LEAF STRUCTURE AND PATTERNS OF PHOTOINHIBITION IN 2 NEOTROPICAL PALMSLN CLEARINGS AND FOREST UNDERSTORY DURING THE DRY SEASON

We studied the leaf structural, water status, and fast fluorescence re sponses of two palms, Socratea exorrhiza and Scheelen zonensis, under natural dry season conditions in a clearing (high light [HL] palms) an d the forest understory (low light [LL] palms) on Barro Colorado Islan d, Panama. HL-Socratea leaves were more shade-adapted, less xeromorphi c, and more strongly affected by drought than HL-Scheelea. F-v/F-m, (t he ratio of variable to maximum chlorophyll fluorescence) and t(1/2) ( the half-rise time of F-m) was lower in HL-leaves of both species, ind icating photoinhibition. In HL-Scheelea, the light-induced reduction o f F-v/F-m was much less than in HL-Socratea, and F-v/F-m recovered com pletely overnight. Patterns of relative water content, specific leaf d ry weight, stable carbon isotope composition, and leaf conductance sug gest that increased drought resistance in Scheelea reduces susceptibil ity to photoinhibition. An increase in F-0 indicated the inactivation of PSII reaction centers in HL-Socratea. The very low chlorophyll a/b ratio and alterations in chloroplast ultrastructure in HL-Socratea are consistent with photoinhibition. Under LL, the species showed no appr eciable interspecific differences in chlorophyll fluorescence. Excess light leads to low values of F-v/F-m in HL-plants relative to LL-plant s on both leaf surfaces, particularly on the lower surface, due to a d ecrease of F-m in both surfaces and an increase in F-0 of lower surfac e. For both species, F-0 for the lower surfaces of HL-plants was highe r and t(1/2) was markedly lower than for the upper surface, as is typi cal for shade-adapted leaves. Xeromorphic leaf structure may reduce su sceptibility to photoinhibition during the dry season. Drought-enhance d photoinhibition could limit the ability of some species to exploit t reefall gaps.