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 O2 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 zmax (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/zmax 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/zmax, respectively, using a quick P700 absorbance protocol compatible with modern P700 instruments.