'Photoinhibition' During Winter Stress: Involvement of Sustained Xanthophyll Cycle-Dependent Energy Dissipation

Abstract

Sustained decreases in intrinsic photosystem II efficiency (i.e. F_v/F_m) in response to high light and chilling temperatures were examined in eight species, and were found to be accompanied by the retention of zeaxanthin (Z) and antheraxanthin (A) overnight. The quantitative relationship between changes in F_v/F_m and the A + Z level during these sustained changes on cold days was similar to that obtained for rapidly reversible changes on warm days. Furthermore, upon removal of leaves from the field, recovery from 'photoinhibition' (the reversal of the depression of F_v/F_m) matched the timecourse of the epoxidation of Z and A to violaxanthin (V). These findings suggest that the 'photoinhibition' occuring in these species might be due to the sustained engagement of these de-epoxidised components of the xanthophyll cycle in photoprotective energy dissipation. When examined over the course of several days during the winter, the predawn conversion state of the xanthophyll cycle responded to the daily changes in minimum air (and leaf) temperature, such that the xanthophyll cycle was largely de-epoxidised prior to sunrise on cold nights and was present predominantly as V after nights when the nocturnal temperatures were above freezing. In addition, in some of the species examined, there was a large acclimation of the xanthophyll cycle pool size to the level of excessive light, with a much larger pool present in the leaves examined during the winter and that pool being de-epoxidised to Z and A to a much greater degree at midday than from similar leaves examined during the summer. The xanthophyll cycle, and the photoprotective energy dissipation process associated with it, would thus appear to provide plants the flexibility required to deal with the excessive levels of light absorbed by chlorophyll under a wide range of climatic conditions, and can quite possibly account for the 'photoinhibition' observed during winter stress.