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 sev eral 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 ca rotenoids and xanthophyll cycle pigments and high ratios of chlorophyl l a to b. During 1-2 h periods of exposure to direct mid-day sun, all leaves experienced substantial photoinhibition as indicated by a marke d decline in the ratio of variable to maximum chlorophyll a fluorescen ce 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 b iphasic kinetics, similar to previous findings under controlled condit ions in the laboratory, A phase lasting about 1 h accounted for most o f the recovery of F-V/F-M and was followed by a slow phase which proce eded until sunset. The decline in F-V/F-M during photoinhibition and t he fast phase of recovery correlated closely with the amounts of zeaxa nthin in the leaves. Given the small portion of the second recovery ph ase which has previously been attributed to turnover of the D1 protein in Photosystem II, high xanthophyll cycle activity in these gap leave s is probably responsible for the major part of photoinhibition, provi ding an efficient energy dissipation pathway during periods of high su nlight exposure.