Pod wall photosynthesis is known to contribute to nearly one-third of rape (Brassica napus) seed weight. The relationship between the number of seeds and the growth of the pod wall, and the effects of single and mixed applications of gibberellic acid (GA), naphthyl acetic acid (NAA) and benzyl adenine (BA) on development of the pod wall were, therefore, examined in potted plants of spring rape under glasshouse conditions.
The size and weight of the pod wall were closely associated with the number of seeds per pod, and a stronger development of tissues was observed in the presence of seeds. Application of BA alone or in combination with GA (GA+BA) to emasculated flowers induced the development of parthenocarpic pods similar in size and shape to the untreated seeded pods. NAA applied singly or in mixtures produced abnormal pods. BA enhanced cell division in all tissues while GA and NAA caused expansion of parenchyma in the mesocarp. Application of BA or GA+BA at 1-5 rather than at 11-15 days after flower opening led to the development of bigger pods. Younger pods responded more than the older ones when they were treated simultaneously. BA or GA+BA application to non-emasculated flowers on the terminal raceme delayed senescence in both basal and apical pods but it increased pod size only in the latter. The balance of cytokinins and gibberellins within a pod seems to play an important role in regulating the growth of the pod wall.
INTRODUCTION
Regulation of the growth of the pod wall in oilseed rape (Brassica napus L.) is important agronomically because as much as 32% of the seed weight within a pod is accounted for by photosynthesis of the pod wall (Brar & Thies 1977). Further, pod surface area and seed yield on a plant basis are known to be positively correlated (Li & Wang 1988). The regression of pod length on the number of seeds per pod at maturity is reportedly linear (Pechan & Morgan 1985). It is not clear, however, when such close association developed, whether it holds under varied conditions of assimilate supply, and if development of the pod wall tissues is associated with the presence of seeds. On the basis of a strong positive correlation between size and shape of the fruit and the number and position of seeds, several studies (e.g. Hedden & Hoad 1985) suggest that phytohormones, probably of seed origin, are important determinants of fruit growth. Rape pods contain many hormones at varying concentrations depending on the developmental stage (Bouille et al. 1989), but the effects of exogenous plant growth substances (PGS) on pod wall development have not yet been examined.
This study was conducted initially to examine the relationship between the number of seeds and the growth of the pod wall. The effects of gibberellic acid (GA), naphthyl acetic acid (NAA) and benzyl adenine (BA) applied singly or in mixtures on morphology, anatomy and senescence of the pod wall were then analysed. Factors influencing the response to applied GA and BA were also investigated.
MATERIALS AND METHODS
All experiments were conducted under glasshouse conditions, using single plants of spring rape (Brassica napus L. cv. Maris Haplona) grown on John Innes compost in plastic pots (11 cm diameter). Plants were selected for uniformity 1 week prior to anthesis and randomly allocated to treatments arranged in a randomized block design. The objectives and treatments in various experiments are summarized in Table 1.
X-radiography
Flowers at nodes 3+2 and 15+2 on the terminal raceme were x-rayed in vivo (Pechan & Morgan 1985) daily for 15 days after flowering (DAF) and at weekly intervals thereafter until maturity to test differences in development of pods with varying number of seeds (Expt 1).
Histology
Pods sampled at varying intervals (Expts 1d and 2b) were fixed in FAA (Formalin - acetic acid - 70% alcohol 1:1:18 v/v) for at least 24 h, dehydrated through an alcohol series, cleared in toluene and embedded in paraffin wax. Serial sections of 10 µm thick were cut on a microtome, dewaxed in xylene, hydrated through a series of alcohols, and differentially stained with Safranine and Fast Green before mounting in DePex. In Expt 1d, c. 5 mm pod wall on each side of the replum was sectioned longitudinally and transversely in regions where (a) no seed was developing on either side of the replum, (b) only one seed was developing on one of the two sides of the replum, and (c) where two seeds were developing on either side of the replum. In Expt 2b, pod wall segments halfway along the length of the pod were used.
