A novel P700 redox kinetics probe for rapid, non‐intrusive and whole‐tissue determination of photosystem II functionality, and the stoichiometry of the two photosystems in vivo

We sought a rapid, non‐intrusive, whole‐tissue measure of the functional photosystem II ( PS II ) content in leaves. Summation of electrons, delivered by a single‐turnover flash to P700 + (oxidized PS I primary donor) in continuous background far‐red light, gave a parameter S in absorbance units after taking into account an experimentally determined basal electron flux that affects P700 redox kinetics. S was linearly correlated with the functional PS II content measured by the O 2 yield per single‐turnover repetitive flash in Arabidopsis thaliana expressing an antisense construct to the PsbO (manganese‐stabilizing protein in PS II) proteins of PS II ( PsbO mutants). The ratio of S to z max (total PS I content in absorbance units) was comparable to the PS II / PS I reaction‐center ratio in wild‐type A. thaliana and in control Spinacea oleracea . Both S and S/z max decreased in photoinhibited spinach leaf discs. The whole‐tissue functional PS II content and the PS II /photosystem I ( PS I) ratio can be non‐intrusively monitored by S and S/z max , respectively, using a quick P700 absorbance protocol compatible with modern P700 instruments.