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 mu mol absorbed quan ta m(-2) and from multiple turnover pulses of 8.6 and 38.6 ms duration and 12 800 and 850 mu mol 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 backg round of 10-50 mu mol O-2 mol(-1). Before the measurements leaves were pre- adapted either at 30-60 or 1700 mu mol quanta m(-2) s(-1) to induce differe nt non-photochemical excitation quenching (q(N)). Short (1 min) exposures a t the high light that created only energy-dependent, q(E) type quenching, c aused no changes in the O-2 yield from saturating flashes or pulses that co uld be related to the q(E) quenching, but the yield from low intensity flas hes 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 mo re from the limiting ones), which reversed within 15 min under the low ligh t. The results are in agreement with the notion that q(E) is caused by a qu enching process in the PSII antenna and no changes occur in the PSII centre s, but the reversible (15-30 min) q(I) quenching is accompanied by inactiva tion of a part of PSII centres. (C) 2000 Elsevier Science B.V. All rights r eserved.