Application of growth regulators
GA and NAA were dissolved in 50% aqueous ethanol (Solvent A), and BA in 0.1M HCl (Solvent B) before making up the volume with 0.1% aqueous Tween 20 (surfactant). The mixtures were made by adding equal amounts of stock solutions of single chemicals.
In Expts 2 and 3, emasculated flowers were treated to separate the effects of applied PGS from those produced by the developing seeds themselves. Branches were removed 1 week prior to anthesis and PGS treatments were restricted to the first 15 basal flowers on the terminal raceme. Emasculation was done 2 days before flower opening. The apex and buds were removed after opening of the 15th flower to exclude the effects of non-emasculated flowers (seeded pods) at later stages. In Expt 4, branches were not removed and PGS were applied to non-emasculated flowers.
PGS were applied at 1 g l-1 (except in Expts 2a and 3a) using an Agla micrometer syringe as droplets of 5 µl each on distal and proximal ends of the pod from 1 DAF for 5 days consecutively. Each pod, therefore, received 50 µg of PGS. In Expt 3b, the solvent control involved application of mixture of solvents A and B (1:1 v/v). To apply PGS through lanolin, c. 10 g lanolin weighed into a vial was immersed in hot water at 60oC, and GA or BA (2.5 mg each) or GA and BA (1.25 mg each) were mixed by stirring with a glass rod until the mixture became semi-solid. About 200 mg lanolin was smeared over the pod surface at 2 DAF.
In Expt 3d, the flowers that opened on the terminal raceme during the first 8 days after anthesis (DAA) were treated simultaneously with PGS for 5 consecutive days and the remainder of the raceme was decapitated. Treatments were started 7 DAA so that the basal pods received treatment from 7 to 11 DAF and the uppermost ones from 1 day before flower opening to 3 DAF.
Data collection and statistical analyses
Pod growth was measured in terms of length, diameter and dry weight. By considering the shape of pod as cylindrical, surface area was estimated using the equation ¹DL where D is the diameter, and L is the cylindrical length. Correlation and regression were used to examine the relationships between pod size components and seed number per pod. Data were subjected to analysis of variance using the GENSTAT V (Rothamsted Experimental Station).
RESULTS AND DISCUSSION
Seed number and development of the pod wall (Expt 1)
Irrespective of pod location on the plant, a strong positive association between seed number per pod and components of pod size was found at maturity. It was seen in pods located not only on the terminal and axillary racemes 1, 4 and 7 (Table 2A) but also on other racemes, and in all regions within a raceme (data not shown). The variation in seed number accounted for as much as 80% of the variation in pod length, surface area, and dry weights of pod and pod wall. The strength of correlation with pod diameter was, however, less than that for other components of pod size. The correlation was evident from as early as 5 DAF for pod length and 10 DAF for other components (Table 2B). All components of pod size except diameter were strongly correlated with seed number even after increasing the supply of assimilates to pods by reducing competition from other organs. The strength of correlation was fairly constant but the slope of regression was higher (for pod surface area, and dry weights of pod and pod wall) in pods supplied with additional carbon assimilates (Table 2C).
X-radiography showed that pod wall development was associated with the presence of a developing seed nearby. The region devoid of seeds grew only slightly (Fig. 1). The close association between the number of seeds and pod elongation was evident from 4 DAF. When a seed aborted by 5-6 DAF, the growth of the pod wall surrounding the seed had ceased and the width in that area contracted within a few days. When abortion occurred later, there was only a small reduction in the growth of the pod wall (data not shown).
A rape pod is made up of two carpels separated by a false septum which, in turn, is surrounded by a thick-ribbed replum. Each carpel (pod wall) is composed of (a) the exocarp consisting of a thin walled cuticle and epidermis, (b) the mesocarp comprising chlorenchyma and parenchyma, and (c) the endocarp comprising sclerenchyma and endodermis. The mesocarp is interspersed with vascular bundles, the central one being the largest of all. Cell division and expansion in longitudinal, circumferential and radial planes (to the centre of the pod axis) account for variations in length, diameter and thickness of the pod wall respectively.
