Temperature dependencies of the rate of electron flow and of the formation of the high energy state in spinach chloroplasts and leaves

Abstract

The relationship between the rate of electron transport and the rate of formation of the high energy state during illumination was studied by measuring temperature dependencies of oxygen uptake and of the induction of millisecond delayed fluorescence of chlorophyll in isolated spinach chloroplasts and in spinach leaves. Measurements in chloroplasts with and widiout added electron acceptors showed that the decrease in the rate of formation of the high energy state caused by cooling was due to the decrease in the rate of electron transport linked to the Mehler reaction. Measurement of the induction of delayed fluorescence in leaves suggested the existence of a similar temperature-sensitive step in the electron transport system. Delayed fluorescence of chlorophyll in green plants is known to be generated by reversal of the photosynthetic reactions (6~, 11). Recent studies (4, 13) on the phenomenon showed that the intensity of delayed fluorescence emitted within a few milliseconds after illumination of chloroplasts sensitively reflects formation of the high energy state for photophosphorylation. In the previous study (5) it was found that the induction in the intensity of delayed fluorescence during illumination (a transient time course of the intensity of delayed fluorescence at the onset of illumination) is highly sensitive to temperature. By decreasing temperature the increase in the intensity during illumination, which reflects formation of the high energy state, became quite slow in the absence of added electron acceptors (5). In isolated chloroplasts molecular oxygen is known to serve as an electron acceptor at room temperature (2, 7, 8). It seefns likely that the rate of electron transport supported by the reduction of oxygen molecules, the Mehler reaction (7), is temperature dependent when no external electron acceptor is added. At lower temperatures, the lower rate of electron transport may result in slow formation of the high energy state. In-this study temperature dependencies of the rates of formation of the high energy state and of the Mehler reaction were studied by measuring the induction of delayed fluorescence and the oxygen uptake in isolated chloroplasts. The temperature dependencies were analyzed in order to examine the above assumption. Temperature dependency of the induction of delayed fluorescence was also studied in intact leaves.