Mechanism of Photosynthate Efflux fromVicia fabaL. Seed Coats

In order to develop a tentative model of the mechanism of photosynthate efflux from the vascular region of Vicia faba L. seed coats, wash-out experiments were performed after removal of the embryo. The sulphydryl group modifiers, pCMBS and NEM, reduced 14C-photosynthate efflux by 40% and 50%, respectively. Their inhibitory effect could be prevented or reduced (in the latter case) by including DTT in the bathing solution. Maltose competed with sucrose for efflux; a concentration of 300 mol m−3 inhibited 14C-photosynthate release by 35%. The cations K+ , Na+ Mg2+ and TPP+ enhanced efflux significantly, whereas the countenon Cl− had no effect. The presence of the protonophore CCCP (0·1 mol m−3) led to a reduction of efflux by 50% net proton extrusion dropped by 34%. To a lesser extent, an efflux inhibition was also achieved by decreasing the cytoplasmic pH with the weak acid DM0. In contrast, alterations in the external pH caused only a feeble response. The ATPase inhibitor, EB, decreased photosynthate efflux and H+ extrusion. DES reduced efflux slightly, presumably by affecting ATPase activity as well as energy metabolism. Based on these findings, it is proposed that a sucrose/proton antiport mechanism could be responsible for photosynthate efflux from Vicia faba seed coats.