The role of Ca2+ ions in modulating changes induced in bean plants by an excess of Cu2+ ions. Chlorophyll fluorescence measurements

Ca2+-dependent influence of excess Cu2+ on the photosynthetic apparatus mon itored through chlorophyll fluorescence measurements was investigated in ru nner bean plants (Phaseolus coccineus L. cv, Piekny Jas) at three different growth stages, It was observed that the toxic effect of excess Cu2+ on pla nts depends both on their growth stages and the Ca2+ content in the medium. increased Ca2+ content limits Cu2+ action on plants at their initial growt h stage (I) through: stabilization of the PSII complex (increase of the rat io of variable to minimal fluorescence [F-v/F-0]), improved electron flaw a nd reoxidative processes of the quinone primary electron acceptor of PSII ( Q(A)) (increase of quantum yield of PSTI electron transport [phi(c)] and ph otochemical quenching of fluorescence [q(p)] values) and elimination of non photochemical energy dissipation (decrease of nonphotochemical fluorescence quenching from the Stern-Volmer equation [NPQ] and fraction of the absorbe d light energy not used for photochemistry [LNU] values). At this growth st age excess Cu2+ decreases the rates of Q(A) reduction as a result of decrea sed PSII activity at its donor side only at lower Ca2+ level. At the interm ediate growth stage (II) the plants were less sensitive to Cu2+ treatment a nd also to changed Ca2+ content. A weakening of some photochemical processe s by excess Cu2+ could be observed only at a higher Ca2+ dose. Ar the final growth stage of plants (III) Ca2+ ions exerted a decisively different effe ct on the mechanism of excess Cu2+ action on bean plants, visualized by dec reased PSII stabilization and utilization of absorbed light energy at incre ased Ca2+ content in the medium.