CELLULAR PATHWAY OF PHOTOSYNTHATE TRANSPORT IN COATS OF DEVELOPING SEED OF VICIA-FABA L AND PHASEOLUS-VULGARIS L .2. PRINCIPAL CELLULAR SITE(S) OF EFFLUX

The cells responsible for the photosynthate efflux from coats of devel oping seed of Vicia faba L. and Phaseolus vulgaris L. were elucidated using known properties of the efflux mechanism. Sensitivity of sucrose efflux to NEM and high potassium concentrations was retained by seed- coat halves of Phaseolus following pectinase removal of the branch par enchyma cell layer. In contrast, removal of the thin-walled parenchyma transfer cell layer from Vicia seed-coat halves abolished this sensit ivity. The membrane-impermeant thiol-binding fluorochrome, qBBr, selec tively stained the surface of the thin-walled parenchyma transfer cell s. This phenomenon was inhibited by the slowly permeable sulphydryl ag ent, PCMBS, indicating that the plasma membranes of these cells are en riched in sulphydryl groups characteristic of membrane porter proteins . On the basis that carrier-mediated sucrose efflux from seed coats ap pears to be proton coupled, the putative plasma membrane H+-ATPase was used as a marker for the cells responsible for carrier-mediated photo synthate efflux, When seed-coat halves were exposed briefly at pH 8.5 to the weak acid fluorochrome, SRG, the ground parenchyma and thin-wal led parenchyma transfer cell layers selectively accumulated the dye, T he apparent low pH environment in the walls of these cells that render s SRG membrane permeant appeared to be maintained by a VAM-sensitive p roton pump. The findings with SRG were corroborated by the cytochemica l localization of plasma membrane ATPase activity to the ground parenc hyma and thin-walled parenchyma transfer cells using precipitation of cerium phosphate. Together, our observations provide qualified support for the conclusion that carrier-mediated photosynthate efflux from co ats of Phaseolus and Vicia seed is primarily restricted to the ground parenchyma and thin-walled parenchyma transfer cell layers, respective ly,