A comparative study of the pod wall anatomy in the presence and absence of seeds showed that cells in chlorenchyma and parenchyma divided more, and cells in most tissues expanded more in all planes in the areas with developing seeds than in regions devoid of seeds (Table 3). However, there were no differences in expansion of sclerenchyma circumferentially, and of endodermis circumferentially and longitudinally. The thickness and perimeter, and the size of vascular bundles in the adjacent carpel were higher in the presence of seeds than in the absence of seeds. When a seed was present only on one side of the replum, parenchyma and endodermis on the other side expanded more radially but other cells were of similar size to those in the region without seeds (data not shown).
The growth of the pod wall is influenced by both physical (cell number and size) and physiological factors (metabolism, storage and utilization of assimilates). A higher cell division and expansion in the presence of a seed suggests that seeds act as important determinants of the pod wall development. The developing seeds, especially at the liquid endosperm stage, are rich sources of PGS (Hedden & Hoad 1985). When a seed develops, it is likely that some of these PGS translocate and stimulate pod wall development. In contrast, when a seed aborts or does not develop, the production of PGS may cease in that seed and very small amounts are translocated, thereby resulting in cessation of the growth of the pod wall.
Since pod length was closely associated with the number of seeds from very early on, and a higher cell division in chlorenchyma and parenchyma of the pod wall was found in the presence of seeds, it is reasonable to assume that seeds initially produce cytokinin-like substances that induce cell division. Studies in winter rape showed that concentrations of cytokinins (zeatin and zeatin riboside) reached peak levels on the third day when pods were c. 10 mm in length (Bouille et al. 1989). A major peak in cytokinin concentration of lupin seeds was shown to coincide with the stage of cell division in the fruit wall (Readman 1983). Thus the existence of a strong correlation between the presence of seeds and the development of the pod wall in rape, and between PGS concentration in seeds and the growth of the fruit wall in other crops, prompted us to test whether exogenous hormones could partly or fully replace the role of seeds in promoting the growth and development of the pod wall.
Exogenous phytohormones and development of the pod wall (Expt 2)
Morphology:
When PGS were not applied to the emasculated flowers, growth of the pod wall was small. In contrast, all PGS treatments induced parthenocarpy but the nature and extent of response varied with the type and the concentration of the chemical(s) applied.
GA induced the formation of normal but small pods. NAA also promoted pod growth but it caused abnormalities such as contorted pod shape, callus development at the base, and sepal and petal retention. The pods were of similar length but were lighter than the GA-treated pods. BA at 1 g l-1 led to the development of pods of normal appearance but pod growth was inferior to that in the seeded pods. The promotive effect of BA was less at 10 g l-1 but the overall response was still higher than in the GA or NAA treatments (Table 4).
Of all mixtures, GA+BA led to the maximum growth of the pod wall, which was similar to that in the seeded pods. Application of NAA with either GA or BA reduced growth. When all three substances were applied as a mixture, the effects were similar to those in the BA+NAA treatment. The overall ranking of mixtures at 10 g l-1 was similar to that at 1 g l-1 but the degree of response was less, especially in the GA+BA and BA+NAA treatments (Table 4).
The results indicated that normal growth of the pod wall requires the presence of seeds but such a requirement can be partly or fully replaced by exogenous PGS application. The greater effectiveness of BA in inducing development of the pod wall may be related to an increased competitive ability of the treated pods to attract carbon assimilates (Keiller & Morgan 1988) and nutrients (Crosby et al. 1981). The addition of GA to BA increased the growth promoting effects of the latter. Similar beneficial effects of such a mixture were reported in relation to flowering and fruit set in horticultural crops (e.g. tomato, Satti & Oebker 1986).
