Light emission originating from photosystem II radical pair recombination is sensitive to zeaxanthin related non-photochemical quenching (NPQ).

We have used chlorophyll fluorescence, delayed luminescence and thermoluminescence measurements to study the influence of an artificial DeltapH in the presence or absence of zeaxanthin on photosystem II reactions. Energization of the pea thylakoid membranes induced non-photochemical fluorescence quenching and an increase in the overall luminescence emission of PSII during delayed luminescence and thermoluminescence measurements. This DeltapH-induced overall luminescence increase was caused by a strongly enhanced delayed luminescence in the seconds range before sample heating. In the subsequent thermoluminescence measurements the intensity of the B-band decreased after one and increased after two or more single turnover flashes. We propose that strong membrane energization shifted the redox potential of photosystem II radical pairs to more negative values causing the high delayed luminescence. The zeaxanthin-dependent non-photochemical fluorescence quenching component, however, did not alter thermoluminescence B-bands but decreased the delayed luminescence intensity by 30%. To our knowledge this is the first report that the radiative radical pair recombination, exhibited as delayed luminescence but not thermoluminescence emission, is sensitive to the antenna located zeaxanthin related non-photochemical fluorescence quenching. Our data can be interpreted within the frame of the exciton/radical pair equilibrium model that describes photosystem II as a shallow trap and incorporates the transfer of energy from the re-excitated reaction centre to the antenna of photosystem II.