A pronounced light-induced zeaxanthin formation accompanied by an unusually slight increase in non-photochemical quenching: a study with barley leaves treated with methyl viologen at moderate light.

Light-induced deepoxidation of violaxanthin to antheraxanthin and zeaxanthin in plants is associated with the induction of pronounced xanthophyll-dependent non-photochemical quenching (NPQ). To date, a misbalance between a high amount of zeaxanthin in thylakoid membranes and low NPQ has been explained by an absence of lumen acidification (e.g. when NPQ is measured in the dark after high light stress). In this study, we report that this misbalance can also be observed under moderate light. We found this result (deepoxidation state, DEPS, above 55% and NPQ approximately 0.9) in barley leaves treated with 10 microM methyl viologen (MV) under white light (100 micromol photonsm(-2)s(-1), photosynthetically active radiation (PAR), growth irradiance). The addition of MV at this moderate light did not accelerate electron transport in thylakoid membranes, and induced only slight oxidative stress (no lipid peroxidation, almost unchanged maximum yield of photosystem II photochemistry, a decrease in activity of ascorbate peroxidase, and an increase in that of glutathion reductase). We suggest that, in leaves treated under the conditions used here, the lumen acidification induced by light-limited electron transport in thylakoid membranes was high enough to activate violaxanthin deepoxidase, but not sufficiently high to form the expected number of zeaxanthin-dependent quenching centers in photosystem II antennae.