In situ study of photoinhibition of photosynthesis and xanthophyll cycle activity in plants growing in natural gaps of the tropical forest

Photoinhibition of photosynthesis was studied in situ in leaves of several species of plants growing in natural treefall gaps of a tropical lowland forest (Barro Colorado Island, Panama). Leaves showed several features typical of sun-acclimation: relatively high pools of total carotenoids and xanthophyll cycle pigments and high ratios of chlorophyll a to b. During 1-2 h periods of exposure to direct mid-day sun, all leaves experienced substantial photoinhibition as indicated by a marked decline in the ratio of variable to maximum chlorophyll a fluorescence emission, F-V/F-M, detected after 10 min of dark adaptation. After return to shade, these 'dark-adapted' F-V/F-M, ratios increased with biphasic kinetics, similar to previous findings under controlled conditions in the laboratory, A phase lasting about 1 h accounted for most of the recovery of F-V/F-M and was followed by a slow phase which proceeded until sunset. The decline in F-V/F-M during photoinhibition and the fast phase of recovery correlated closely with the amounts of zeaxanthin in the leaves. Given the small portion of the second recovery phase which has previously been attributed to turnover of the D1 protein in Photosystem II, high xanthophyll cycle activity in these gap leaves is probably responsible for the major part of photoinhibition, providing an efficient energy dissipation pathway during periods of high sunlight exposure.