THE APPEARANCE OF QUENCHING CENTERS 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 II-alpha units and 50% of the alpha 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 II-alpha 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 II-alpha units were preferentially inactivated. These light-induced changes may relate to an adaptation in leaves to increasing light intensity.