Table 1. Effect of various PQs on oxygen uptake measured in control, low-pH-incubated and Tris-treated spinach PSII membrane particles.
Oxygen uptake was measured under saturating white light in the presence of either 400 mM sucrose, 15 mM NaCl, 5 mM MgCl2 and 40 mM Mes (pH 6.5) (control and Tris-treated PSII membrane particles), or 400 mM sucrose, 15 mM NaCl, 5 mM MgCl2 and 40 mM acetate buffer (pH 4.5) (low-pH incubation). In Tris-treated PSII membranes, electron transport was supported by the addition of 1 mM DPC (diphenylcarbazide). The values in parentheses are the percentage of O2 uptake measured in the absence of PQs. The amount of HP cyt b559 and LP cyt b559 was determined from difference absorption spectra at 559–570 nm. The amount of HP cyt b559 was calculated from the absorbance change after the addition of potassium ferricyanide (control-FeCy), whereas the oxidized HP and LP cyt b559 content was estimated from the absorbance change after sequential reduction by hydroquinone (HQ-control) and sodium ascorbate (Asc-HQ) respectively. No oxidized HP cyt b559 was found.
| O2 uptake [μmol of O2·(mg of chl)−1·h−1] | ||||
|---|---|---|---|---|
| LP cyt b559 | PSII… (%)… | Control 28 | Low-pH-incubated 49 | Tris-treated 75 |
| −PQ | 2.18 (100%) | 5.69 (100%) | 6.01 (100%) | |
| +33 μM PQ-9 | 2.45 (112%) | 6.86 (121%) | 7.99 (133%) | |
| +33 μM PQ-2 | 2.66 (122%) | 8.56 (150%) | 16.43 (274%) | |
| +33 μM PQ-1 | 2.76 (126%) | 18.48 (324%) | 20.00 (333%) | |