XANTHOPHYLL-CYCLE PIGMENTS AND PHOTOSYNTHETIC CAPACITY IN TROPICAL FOREST SPECIES - A COMPARATIVE FIELD-STUDY ON CANOPY, GAP AND UNDERSTORYPLANTS

Xanthophyll-cycle pigments and photosynthetic capacity (PSmax) were an alyzed in 25 species from different light environments (canopy, gap, u nderstory) within a Panamanian tropical forest. (1) Sun-exposed leaves of canopy tree species showed the highest photosynthetic capacities a nd largest xanthophyll-cycle pools (violaxanthin, antheraxanthin, zeax anthin) of about 87 mmol mol(-1) chlorophyll with only small amounts o f alpha-carotene [about 7 mmol mol(-1) chlorophyll = 8% of total (alph a+beta) carotene pool]. Under high natural photon flux densities (PFDs ) canopy leaves rapidly converted up to 96% of the xanthophyll-cycle p ool into zeaxanthin. The back reaction to violaxanthin occurred much f aster in low light than in complete darkness. At the end of the night, zeaxanthin still accounted for, on average, 14% of the total xanthoph yll-cycle pigments. (2) Leaves of gap plants had intermediate values o f PSmax and a 43% lower total carotenoid content than canopy leaves. T he average size of the xanthophyll-cycle pool was 35 mmol mol(-1) chlo rophyll, and alpha-carotene accounted for up to 66% of the total (alph a+beta) carotene pool. Under high light conditions gap plants converte d, on average, 86% of the xanthophyll-cycle pigments into zeaxanthin. The back reaction, following a decrease in ambient PFD, was slower tha n the forward reaction. At the end of the night, zeaxanthin accounted for, on average, 7% of the xanthophyll-cycle pigments in gap plants. ( 3) Understory plants showed the lowest values of PSmax and the smalles t xanthophyll-cycle pool of about 22 mmol mol(-1) chlorophyll. alpha-C arotene accounted for up to 70% of total carotene. The conversion of x anthophyll-cycle pigments into zeaxanthin was negligible during short sunflecks of 1-2 min duration and PFDs up to about 400 mu mol m(-2) s( -1). At predawn, leaves of understory plants rarely contained any dete ctable zeaxanthin. Aechmea magdalenae, an understory CAM plant, showed exceptionally high rates of PSmax per unit leaf area compared to symp atric C-3 understory species.