Light-dependent photosynthetic characteristics indicated by chlorophyll fluorescence in five mangrove species native to Pohnpei Island, Micronesia.

To investigate the adaptive capacity to strong light among mangrove species, we examined light-dependent properties of photosynthesis in relation to photoinhibition using chlorophyll fluorescence for sun-leaves of five mangrove tree species; Sonneratia alba, Rhizophora stylosa, Rhizophora apiculata, Bruguiera gymnorrhiza and Xylocarpus granatum. These species were classified into three groups; pioneer -S. alba and R. stylosa, intermediate -R. apiculata and B. gymnorrhiza and climax species -X. granatum. A further distinction can be made between the two intermediate species, the less shade-tolerant R. apiculata and the shade-tolerant B. gymnorrhiza. There was a clear association between successional status and light-saturated electron transport rate (ETR) where ETR was higher in S. alba and R. stylosa > R. apiculata and B. gymnorrhiza > X. granatum. Based on its lower initial slope of light-dependent ETR and lower ratio of variable to maximum fluorescence (Fv/Fm) after a dark-adaptation, X. granatum appeared to suffer greater photoinhibition than other species. The response of qP, which represents PSII openness, to incident light indicates a ranking of tolerant capacity to photoinhibition of S. alba and R. stylosa > R. apiculata > B. gymnorrhiza and X. granatum. The difference in qP response between R. apiculata and B. gymnorrhiza might be attributed to the higher leaf absorptance and the lower degree of thermal dissipation indicated by the higher F'v/F'm in B. gymnorrhiza. It appears that the co-ordination of leaf absorptance, thermal dissipation and consumption of absorbed light energy through electron transport is important in the acclimation of mangrove species to exposed habitats.