Photosynthetic Electron Transport During Senescence of the Primary Leaves ofPhaseolus vulgarisL.: I. NON-CYCLIC ELECTRON TRANSPORT

Abstract

Coupled, non-cyclic electron transport was measured for chloroplasts isolated from the primary leaves of Phaseolus vulgaris. Preparations from young, fully expanded leaves gave good rates of electron transport, but the rates obtained decreased by approximately 80% during leaf senescence. Higher rates of electron transport were recorded for chloroplasts isolated from primary leaves which had regreened following removal of the remainder of the shoot. With preparations from leaves of all ages, photophosphorylation was coupled to electron transport with a mean P/2e ratio of approximately 1.3. No evidence was obtained for inactivation of chloroplasts from older leaves during isolation or assay, and it is suggested that the decrease in rate of electron transport over the period of senescence, and its increase during regreening, were consequences of changes in the composition and physical properties of the thylakoid membrane which occur in vivo. The decrease in rate of non-cyclic electron transport may be important in limiting the rate of photosynthesis in the senescing leaves.