Dry weight:
All PGS treated pods accumulated less dry matter in the pod wall than the corresponding untreated pods with seeds (Table 4). The reasons are not clear but several possibilities exist. It is likely that the duration of availability of endogenous PGS is longer than that achieved through application of PGS in the early stages. Seeds may produce complex mixtures of PGS which are probably more effective than the applied ones. Furthermore, abscisic acid (ABA) of seed origin is known to be involved in unloading of carbon assimilates through the pod wall (e.g. in peas, Readman 1983). It is likely that parthenocarpic pods contain less ABA than seeded pods, thereby leading to less accumulation of photosynthates.
Senescence:
The seedless controls and GA, NAA, GA+NAA treated pods senesced 4-6 days earlier than the untreated seeded pods. However, when BA was applied singly or in mixture with GA, senescence was delayed by as much as 30-34 days (Table 4). Kelly (1985) also reported a delay in senescence after dipping the seeded pods in BA or GA+BA solutions at 28-30 DAF. Cytokinins are known to retard senescence due to their role in chloroplast differentiation and synthesis (Stoddart & Thomas 1982), but it is interesting to note that the effects of BA applied very early on were evident for a long time. The delay in senescence was reduced when BA was applied along with an auxin, NAA. This is contrary to the findings of Picart & Morgan (1986), who observed a significant delay in senescence after treating 36-39 day old pods with another auxin, 4-chlorophenoxy acetic acid. Seeded pods senesced earlier than the BA or GA+BA treated pods. This is probably due to (a) a transient peak-release of seed-produced ethylene, which presumably increases cellulase activity and senescence in rape pods during maturation (Meakin & Roberts 1990), and/or (b) an increase in the endogenous ABA in seeds just before the embryo begins to desiccate, which induces senescence in the adjacent photosynthetic tissue (Finkelstein et al. 1985).
Anatomy:
Many histological changes occurred after PGS application. However, the effects on thickness and perimeter of the pod wall, number and size of cells, cellular organization in the dehiscence zone, and ovule growth were examined in detail (Table 5).
Thickness and perimeter of the pod wall, number and size of cells, and ovule growth were higher in all PGS treatments than in the seedless controls (Table 5). GA and NAA, singly or in mixture, caused broadly similar effects on histology of the pod wall. Thickness and perimeter of the pod wall, and number and size of cells were, however, much less than in the untreated seeded pods. The higher cell size, especially of endodermis, in the treated pods than in the untreated seedless and seeded pods may be related to an increased multiplication of membranes and endoplasmic reticulum following GA application (Srivastava et al. 1975), and cell wall loosening after NAA application (Jones 1985).
BA application considerably increased cell number in epidermis, chlorenchyma and parenchyma of the pod wall (Fig. 2), and in ovules. Cells in endodermis were larger than in the seeded pods but were smaller than in the GA treated pods (Table 5). Similar effects of BA on cell number and size were seen in cucumber by Takeno et al. (1992). The increased cell division and expansion may be due to an accelerated synthesis of nucleic acids and proteins (Jacqmard et al. 1994). The area of vascular bundles was higher and parenchyma in the dehiscence zone was more intact in the BA treated pods than in the GA or NAA treated pods.
A mixed application of GA and BA showed complementary effects. While GA increased cell size in parenchyma, sclerenchyma and endodermis (Fig. 2), BA seemed to reduce the negative effects of GA on parenchyma in the dehiscence zone (Table 5). The thickness and perimeter of the pod wall, and the number and size of vascular bundles were much higher than in the seeded pods. The growth of ovules was similar to that in the seeded pods. The addition of NAA reduced the promotive effects of BA in terms of cell number in all tissues of the pod wall. Similar adverse effects of BA+NAA were observed in Brassica juncea (Guo et al. 1994). When all three substances were applied as a mixture, the pod wall was still thicker than in the untreated seeded pods but parenchyma in the dehiscence zone showed symptoms of autolysis. Ovule growth was moderate and similar to that in the BA+NAA treated pods (Table 5).
