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Effects of Temperature on the Hill Reaction and Photophosphorylation in Isolated Cactus Chloroplasts ¹

^a Department of Botany, Washington State University, Pullman, Washington 99164

Chloroplasts isolated from Opuntia polyacantha Haw. (Cactaceae) are capable of noncyclic electron transport and ATP synthesis. Hill reaction rates, measured by O₂ evolution or by ferricyanide reduction, increase with increasing temperature to approximately 40 C. The temperature optimum of NADP reduction is 42 C while the optimum for noncyclic photophosphorylation is 35 C. NADP-linked phosphorylation exhibits a higher coupling ratio (P/e₂) than ferricyanide-linked photophosphorylation. The temperature optima for photochemical energy production correlate with photosynthetic properties of Crassulacean acid metabolism (CAM) plants and are discussed in relation to the operation of CAM at high tissue temperature.