Xanthophyll-cycle pigments and photosynthetic capacity in tropical forest species: a comparative field study on canopy, gap and understory plants.

Xanthophyll-cycle pigments and photosynthetic capacity (PSmax) were analyzed in 25 species from different light environments (canopy, gap, understory) within a Panamanian tropical forest. (1) Sun-exposed leaves of canopy tree species showed the highest photosynthetic capacities and largest xanthophyll-cycle pools (violaxanthin, antheraxanthin, zeaxanthin) of about 87 mmol mol-1 chlorophyll with only small amounts of α-carotene [about 7 mmol mol-1 chlorophyll = 8% of total (α+β) carotene pool]. Under high natural photon flux densities (PFDs) canopy leaves rapidly converted up to 96% of the xanthophyll-cycle pool into zeaxanthin. The back reaction to violaxanthin occurred much faster in low light than in complete darkness. At the end of the night, zeaxanthin still accounted for, on average, 14% of the total xanthophyll-cycle pigments. (2) Leaves of gap plants had intermediate values of PSmax and a 43% lower total carotenoid content than canopy leaves. The average size of the xanthophyll-cycle pool was 35 mmol mol-1 chlorophyll, and α-carotene accounted for up to 66% of the total (α+β) carotene pool. Under high light conditions gap plants converted, on average, 86% of the xanthophyll-cycle pigments into zeaxanthin. The back reaction, following a decrease in ambient PFD, was slower than 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 smallest xanthophyll-cycle pool of about 22 mmol mol-1 chlorophyll. α-Carotene accounted for up to 70% of total carotene. The conversion of xanthophyll-cycle pigments into zeaxanthin was negligible during short sunflecks of 1-2 min duration and PFDs up to about 400 μmol m-2 s-1. At predawn, leaves of understory plants rarely contained any detectable zeaxanthin. Aechmea magdalenae, an understory CAM plant, showed exceptionally high rates of PSmax per unit leaf area compared to sympatric C3 understory species.