Factors influencing the response to GA and BA (Expts 3 and 4)
The pod growth response in terms of length to BA and GA+BA started at 10, and to GA at 100 mg l-1 and was maximum at 500 mg l-1. Diameter, surface area and dry weight of the pod, however, increased up to 1 g l-1 (Table 6). The results suggest that smaller amounts of PGS were required to restore pod length than those needed for restoring other components.
The response to BA and GA+BA was higher when applied through solvent than through lanolin but the response to GA was unaffected (Table 7). Although lanolin is known to ensure a continuous supply of PGS, a reduction in response was seen here probably due to differences in diffusion, penetration and availability of BA to receptor sites in the pod.
All components of pod size except diameter were greater when BA or GA+BA were applied at 1-5 rather than at 11-15 DAF (Table 8). This could be due to several factors: differences in ability of pods to receive, metabolize, or retain applied BA, differences in ability of pods to synthesise cytokinins in situ, etc. Further, BA is known to stimulate cell division during early stages of development only (Kinet & Leonard 1983). The variations in the metabolic and physiological status of the pod wall tissues at the time of treatment and/or temporal changes in the levels of cytokinins within a pod (Lis & Antoszewski 1982) may be another cause. The delayed application may also have led to an unfavourable hormonal balance that was less conducive to the growth of the pod wall. In addition, the cuticle may be thicker at 11-15 than at 1-5 DAF and this may reduce the rate of translocation and the availability of BA in older pods.
The pod growth response to GA at maturity was more or less uniform despite the variations in the age of the floral organ at the time of treatment. In contrast, the response to BA and GA+BA in terms of length, surface area and dry weight of pod was higher in young (F-1 to F+2) than in old flowers (Fig. 3). The greater response to BA in the earlier than in the later stages suggests that BA is more effective in activating cell division in younger than in older pods and that cytokinins are probably important during the early stages of rape pod development. Kinet & Leonard (1983) showed that cytokinins were important in the early stages and gibberellins in the later stages of inflorescence development in tomato. Similarly, Bennici & Cionni (1979) reported that embryo development in Phaseolus coccineus was promoted by exogenous cytokinins during the early stages but not in the later stages. A thicker cuticle in the older pods might further restrict the availability of BA to the receptor sites. Thus the later formed pods, which were young at the time of treatment but which were positionally more disadvantaged with respect to assimilates from the leaves, grew bigger and accumulated more dry matter at maturity than the earlier formed pods which were better positioned with respect to leaf assimilates. Pod diameter at maturity was, however, unaffected by the variations in the age of flower. This may be related to differences in the period of determination of various components of pod size. While pod length, surface area and weight, are largely determined by cell division during early stages, pod diameter is determined over a long period. The relatively weak correlation between seed number and pod diameter, as observed earlier (Table 2), further supports this view.
The response of seeded pods to exogenous applications of GA, BA and their mixture were then investigated. All pod and seed characters except senescence were unaffected by PGS application in the basal region. In the apical pods however, pod length was higher in the BA and GA+BA treated pods while diameter and surface area were higher in all PGS-treated pods than in the controls. Number and weight of seeds were unaffected. Senescence was delayed in the BA and GA+BA treated pods both in the basal and apical regions (Table 9).
GA was less effective in enhancing the growth of the pod wall both in the basal and apical pods. It is likely that the developing seeds in them may be producing gibberellins in adequate or excess amounts than would normally be required for pod development and any exogenous supply may have a limited effect (Pharis & King 1985). Application of BA or GA+BA increased pod size in the apical but not in the basal pods. This is probably related to ontogenetic variations in the levels of endogenous cytokinins within pods of different positions in a raceme. The apical pods may contain less amounts of cytokinins than the basal pods probably due to (a) a reduced amount of seed-produced cytokinins because of a low seed number and (b) a decrease in the cytokinin supply from roots during late flowering (Carlson et al. 1987). The exogenous supply of BA may therefore enhance the development of the pod wall in the apical pods. In contrast, the basal pods, which normally have many seeds and are formed very early on, may contain cytokinins in adequate amounts that would ensure the potential development of the pod wall realised. It is also likely that the apical pods accumulate growth inhibitors either due to transport from other parts of the plant including the basal developing pods or due to production in situ by aborting ovules/seeds within them (Pechan & Morgan 1985). In such cases, BA application may counteract the effects of growth inhibitors to some extent and create a more favourable hormonal balance, which would lead to a greater development of the pod wall.
