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 dif-ferences in seed growth rates of Phaseolus vulgaris L. cultivars was accounted for by variation in dry matter flux and seed size. Bulk cotyledon saps contained identical concentrations of sucrose across cultivars suggesting that geno-typic variation in capacities for sucrose transport and metabolism are equally matched. Cotyledon sucrose transport, monitored as in vitro uptake of [14C]sucrose, exhibited genotypic variation and this was abolished by para-chloromercuribenzene- sulfonate. Eadie–Hofstee transformations of concentration-dependent [14C]sucrose uptake showed that genotypic variation in sucrose flux resulted from differences in maximal transporter activity. Maximal sucrose fluxes and levels of transcript and microsomal protein for the sucrose/H+ symporter and H+-ATPase were positively correlated. In contrast, sucrose binding protein transcript and microsomal protein levels correlated negatively with sucrose fluxes. In all cultivars, a sucrose/H+ symporter and H+-ATPase were co-localised to plasma membranes of the dermal cell complexes. Total plasma membrane surface areas of the dermal cell complexes and total volume of storage parenchyma cells correlated with cultivar variation in seed growth rates. Differences in cell number and size accounted for cultivar variation in total plasma membrane surface area of the dermal cell complexes and total storage parenchyma cell volume.