weight, d i a m e t e r a n d p e r i c a r p thickness were significantly l o w e r in the c o n t r o l t h a n in the d e f r u i t i n g t r e a t m e n t . F r u i t length was also r e d u c e d significantly as c o m p e t i t i o n effect i n t e n s i f i e d in the field, b u t r e d u c t i o n was n o t significant in the glasshouse. D e f r u i t i n g the first a n d s e c o n d f r u i t i n g n o d e s 2 weeks after set significantly i n c r e a s e d the fruit d i a m e t e r , weight a n d p e r i c a r p thickness in the field as c o m p a r e d with d e f r u i t i n g 4 weeks a f t e r set ( T a b l e 2). H o w e v e r , n o signifi-
INTERFRUITCOMPETITIONON THE SIZEOF SWEETPEPPER
73
TABLE2 Effect of defruiting the first and second flowering nodes on the fruit weight, length, diameter and pericarp thickness in the 'New Ace' and 'Staddon's Select' cultivars grown in the field, (Experiment B) Treatments
Cultivars New Ace Staddon's Select SE Defruiting DFS DF2W DF4W Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (mm)
Pericarp thickness (ram)
111 164 9.2
91 92 3.6
67 77 2.6
3.9 5.0 0.93
161 147 126 119 8.5
96 93 89 90 3.3
77 73 69 69 1.8
4.8 4.6 4.2 4.2 0.22
TABLE 3 Effect of inter-fruit competition on fruit weight, length, diameter and pericarp thickness in 'New Ace' and 'Staddon's Select' cultivars grown in the field, (Experiment C) Treatments
Cultivars New Ace Staddon's Select SE Competition DBU DFL DFR Control SE
Fruit weight (g)
Fruit length (mm)
Fruit diameter (ram)
Pericarp thickness (ram)
133 152 7.9
92 81 3.7
68 73 2.3
4.0 5.1 0.65
177 155 134 106 7.1
93 86 86 83 2.9
77 74 69 62 1.7
4.9 4.6 4.5 4.1 0.26
cant difference between the same treatments was observed in the glasshouse (Table 1 ).
Staddon's Select cultivar gave significantly higher fruit diameter, weight and pericarp thickness than 'New Ace'. On the other hand, fruit length was significantly higher in 'New Ace' than 'Staddon's Select' (Table 3 ). All parameters were inversely related to the intensity of competition. That is, when there was none (Debudding) or little (Deflowering) competi-
E x p e r i m e n t C. -
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A.M.ALIANDW.C.KELLY
Fig. 7. Longitudinal sections in the ovary wall at the bud and anthesis stages of New Ace cultivar grown in the field, ( × 2 6 6 ) . (a) and (b) are bud stages from no (DBU) and maximum (control) competition treatments respectively. (c) and (d) are anthesis stages from no (DBU) and maximum (control) competition treatments respectively. Mean number of cell layers were 19 _+3.6 and 20 + 3.4 for DBU treatment at the bud (a) and anthesis (c) stages. For the control treatment at the bud (b) and anthesis (d) stages these numbers were 14+ 3.7 and 15+3.9 respectively. ( _+) standard deviation of layer counts.
tion from fruits on the first three nodes, those on the fourth node tended to be blocky; as the competition effect increased (defruiting, control), fruits became shorter and narrower (Table 3 ). Microscopic observations showed that the carpel wall of the bud stage was thicker and cells were more meristematic when no competition was exerted at the bud stage (Fig. 7 ( a ) ) as compared with the thin pericarp of small and less meristematic cells (Fig. 7 ( b ) ) of the m a x i m u m competition. The same effect was maintained through anthesis stage (Fig. 7 (c) vs. ( d ) ) . DISCUSSION
Outlining the nature of the interfruit competition, Nitsch ( 1965 ) reported that competitive limitations on the growth rates of fruits began as the increasing numbers of fruits mobilize nutrient supplies for their growth. Translocation of such nutrients was found to be a function of the capacity and proximity of the sink organs to the source (Szynkler, 1974; McArther et al., 1975; Walker and Ho, 1977; Cook and Evans, 1978 ). Results of our 3 experiments have, likewise, demonstrated the limitations excerted by the lower fruiting nodes on fruits of the third and fourth nodes and the negative consequences on their size. That is, at the time when the lower fruits were actively mobilizing assimilates and nutrients for their growth, those on the upper nodes were at a less competitive flower bud stage. This observation agrees with Hale and Weaver's ( 1962 ) findings which showed that grape flowers were a weak sink,
INTERFRUIT COMPETITION ON THE SIZE OF SWEET PEPPER
