Evidence for the role of the light-harvesting chlorophyll [formula omitted] protein complex in photosystem II heterogeneity

Abstract

Analyses of chlorophyll fluorescence induction kinetics from DCMU-poisoned thylakoids were used to examine the contribution of the light-harvesting chlorophyll a b protein complex (LHCP) to Photosystem II (PS II) heterogeneity. Thylakoids excited with 450 nm radiation exhibited fluorescence induction kinetics characteristic of major contributions from both PS II α and PS II β centres. On excitation at 550 nm the major contribution was from PS II β centres, that from PS II α centres was only minimal. Mg 2+ depletion had negligible effect on the induction kinetics of thylakoids excited with 550 nm radiation, however, as expected, with 450 nm excitation a loss of the PS II α component was observed. Thylakoids from a chlorophyll- b-less barley mutant exhibited similar induction kinetics with 450 and 550 nm excitation, which were characteristic of PS II β centres being the major contributors; the PS II α contribution was minimal. The fluorescence induction kinetics of wheat thylakoids at two different developmental stages, which exhibited different amounts of thylakoid appression but similar chlorophyll a b ratios and thus similar PS II:LHCP ratios, showed no appreciable differences in the relative contributions of PS II α and PS II β centres. Mg 2+ depletion had similar effects on the two thylakoid preparations. These data lead to the conclusion that it is the PS II:LHCP ratio, and probably not thylakoid appression, that is the major determinant of the relative contributions of PS II α and PS II β to the fluorescence induction kinetics. PS II α characteristics are produced by LHCP association with PS II, whereas PS II β characteristic can be generated by either disconnecting LHCP from PS II or by preferentially exciting PS II relative to LHCP.