Inhibition of photosynthesis in Phaseolus vulgaris by treatment with toxic concentrations of zinc: effects on electron transport and photophosphorylation

Dwarf beans ( Phaseolus vulgaris L. cv. Limburgse Vroege) were grown on a nutrient medium containing a toxic non-lethal ZnSO₄ concentration. The electron transport and photophosphorylation activities of chloroplasts, isolated from these beans, and from control plants, grown under standard nutrient conditions, were compared. Electron transport was significantly inhibited by Zn²⁺ treatment. Photosystem 2 activity proved to be more sensitive than photosystem 1 activity.
Inhibition was dependent on electron flow rate. Activity was fully restored with semicarbazide. EDTA-washed thylakoid membranes were strongly manganese-deficient. The results suggest that photolysis of water was primarily inhibited, due to a zinc-induced deficiency in loosely bound manganese at the water-splitting site. Manganese is probably substituted by zinc, since the zinc content of thylakoids increased five-fold. Non-cyclic photophosphorylation capacity was also limited as a result of inhibition of electron transport. Phosphorylation efficiency (ATP/2e ratio) involving both energy conserving sites was hardly affected.