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# Effects of light on endogenous seed abscisic acid levels and seed growth characteristics in soybean

Abstract (Summary)
The present study investigated the effects of light variation on the endogenous ABA levels in seed tissues, cotyledon cell characteristics and seed growth rate in soybean (Glycine max (L.) Merrill). Yield response of short season soybean Evans of maturity group 0 was studied under light enrichment and reduction treatments. Endogenous ABA levels were determined in seed components and correlated with cotyledon cell characteristics, seed growth rates, seed filling duration and seed size in experiments conducted in 1994-1995. Optimizing light condition during the early flowering stage (beginning flowering, $\rm LE\sb1)$ was more efficacious in determining seed yield than at the later growth stage (beginning pod formation, $\rm LE\sb2).$ The increase in seed yield was largely due to an increase in pod number per plant and per node, although an increase in seed size also partially contributed to increased seed yield in $\rm LE\sb2.$ Endogenous abscisic acid levels of seed components was high during the initial stages of seed growth and development and were significantly correlated with the number of cells in the cotyledons and seed growth rates under the light treatments. Plants subjected to shade early in seed development showed lower seed growth rates and a reduction in both final seed weight and cotyledon cell number along with a significant lowering of endogenous ABA levels in the seed components. This trend in seed size variation was consistent across all node positions on the main axis regardless of the light treatments. Position of a seed in a pod influenced its growth characteristics. Seed growth rate, seed size and cotyledon cell number of the middle and terminal seeds were significantly higher than that of the basal seed. This difference in seed size was consistent across node position, genotypes and irrespective of the number of seeds in pods, thus suggesting that the seeds growth rate is at least partially determined by the genetic constitution of the seed. Thus, within a single genotype, seed size variation within a pod may be influenced more by the rate of seed growth and cotyledon cell number than the filling duration and cell size. Results indicate that seed growth rate and cotyledon cell number can be influenced by light availability to individual plants, this helps explain variations in seed size within a single genotype that have been observed between years and locations. Reduction in light availability along with lowering of ABA levels during critical cell division period may decrease assimilate availability to the developing seed, and thus may regulate cotyledon ceil number and seed growth rates during the remainder of seed filling period.
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