The delay in senescence due to BA or GA+BA was not accompanied by an increased seed weight. Dybing & Lay (1981) also reported that there was no increase in seed yield in soya bean despite a delay in leaf senescence after BA application. The results suggest that other factors may be important in remobilization of reserve carbohydrates from the pod wall. The extra carbon assimilates produced by pod wall photosynthesis in the later stages may have been utilized in respiration, leading to no increase in mobilizable reserves. The maturation of the pod wall and the seeds within were found to be independent of each other (data not shown). While the pod wall was green, the funiculus was almost dry and the seeds were fully ripe. At this stage, the seeds may not attract any carbon assimilates further.
Conclusion
The strong positive correlations between the number of seeds and size or weight of the pod wall provide circumstantial evidence that seeds play an important role in regulating the growth of the pod wall. Experiments with emasculated flowers showed that phytohormones of more than one group could regulate the growth. It is likely that in normal developing pods, seeds may be producing such substances. The lack of response to PGS in the basal pods, which contain many seeds, further supports this view. Of the three kinds of substances, cytokinins and gibberellins and their balance within a pod, seem to be major determinants of development. BA application showed far reaching effects on pod growth at different stages, primarily through effects on cell division early on and on pod senescence later on. On the other hand, GA application influenced pod growth primarily through promoting cell expansion. NAA promoted pod growth but caused abnormalities at the concentrations tested. Further experiments demonstrating that seed-derived growth substances are required for the growth of the pod wall are necessary before a definite conclusion can be drawn. However, the finding that applied PGS apparently substitute for an endogenous source of hormones in maintaining the growth of the pod wall does lend support to a causal connection.
We wish to thank R. Day, A. Hilton and S. Revell for their excellent technical help.
REFERENCES
BENNICI, A. & CIONNI, P.G. (1979). Cytokinins and in vitro development of Phaseolus coccineus embryos. Planta 147, 27-29.
BOUILLE, P. DE, SOTTA, B., MIGINIAC, E. & MERRIEN, A. (1989). Hormones and pod development in oilseed rape (Brassica napus). Plant Physiology 90, 876-880.
BRAR, G.S. & THIES, W. (1977). Contribution of leaves, stem, siliques and seeds to dry matter accumulation in ripening seeds of rapeseed, Brassica napus L. Zeitschrift fur Pflanzenphysiologie 82, 1-13.
CARLSON, D.R., DYER, D.J., COTTERMAN, C.D. & DURLEY, R.C. (1987). The physiological basis for cytokinin-induced increases in pod set in IX93-100 soybeans. Plant Physiology 84, 233 -239.
CROSBY, K.E., AUNG, L.H. & BUSS, G.R. (1981). Influence of 6-benzylaminopurine on fruit set and seed development in two soybean, Glycine max (L.) Merr. genotypes. Plant Physiology 68, 985-988.
DYBING, C.D. & LAY, C. (1981). Yield and yield components of flax, soybean, wheat, and oats treated with morphactins and other growth regulators for senescence delay. Crop Science 21, 904-908.
FINKELSTEIN, R.R., TENBARGE, K.M., SHUMWAY, J.E. & CROUCH, M.L. (1985). Role of ABA in maturation of rapeseed embryos. Plant Physiology 78, 630-636.
