Genotypic differences in seed growth rates of Phaseolus vulgaris L. - II. Factors contributing to cotyledon sink activity and sink size

A previous study [Thomas et al. (2000) Aust. J. Plant Physiol. 27, 109-118] showed that genotypic differences in seed growth rates of Phaseolus vulgar is L. cultivars was accounted for by variation in dry matter flux and seed size. Bulk cotyledon saps contained identical concentrations of sucrose acr oss cultivars suggesting that genotypic variation in capacities for sucrose transport and metabolism are equally matched. Cotyledon sucrose transport, monitored as in vitro uptake of [C-14] sucrose, exhibited genotypic variat ion and this was abolished by parachloromercuribenzene-sulfonate. Eadie-Hof stee transformations of concentration-dependent [C-14] sucrose uptake showe d that genotypic variation in sucrose flux resulted from differences in max imal transporter activity. Maximal sucrose fluxes and levels of transcript and microsomal protein for the sucrose/H+ symporter and H+-ATPase were posi tively correlated. In contrast, sucrose binding protein transcript and micr osomal protein levels correlated negatively with sucrose fluxes. In all cul tivars, a sucrose/H+ symporter and H+-ATPase were co-localised to plasma me mbranes of the dermal cell complexes. Total plasma membrane surface areas o f the dermal cell complexes and total volume of storage parenchyma cells co rrelated with cultivar variation in seed growth rates. Differences in cell number and size accounted for cultivar variation in total plasma membrane s urface area of the dermal cell complexes and total storage parenchyma cell volume.