PHOTOINHIBITION AND RECOVERY IN RELATION TO HETEROGENEITY OF PHOTOSYSTEM-II

Photosystem II (PS II) heterogeneity during photoinhibition at 4-degrees-C and subsequent recovery at 20-degrees-C was investigated in spinach leaves and chloroplasts. The population of inactive, Q(B)-nonreducing centers was estimated by means of fluorescence induction in the presence of ferricyanide and was only 5-10% of the total PS II population in the untreated material, when assuming that the fluorescence yield from all PS II units was identical. No significant changes in this population of Q(B)-nonreducing centers after photoinhibition treatment at 4-degrees-C of spinach leaves or chloroplasts was found. During subsequent recovery of the inhibited leaves in low light at room temperature no changes in this Q(B)-nonreducing pool could be detected, while electron transport measured in the presence of methylviologen or 1,4-benzoquinone and the yield of variable fluorescence recovered in 6-8 h. During photoinhibitory irradiation of the leaves and thylakoids, the PS II(alpha), population became strongly inhibited while PS II(beta) was inhibited to a lesser extent. The inhibition treatment did not affect the photochemical efficiency of the remaining PS II(alpha) and PS II(beta) centers, but diminished the cooperativity between the PS Il. centers. In leaves, a small population of inhibited PS II centers seemed to have an impairment of the donor side, which disappeared during recovery in low light but not in darkness, concomitant with recovery of electron transport and variable fluorescence. No significant changes in the Q(B)-nonreducing PS II, PS II(alpha) and PS II(beta) populations or electron transport activities occurred when the inhibited leaves were transferred to room temperature in darkness. In conclusion, no evidence for a repair cycle that proceeds via Q(B)-nonreducing centers could be obtained. However, we cannot exclude the possibility that a dynamics of PS II heterogeneity was concealed by synchronous turnover of different populations of PS II. Our electron transport data show that rate measurements with different quinone acceptors cannot be used to discriminate between effects of photoinhibition on 'active' and Q(B)-nonreducing centers.