Inhibition of light-induced stomatal opening and of guard-cell-protoplast swelling in Vicia faba L. by tentoxin, an inhibitor of photophosphorylation

Abstract

The fungal phytotoxin tentoxin and its natural derivative dihydrotentoxin impair light-induced stomatal opening in epidermal strips of broad bean (Vicia faba L.) incubated in a potassium-rich medium. Swelling of guard-cell protoplasts (GCPs) of the same species is inhibited in the presence of both substances. Swollen GCPs shrink after tentoxin or dihydrotentoxin treatment and these effects cannot be fully compensated by the phytoeffector fusicoccin. A comparison with the potassium carrier valinomycin shows that tentoxin acts in a different manner, because it is effective in the light only, whereas valinomycin causes shrinkage of GCPs also in the dark. Determination of adenine nucleotides in GCPs indicates a reduced ATP content and an enhanced ADP level after addition of tentoxin. At the same time, tentoxintreated GCPs contain more NADPH and less NAD(+) than the control (NADP(+) and NADH content does not differ). The results presented are consistent with the hypothesis that tentoxin closes stomata as a consequence of its inhibitory action on photophosphorylation.