Flash-induced redox changes of P700 and plastocyanin in chloroplasts suspended in fluid media at sub-zero temperatures

Abstract

Following a short flash of light, the reaction centre chlorophyll of photosystem I, WOO, becomes oxidised. Measurement of the dark reduction of this oxidised species generated by a single turnover give information about the mechanism of electron donation. Studies of this type have shown that the kinetics of P700’ reduction do not follow a single exponential [l-5]. As a result of this both plastocyanin and cytochrome fhave been implicated as direct donors to P700’ [ 1,2,5,6] although a claim has also been made for another more primary donor [4]. Part of the reason for the lack of clear understanding of electron donation on the oxidising side of photosystem 1 is due to the speed of the decay of the P700’ signal and complications associated with detecting it spectrophotometrically. One possible way to simplify the analysis of the signal is to measure its kinetics at low temperatures where the electron-transfer rates are likely to be slowed down. It has convincingly been shown that all properties of photosynthetic electron transport and associated proton translocation are preserved when chloroplasts are suspended in suitable aqueous/organic media at subzero temperatures [7-121. Here, we have made use of this low temperature approach to investigate the flash-induced redox changes of P700 and plastocyanin. It has been found that the electron-transfer reaction involving these components are qualitatively similar to those observed