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Increase in the Quantum Yield of Photoinhibition Contributes to Copper Toxicity in Vivo1

Eija Pätsikkä, Eva-Mari Aro, and Esa Tyystjärvi*

Department of Biology, Plant Physiology, and Molecular Biology, University of Turku, FIN-20014 Turku, Finland

The effect of copper on photoinhibition of photosystem II in vivo was studied in bean (Phaseolus vulgaris L. cv Dufrix). The plants were grown hydroponically in the presence of various concentrations of Cu2+ ranging from the optimum 0.3 µM (control) to 15 µM. The copper concentration of leaves varied according to the nutrient medium from a control value of 13 mg kg-1 dry weight to 76 mg kg-1 dry weight. Leaf samples were illuminated in the presence and absence of lincomycin at different light intensities (500-1500 µmol photons m-2 s-1). Lincomycin prevents the concurrent repair of photoinhibitory damage by blocking chloroplast protein synthesis. The photoinhibitory decrease in the light-saturated rate of O2 evolution measured from thylakoids isolated from treated leaves correlated well with the decrease in the ratio of variable to maximum fluorescence measured from the leaf discs; therefore, the fluorescence ratio was used as a routine measurement of photoinhibition in vivo. Excess copper was found to affect the equilibrium between photoinhibition and repair, resulting in a decrease in the steady-state concentration of active photosystem II centers of illuminated leaves. This shift in equilibrium apparently resulted from an increase in the quantum yield of photoinhibition (Phi PI) induced by excess copper. The kinetic pattern of photoinhibition and the independence of Phi PI on photon flux density were not affected by excess copper. An increase in Phi PI may contribute substantially to Cu2+ toxicity in certain plant species.

1   This study was supported by the Academy of Finland.
*   Corresponding author; e-mail esatyy{at}utu.fi; fax 358-2- 333-8075.

Plant Physiol. (1998) 117: 619-627
Copyright Clearance Center:   0032-0889/98/117/0619/09
© 1998 American Society of Plant Physiologists




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E. Patsikka, M. Kairavuo, F. Sersen, E.-M. Aro, and E. Tyystjarvi
Excess Copper Predisposes Photosystem II to Photoinhibition in Vivo by Outcompeting Iron and Causing Decrease in Leaf Chlorophyll
Plant Physiology, July 1, 2002; 129(3): 1359 - 1367.
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