MECHANISM OF PHOTOSYNTHATE EFFLUX FROM VICIA-FABA L 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 C-14-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 C-14-photosynthate release by 35%. The cations K+, Na+, Mg 2+, and TPP + enhanced efflux significantly, whereas the counterion 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 DMO. 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.