GUO, D.P., JIANG, Y.T., ZENG, G.W. & SHAW, G.A. (1994). Stem swelling of stem mustard as affected by temperature and growth regulators. Scientia Horticulturae 60, 153-160.
HEDDEN, P. & HOAD, G.V. (1985). Hormonal regulation of fruit growth and development. In Regulation of Sources and Sinks of Crop Plants (Eds B. Jeffcoat, A.F. Hawkins & A.D. Stead), pp. 211-224. Bristol, UK: British Plant Growth Regulator Group, Monograph No. 12.
JACQMARD, A., HOUSSA, C. & BERNIER, G. 1994. Regulation of the cell cycle by cytokinins. In Cytokinins: Chemistry, activity and function (Eds D.W.S. Mok, & M.C. Mok), pp. 197-215. Boca Raton, USA: CRC press.
JONES, R.L. (1985). The control of plant cell elongation by auxin and gibberellin. In Plant Growth Substances 1985 (Ed. M. Bopp), pp. 275-283. Heidelberg, Germany: Springer-Verlag.
KEILLER, D.R. & MORGAN, D.G. (1988). Effect of pod removal and plant growth regulators on the growth, development and carbon assimilate distribution in oilseed rape (Brassica napus L.). Journal of Agricultural Science, Cambridge 111, 357-362.
KELLY, C.C. (1985). Plant growth substances and the later stages of pod development in oilseed rape. MPhil thesis, University of Cambridge.
KINET, J.M. & LEONARD, M. (1983). The role of cytokinins and gibberellins in controlling inflorescence development in tomato. Acta Horticulturae 134, 117-124.
LI, C.F. & WANG, C.F. (1988). Effect of nitrogen nutrition on pod development in rape (Brassica napus). Acta Agronomica Sinica 14, 329-335.
LIS, E.K. & ANTOSZEWSKI, R. (1982). Do growth substances regulate the phloem as well as the xylem transport of nutrients to the strawberry receptacle? Planta 156, 492-495.
MEAKIN, P.J. & ROBERTS, J.A. (1990). Dehiscence of fruit in oilseed rape (Brassica napus L.). II. The role of cell wall degrading enzymes and ethylene. Journal of Experimental Botany 41, 1003-1011.
PECHAN, P.M. & MORGAN, D.G. (1985). Defoliation and its effects on pod and seed development in oilseed rape (Brassica napus L.). Journal of Experimental Botany 36, 458-468.
PHARIS, R.P. & KING, R.W. (1985). Gibberellins and reproductive development in seed plants. Annual Review of Plant Physiology 36, 517-568.
PICART, J.A. & MORGAN, D.G. (1986). Preliminary studies on the effects of 4-chlorophenoxy acetic acid on the senescence and dehiscence of pods in oilseed rape (Brassica napus L.). Plant Growth Regulation 4, 169-175.
READMAN, J.E. (1983). Plant hormones and fruit development in Lupinus albus L. PhD thesis, University of Bristol.
SATTI, S.M.E. & OEBKER, N.F. (1986). Effects of benzyladenine and gibberellin (GA4/7) on flowering and fruit set of tomato under high temperature. Acta Horticulturae 190, 347-354.
SRIVASTAVA, L.M., SAWHNEY, V.K. & TAYLOR, I.E.P. (1975). Gibberellin-induced cell elongation in lettuce hypocotyls. Proceedings of the National Academy of Sciences (USA) 72, 1107-1111.
STODDART, J.L. & THOMAS, H. (1982). Leaf senescence. In Encyclopaedia of Plant Physiology, New Series, Vol. 14A (Eds D. Boulter & B. Parthier), pp. 592-636. Berlin, Germany: Springer-Verlag.
TAKENO, K., ISE, H., MINOWA, H. & DOUNOWAKI, T. (1992). Fruit growth induced by benzyladenine in Cucumis sativus L.: Influence of benzyladenine on cell division, cell enlargement and indole-3-acetic acid content. Journal of the Japanese Society for Horticultural Science 60, 915-920.
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