Fig. 2.

Kinetics of ${\mathrm{Q}}_{\mathrm{A}}^{•–}$ reoxidation by O₂. Fraction of ${\text{Q}}_{\text{A}}^{•-}$ obtained after three saturating flashes of a degassed sample of PSII membrane fragments (60 nM PSII cores) in the presence of 250 µM hydroxylamine in a 40 mM MES buffer, 5 mM MgCl, pH 6.5. The data in A, C, and D were fitted to Eq. 1. (A) Orange symbols represent the decay in the absence of added oxygen, and the data were fitted with a linear decay. Other symbols represent the decay in the presence of oxygen in the concentration range from 40 ± 3 µM (black symbols) to 155 ± 6 µM (blue symbols). Data were fitted with Eq. 1. (B) Observed kinetic rate constants obtained from the fitting of the kinetic data with Eq. 1 plotted in function of the concentration of added oxygen. Data were fitted with the hyperbole of Eq. 2. (C) Effect of the addition of Q_B site inhibitors. Control experiments with (open black symbols) and without (closed orange symbols) oxygen are shown for comparison; 10 µM DCMU (open blue symbols) or 100 µM bromoxynil (open red symbols) was added prior to the degassing step and the kinetics were measured upon the addition of 110 ± 5 µM oxygen. (D) Effect of the addition of 1 mM bicarbonate (open circles) at the degassing step prior to the addition of oxygen is compared with a control kinetics in the absence of bicarbonate (open squares). Kinetics were recorded upon addition of 110 ± 5 µM oxygen.