LACK OF CORRELATION BETWEEN TRANSPLASMALEMMA ELECTRON-TRANSPORT RATE AND DEPOLARIZATION IN EGERIA-DENSA LEAF-CELLS

Transplasmalemma electron transport induced by the impermeable electro n accepters, Na-3[Fe(CN)(6)], K-3[Fe(CN)(6)], K-2[IrCl6] and K-2[IrBr6 ], was investigated by means of microelectrode technique, pH-stat and colorimetric experiments using the waterweed, Egeria densa. No fix sto ichiometry between protons and electrons exported was observed during acceptor reduction. The H+/e(-) ratio changed during the experiments w ith time and depended on a number of factors such as light conditions, preincubation and incubation medium. Application of ion channel block ers (BaCl2, tetraethylammonium, charybdotoxin), inhibitors (erythrosin B, NaCN+salicylhydroxamic acid) and stimulators of proton transport ( ethanol, fusicoccin) showed that ions including protons are transporte d in order to compensate for the electron acceptor-induced charge imba lance. Electrically, acceptor reduction was accompanied by depolarisat ion of the plasmalemma at acid and neutral pH. Both, reduction and dep olarisation displayed saturation kinetics under these conditions. Surp risingly, the depolarisation disappeared at alkaline pH, whereas the a cceptor reduction continued. A different mechanism of charge compensat ion is proposed. The natural electron acceptor, nitrate, did not compe titively inhibit ferricyanide reduction, by contrast, it enhanced the reduction rate. Furthermore, in the presence of nitrate no nitrite was produced in the medium. Nitrate did not depolarise the plasmalemma no twithstanding the required charge compensation. Nitrate uptake is acco mpanied by alkalinization of the medium and acidification of the cytos ol. Hence, nitrate is not a substrate of the plasmalemma redox system and it is taken up by an apparent 1:1 H+/NO3- cotransport. The results demonstrate that the plasmalemma redox system of Egeria leaf cells is neither a proton pump nor a nitrate carrier or reductase.