Abstract. Uptake of phytotoxic amounts of metal by higher plants or algae can result in inhibition of several enzymes, and in increase in activity (= induction) of others. Two mechanisms of enzyme inhibition predominate: (1) binding of the metal to sulphydryl groups, involved in the catalytic actionor structural integrity of enzymes, and (2) deficiency of an essential metal in metalloproteins or metal‐protein complexes, eventually combined with substitution of the toxic metal for the deficient element. Metal accumulation in the cellular compartment of the enzyme is a prerequisite for enzyme inhibition in vivo. The induction of some enzymes is considered to play a significant role in the stress metabolism, induced by metal phytotoxicity. Peroxidase induction is likely to be related to oxidative reactions at the biomembrane; several enzymes of the intermediary metabolism might be stimulated to compensate for metal‐sensitive photosynthetic reactions. The induction of enzymes and metal‐specific changes in isoperoxidase pattern can be used as diagnostic criteria to evaluate the phytotoxicity of soils, contaminated by several metals. Lines for future research on metal phytotoxicity are proposed, involving the study of inhibition and induction of enzymes at the different cell membranes (especially the plasmamembrane) in vivo.

Inhibition of photosynthesis by heavy metals is well documented. In this review the results are compared between in vitro experiments on isolated systems (chloroplasts, enzymes -.), experiments on excised leaves and intact plants and algae in vivo. In vitro experiments suggest potential sites of heavy metal interaction with photosynthesis at several levels of organisation, which are not necessarily confirmed in vivo. Analytical data on subcellular heavy metal level are generally missing to discuss their mechanism of action in the intact organism. In the field factors such as soil characteristics and air pollution have to be taken into account for assessing the mechanism of action of heavy metals on photosynthesis in plants, growing in a polluted erea.

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Van Assche, F. and Clijsters, H. 1986. Inhibition of photosynthesis in Phaseolus vulgaris by treatment with toxic concentrations of zinc: effects on electron transport and photophosphorylation. [717][718][719][720][721] Dwarf beans {Phaseolus vulgaris L. cv. Limburgse Vroege) were grown on a nutrient medium containing a toxic non-lethal ZaSO, 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 Zn^* 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-washcd 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.Additional key words -Chloroplast, heavy metal.

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