Temperature Dependence of Photosynthesis in Agropyron smithii Rydb. : II. CONTRIBUTION FROM ELECTRON TRANSPORT AND PHOTOPHOSPHORYLATION.

As part of an analysis of the factors regulating photosynthesis in Agropyron smithii Rydb., a C(3) grass, the response of electron transport and photophosphorylation to temperature in isolated chloroplast thylakoids has been examined. The response of the light reactions to temperature was found to depend strongly on the preincubation time especially at temperatures above 35 degrees C. Using methyl viologen as a noncyclic electron acceptor, coupled electron transport was found to be stable to 38 degrees C; however, uncoupled electron transport was inhibited above 38 degrees C. Photophosphorylation became unstable at lower temperatures, becoming progressively inhibited from 35 to 42 degrees C. The coupling ratio, ATP/2e(-), decreased continuously with temperature above 35 degrees C. Likewise, photosystem I electron transport was stable up to 48 degrees C, while cyclic photophosphorylation became inhibited above 35 degrees C. Net proton uptake was found to decrease with temperatures above 35 degrees C supporting the hypothesis that high temperature produces thermal uncoupling in these chloroplast thylakoids. Previously determined limitations of net photosynthesis in whole leaves in the temperature region from 35 to 40 degrees C may be due to thermal uncoupling that limits ATP and/or changes the stromal environment required for photosynthetic carbon reduction. Previously determined limitations to photosynthesis in whole leaves above 40 degrees C correlate with inhibition of photosynthetic electron transport at photosystem II along with the cessation of photophosphorylation.