Oxygen yield from single turnover flashes in leaves: non-photochemical excitation quenching and the number of active PSII.

O(2) evolution from single turnover flashes of up to 96 micromol absorbed quanta m(-2) and from multiple turnover pulses of 8.6 and 38.6 ms duration and 12800 and 850 micromol absorbed quanta m(-2) s(-1) intensity, respectively, was measured in sunflower leaves with the help of zirconium O(2) analyser. O(2) evolution from one flash could be measured with 1% accuracy on the background of 10-50 micromol O(2) mol(-1). Before the measurements leaves were pre-adapted either at 30-60 or 1700 micromol quanta m(-2) s(-1) to induce different non-photochemical excitation quenching (q(N)). Short (1 min) exposures at the high light that created only energy-dependent, q(E) type quenching, caused no changes in the O(2) yield from saturating flashes or pulses that could be related to the q(E) quenching, but the yield from low intensity flashes and pulses decreased considerably. Long 30-60-min exposures at the high light induced a reversible inhibitory, q(I) type quenching that decreased the O(2) yield from both, saturating and limiting flashes and pulses (but more from the limiting ones), which reversed within 15 min under the low light. The results are in agreement with the notion that q(E) is caused by a quenching process in the PSII antenna and no changes occur in the PSII centres, but the reversible (15-30 min) q(I) quenching is accompanied by inactivation of a part of PSII centres.