Figure 6. Kinetics of Q_A ⁻ oxidation in the presence of DCMU following exposure to excess light.

A–C. Cell suspensions (7.5 µg chl ml⁻¹) were exposed to 2000 µmol photons m⁻² s⁻¹ for 0–35 min, at 25°C. Q_A ⁻ oxidation was measured using the FL 3000 fluorimeter following 2 min dark adaptation in the presence of 10 µM DCMU. The t_½ time of Q_A ⁻ oxidations was 0.2–0.4 ms and 1.3–2.0 s for the fast and slow phases, respectively. Flash intensity was 2300 µmol photons m⁻² s⁻¹; flash duration 30 µs. The first sampling point was recorded 215 µs after the flash to minimize the contribution of the flash decay, 5 points per decade and 20% voltage of the measuring beam. Furthermore, since all measurements were performed with the same instrument and setting, the contribution of instrumental artifacts, if any, would equally occur in all the measurements. For the sake of clarity, we provide full data sets for the 0 and 35 min time point in (A) and the Fv values and the ratio of fast to slow decay kinetics, as a function of time of exposure to the excess light in (B), calculated from data such as presented in (A). (C) Same conditions as above but with the cells exposed for 17 min to various light intensities (250 to 2000 µmol photons m⁻² s⁻¹). (D) Same conditions as in (A) but the QA⁻ re-oxidation was measured at 15°C. The t_½ time of Q_A ⁻ oxidations was 0.32–0.45 ms and 3.1–7.1 s for the fast and slow phases, respectively. Note the large rise in the extent of the fast phase and that the t_1/2 of the fast phase was little affected by lowering the temperature from 25°C (A–C) to 15°C (D–E), whereas that of the slow phase declined by about 3-fold (compare with (A).