Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L. Control by Tissue Water Relations

Unloading of (14)C-photosynthates, potassium and amino acids from excised seed-coat halves of Phaseolus vulgaris L. was slowed when the seed coats were exposed to bathing media containing 100 mM sucrose. Several lines of evidence indicated that the plasma membranes of the seed-coat cells retained their functional properties to regulate solute exchange to the apoplast at all external sucrose concentrations. Substituting sucrose with other slowly permeating osmotica established that the sucrose inhibition of unloading was not the result of transinhibition but rather a response to solution osmolality. The use of rapidly and slowly permeating osmotica to manipulate water, osmotic and turgor potentials of the seed-coat tissues demonstrated that the rate of unloading depended on cell turgor potential and was stimulated when this exceeded ca. 5 × 10(5) Pa. Since the major assimilates for cotyledonary growth (sucrose and potassium) were found to constitute the principle osmotica in both the apoplast and the protoplasts of the seed coats, it was tentatively proposed that the turgor-sensitive unloading process provided the appropriate mechanism for sink control of assimilate unloading in the developing bean ovules.