Effects of hydroxylamine and silicomolybdate on the decay in delayed light emission in the 6-100 μs range after a single 10 ns flash in pea thylakoids.

Measurements are reported on μs delayed light emission, following a single 10 ns excitation flash, in Alaska pea thylakoids treated with hydroxylamine (NH2OH) or with silicomolybdate. 1. In thylakoids treated with 2 mM NH2OH in the light, or in the dark, the quantum yield of delayed light emission is considerably enhanced. A 10 μs lifetime component of delayed light emission is not significantly changed, whereas a 50-70 μs lifetime component is increased. MnCl2 and diphenylcarbazide are unable to reverse the above effects of NH2OH treatment. Thus Mn(2+) and diphenylcarbazide must not donate electrons directly to reaction center II but on the oxygen-evolution side of the NH2OH block. 2. When the closed form of photosystem II reaction centers (P680Q(-)), where P680 is the reaction center chlorophyll and Q is a 'stable' electron acceptor, is generated by preillumination of NH2OH-treated thylakoids with diuron present, the μs delayed light emission is inhibited, but a low level residual delayed light emission remains. Possible origins of this emission are discussed. It is believed that the best explanation for residual DLE is the existence of another acceptor besides Q that partakes in charge separation and rapid dissipative recombination when the reaction center is in the P680Q(-) state. 3. The quantum yield of delayed light emission from 'closed' reaction centers (P680 (+)Q(-)) that have all charge stabilization reactions (i.e., flow of electrons to P680 (+) and out of Q(-)) blocked by NH2OH treatment and addition of diuron is 1.1×10(-3) for components measured in a range from 6 to 400 μs and extrapolated to zero time. 4. The addition of silicomolybdate, which accepts electron from Q(-), causes delayed light emission in the μs range to be greatly inhibited.