PHOTOINHIBITION DURING WINTER STRESS - INVOLVEMENT OF SUSTAINED XANTHOPHYLL CYCLE-DEPENDENT ENERGY-DISSIPATION

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 rel ationship 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 rapidl y 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 'photoinhib ition' occuring in these species might be due to the sustained engagem ent of these de-epoxidised components of the xanthophyll cycle in phot oprotective energy dissipation. When examined over the course of sever al days during the winter, the predawn conversion state of the xanthop hyll cycle responded to the daily changes in minimum air (and leaf) te mperature, such that the xanthophyll cycle was largely deepoxidised pr ior to sunrise on cold nights and was present predominantly as V after nights when the nocturnal temperatures were above freezing. In additi on, 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 an d 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 xanth ophyll cycle, and the photoprotective energy dissipation process assoc iated with it, would thus appear to provide plants the flexibility req uired to deal with the excessive levels of Light absorbed by chlorophy ll under a wide range of climatic conditions, and can quite possibly a ccount for the 'photoinhibition' observed during winter stress.