The appearance of quenching centres in photosystem II.

The variable fluorescence quenching found in the presence of DCMU with isolated chloroplasts which have been exposed previously to a prolonged low light intensity (Sinclair and Spence 1988), is accompanied by a loss of the sigmoidal appearance of the fluorescence induction transient. About 80% of the fluorescence decrease is due to the PS IIα units and 50% of the α centres are inactivated by light exposure. Light incubation slows the PS II partial reaction while the PS I partial reaction is unaffected. We propose that in the light, normal PS IIα centres change into quenching centres which degrade excitation energy to thermal energy. This change can be reversed by 30 min of darkness. A higher flash intensity is needed to saturate the steady state O2 flash yield from light-incubated chloroplasts indicating a light-induced decrease of the average photosynthetic unit size as would happen if PS IIα units were preferentially inactivated. These light-induced changes may relate to an adaptation in leaves to increasing light intensity.