文摘
Nonphotochemical quenching (NPQ) of chlorophyll fluorescence plays an important role in the protection ofplants against excessive light. Fluorescence quenching of the major light-harvesting complex (LHCII) providesa model system to study the mechanism of NPQ. The existence of both quenched and nonquenched states ofLHCII has been postulated. We used time-resolved fluorescence and hydrostatic pressure to study differencesbetween these states. Pressure shifts the thermodynamic equilibrium between the two states. The estimatedvolume difference was 5 mL/mol, indicating a local conformational switch. The estimated free energy differencewas 7.0 kJ/mol: high enough to keep the quenched state population low under normal conditions, but lowenough to switch in a controlled way. These properties are physiologically relevant properties, because theyguarantee efficient light harvesting, while at the same time maintaining the capacity to switch to a quenchedstate. These results indicate that conformational changes of LHCII can play an important role in NPQ.