Are Calcium Ions and Calcium Channels Involved in the Mechanisms of Cu<sup>2+</sup> Toxicity in Bean Plants? The Influence of Leaf Age

Abstract

The influence of calcium channel blockers and ionophore on Cu2+-induced changes of the photosynthetic activity of runner bean plants (Phaseolus coccineus L.) was investigated. Excess Cu2+ was applied to leaves by injection or via the roots to examine a short/local or a long time/systemic effect of this metal, respectively. The changes in fluorescence parameters indicated that the mechanism of toxic action of Cu2+ ions on the photosynthetic apparatus was only partially connected with Ca2+ or Ca2+ channels. In young plants Ca2+ diminished especially photochemical and nonphotochemical dissipative processes induced by short- and long-term influence of excess Cu2+. Blocking of Ca2+ channels did not change direct Cu2+ action on the photosynthetic activity, however, their opening distinctly intensified the inhibitory effect of the metal. After a longer accumulation peri od the effect of Cu2+ ions did not change significantly due to modified Ca2+ penetration through membranes (except that caused by La3+). Copper directly introduced into older leaves diminished only at its highest concentration the activity both of the donor and acceptor sides of photosystem 2 (PS2) connected with Rfd decrease and increase of LNU. A similar effect was observed also after a long-term Cu2+ action, but disturbances on the acceptor side of PS2 were observed only at a higher Ca2+ content in the nutrient solution. Ca2+ ions, particularly after openning of channels, intensified direct inhibitory Cu2+ action on the photosynthetic activity expressed by decreased values of Fv/F0 and Rfd. Lanthanum and verapamil, at a lower Ca2+ content in the medium, decreased the photosynthetic activity of Cu2+-treated plants. This effect was also seen after additional Ca2+ supply to the leaves.