Further Studies on Quantifying Photosystem II in vivo by Flash-Induced Oxygen Yield From Leaf Discs

Abstract

It was shown briefly [W. S. Chow, A. B. Hope and J. M. Anderson (1989), Biochirnica et Biophysics Acta, 973, 105-8] that the oxygen evolved per flash from leaf discs, under steady-state flashing conditions and in the presence of background far-red light, gave a valid measure of the number of functional photosystem II (PS II) reaction centres. Further work on this direct and convenient method has been done to optimise conditions for making reliable measurements. It is found that, to obtain the higher estimates of [PS II], corresponding to functionality of practically all PS II reaction centres that bind herbicides, a form of 'light activation' is necessary after a prolonged dark pre-incubation. Without a sufficient number of flashes being given following a long dark incubation, the number of functional PS II reaction centres was underestimated. Provided light activation had occurred, the measured number of functional PS II reaction centres was independent of flash frequencies up to at least 40 Hz. The results strongly suggest that, in steady-state, light-limited photosynthesis, there does not exist any sub- stantial fraction of non-functional or 'slow' PS II reaction centres.