INCREASE IN THE QUANTUM YIELD OF PHOTOINHIBITION CONTRIBUTES TO COPPER TOXICITY IN-VIVO

The effect of copper on photoinhibition of photosystem II in vivo was studied in bean (Phaseolus vulgaris L. cv Dufrix). The plants were gro wn hydroponically in the presence of various concentrations of Cu2+ ra nging from the optimum 0.3 mu M (control) to 15 CCM. The copper concen tration of leaves varied according to the nutrient medium from a contr ol value of 13 mg kg(-1) dry weight to 76 mg kg(-1) dry weight. Leaf s amples were illuminated in the presence and absence of lincomycin at d ifferent light intensities (500-1500 mu mol photons m(-2) s(-1)). Linc omycin prevents the concurrent repair of photoinhibitory damage by blo cking chloroplast protein synthesis. The photoinhibitory decrease in t he light-saturated rate of O-2 evolution measured from thylakoids isol ated from treated leaves correlated well with the decrease in the rati o of variable to maximum fluorescence measured from the leaf discs; th erefore, the fluorescence ratio was used as a routine measurement of p hotoinhibition in vivo. Excess copper was found to affect the equilibr ium between photoinhibition and repair, resulting in a decrease in the steady-state concentration of active photosystem II centers of illumi nated leaves. This shift in equilibrium apparently resulted from an in crease in the quantum yield of photoinhibition (Phi(PI)) induced by ex cess copper. The kinetic pattern of photoinhibition and the independen ce of Phi(PI) on photon flux density were not affected by excess coppe r. An increase in Phi(PI) may contribute substantially to Cu2+ toxicit y in certain plant species.