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Articles

The Cupric Ion as an Inhibitor of Photosynthetic Electron Transport in Isolated Chloroplasts ¹

Robert L. Heath^b

^a Department of Plant Sciences, University of California, Riverside, California 92502, Department of Biology, University of California, Riverside, California 92502

Strong inhibition of uncoupled photosynthetic electron transport by Cu²⁺ in isolated spinach chloroplasts was observed by measuring changes in O₂ concentration in the reaction medium. Inhibition was dependent not only on the concentration of the inhibitor, but also on the ratio of chlorophyll to inhibitor. Binding of Cu²⁺ to the chloroplast membranes resulted in removal of Cu²⁺ from solution. When chloroplasts were exposed to preincubation in light, there was increased inhibition as a result of Cu²⁺ binding to inhibitory sites. Preincubation in the dark resulted in Cu²⁺ binding to noninhibitory sites and decreased inhibition. The degree of inhibition was lower at low light intensities than at high light intensities.

When the photosystems were assayed separately, photosystem I was more resistant to inhibition than photosystem II. The most sensitive site to the inhibitor was the oxidizing side of photosystem II.

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