Genotypic differences in seed growth rates of Phaseolus vulgaris L. I. General characteristics, seed coat factors and comparative roles of seed coats and cotyledons

Abstract

Developing seeds of four cultivars of Phaseolus vulgaris L., raised under glasshouse conditions, exhib-ited a 4-fold range in rates of storage product accumulation by their cotyledons. These growth rate differences were established during seed expansion. Patterns of dry matter distribution were consistent with absolute growth rates of cotyledons being an inherent property of developing seeds and not limited by photoassimilate supply. Seed surface areas and cotyledon volumes exhibited a 3.2-fold cultivar difference and were the principal components contribut-ing to cultivar variation in cotyledon absolute growth rates. The remaining cultivar variation was attributable to a 1.3- fold difference in dry matter fluxes, expressed on a seed surface area basis. Seed coats reflected these properties in terms of fluxes of dry matter released for cotyledon storage and surface areas supporting these fluxes. Seed coat surface areas correlated with estimates of total plasma membrane areas of ground parenchyma cells that are respon-sible for photoassimilate release. Cultivar differences in these membrane areas largely arose from variation in cell size. Coat turgor pressures correlated positively with dry matter fluxes imported into cotyledons. In contrast, sucrose concentration in bulk saps extracted from seed coats was identical across three cultivars but was positively related to growth rate in the remaining cultivar. Overall, these data suggested that cultivar dry matter fluxes were determined by variation in transport conductances for symplasmic movement through the post-sieve element pathway and for release across the plasma membranes of ground parenchyma cells. Comparable sucrose concentrations were found in seed apoplasmic saps across cultivars, and cultivar differences in absolute growth rates of in vitro cultured coty-ledons were retained. Together, these observations support the conclusion that cotyledons intrinsically express geno-typic variation in rates of dry matter transport comparable to those set independently by seed coats.