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

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Abstract
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.