Heat sensitivity of chloroplasts and leaves: Leakage of protons from thylakoids and reversible activation of cyclic electron transport

In illuminated intact spinach chloroplasts, warming to and beyond 40 degree s C increased the proton permeability of thylakoids before linear electron transport through Photosystem II was inhibited. Simultaneously, antimycin A -sensitive cyclic electron transport around Photosystem II was activated wi th oxygen or CO2, but not with nitrite as electron accepters. Between 40 to 42 degrees C, activation of cyclic electron transport balanced the loss of protons so that a sizeable transthylakoid proton gradient was maintained. When the temperature of darkened spinach leaves was slowly increased to 40 degrees C, reduction of the quinone acceptor of Photosystem II, Q(A), incre ased particularly when respiratory CO2 production and autoxidation of plast oquinones was inhibited by decreasing the oxygen content of the atmosphere from 21 to 1%. Simultaneously, Photosystem II activity was partially lost. The enhanced dark Q(A) reduction disappeared after the leaf temperature was decreased to 20 degrees C. No membrane energization was detected by light- scattering measurements during heating the leaf in the dark. In illuminated spinach leaves, light scattering and nonphotochemical quenching of chlorop hyll fluorescence increased during warming to about 40 degrees C while Phot osystem II activity was lost, suggesting extra energization of thylakoid me mbranes that is unrelated to Photosystem II functioning. After P700 was oxi dized by far-red light, its reduction in the dark was biphasic. It was acce lerated by factors of up to 10 (fast component) or even 25 (slow component) after short heat exposure of the leaves. Similar acceleration was observed at 20 degrees C when anaerobiosis or KCN were used to inhibit respiratory oxidation of reductants. Methyl viologen, which accepts electrons from redu cing side of Photosystem II, completely abolished heat-induced acceleration of P700(+) reduction after far-red light. The data show that increasing th e temperature of isolated chloroplasts or intact spinach leaves to about 40 degrees C not only inhibits Linear electron flow through Photosystem II bu t also activates Photosystem I-driven cyclic electron transport pathways ca pable of contributing to the transthylakoid proton gradient. Heterogeneity of the kinetics of P700(+) reduction after far-red oxidation is discussed i n terms of Photosystem I-dependent cyclic electron transport in stroma lame llae and grana margins.