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while berries were a stronger one. However, Walker and Ho, ( 1977 ) demonstrated that tomato fruits at 20% of their maximum size imported carbohydrates at a rate twice that of larger fruits. These two studies imply that sink strength and capacity may also be a function of its metabolic activity rather than its size. The glasshouse experiment (Experiment A ) indicated that the competition effect was severe until 2 weeks after fruit set, thereafter, the effect was not significant. In the identical experiment grown in the field (Experiment B), competition effect lasted up to 4 weeks, after which there was no significant difference when compared with the control where competition was maintained throughout. Shift in sink strength and capacity in response to changes in metabolic activity of sink components seems to have occurred earlier in the glasshouse than in the field. This could be attributed to the more favourable growing conditions in the glasshouse which would enhance the growth rate of one sink component and consequently shortens the duration of the shift in metabolic activity to other components. On the other hand, likely limitations in mineral nutrients and soil moisture as well as the fluctuations in temperature in the field, could have slowed the growth rate of sink components thereby delaying the metabolic shift and consequently extending the duration of competition. To produce high yield and good quality fruits, Kirti and Nettles ( 1961 ) reported that debudding the first five nodes of pepper plants was more effective than deflowering or defruiting, which is in line with the findings of Experiment C in the field. Intuitively they illustrated the importance of competition alleviation very early in the development of the fruits, i.e. when buds were being formed by cell multiplication which is responsible for determining the number of growth units of the fruits. Microscopic observations on the ovary wall did show that competition clearly affected development (Figs. 17 ). It resulted in fewer and less meristematic cell layers than treatments where competition was less. The stress of competition might be attributed to deprivation of the necessary growth factors for cell division in the buds because most of the assimilates would be diverted to the growth of the metabolically more active sinks in older fruits. Consequently, the rate of cell multiplication would be lower and result in smaller buds. Competitive ability acquired after fruit set, would build on the smaller initials established at the time when older fruits were dominating the supply of nutrients. CONCLUSION
The present study revealed that the critical period in fruit development was from flower bud inception to 2 and 4 weeks after set in the optimum (glasshouse) and suboptimum (field) growing environments respectively. One aspect of competition limitations on fruit growth could be a check on the early
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process of cell multiplication that lays down the growth units for the ultimate fruit size. Thus maintenance of vigorous vegetative growth from flower initiation throughout harvesting might ensure sufficient organic nutrients to alleviate competition stress and lead to less variation of fruit size on the upper nodes of pepper plant.
REFERENCES Archibold, D.D., Dennis, F.G. and Flore, J.A., 1982. Accumulation of C~4-1abelled material from foliar-applied C~a-sucrose by tomato ovaries during fruit set and initial development. J. Am. Soc. Hortic. Sci., 107: 19-23. Boodley, J.W. and Sheldrake, R., 1977. Cornell Peat-lite mixes for commercial plant growing. Cornell Bulletin, No. 43. Cook, M.G. and Evans, L.T., 1978. Effect of relative size and distance of competing sinks on the distribution of photosynthetic assimilates in wheat. Aust. J. Plant Physiol., 5: 493-509. Hale, C.R. and Weaver, R.J., 1962. The effect of developmental stage on direction of translocation of photosynthate in Vitis vinifera. Hilgardia, 33:89-131. Johanson, D.A., 1940. Plant Microtechnique. McGraw-Hill, New York, 523 pp. Kirti, S. and Nettles, V.F., 1961. Effect of defloration, defruiting, nitrogen and calcium on the growth and fruiting responses of bell peppers, Capsicum annuum L. Proc. Fla. State Hortic. Soc., 74: 204-209. Marre, E. and Murneek, A.E., 1963. Carbohydrate metabolism in the tomato fruit as affected by pollination, fertilization and application of growth regulators. Plant Physiol., 28: 255266. McArther, J.A., Hesketh, J.D. and Baker, D.N., 1975. Cotton. In: L.T. Evans (Editor), Crop Physiology. Cambridge University Press, London, pp. 297-325. Nitsch, J.P., 1950. Growth and morphogenesis of strawberry as related to auxin. Am. J. Bot., 37:211-215. Nitsch, J.P. and Nitsch, C., 1951. Growth factors in the tomato fruit. In: R.M. Klin (Editor), Plant Growth Regulation. Iowa State University Press, Iowa, pp. 687-705. Nitsch, J.P., 1965. Physiology of flower and fruit development. Encyco. Plant Physiol., 15:15371647. Perkins, D.Y., Miller, J.C. and Dallyn, S.L., 1952. Influence of pod maturity on the vegetative and reproductive behaviour of okra. Proc. Am. Soc. Hortic. Sci., 50:311-314. Rosa, J.T., 1924. Fruiting habit and pollination of cantaloupe. Proc. Am. Soc. Hortic. Sci., 21: 51-57. Statistical Analysis Systems, 1979. SAS User's Guide; Statistics, 1979 Edition. Cary, NC, 494 PP. Szynkler, K., 1974. The effect of removing of supply or sink organs on the distribution of assimilates in two varieties of garden pea, Pisum sativum L. Acta Agric. Scand., 24: 7-12. Walker, A.J. and Ho, L.C., 1977. Carbon translocation in the tomato: Carbon import and fruit growth. Ann. Bot., 41: 813-824.