Capacity of protection against ultraviolet radiation in sun and shade leaves of tropical forest plants

Protection of leaves of tropical forest plants against UV-A and -B radiation was studied in three lowland forests, a montane cloud forest and a mangrove stand in Panama. Leaves were classified as sun or shade leaves according to their chlorophyll a/b ratio, pool size of xanthophyll cycle pigments and alpha- and beta-carotene contents. The capacity of the leaves for protection against UV radiation was assessed by estimating epidermal UV-A shielding, by a non-invasive fluorometric method, and by the absorbance of ethanolic/aqueous leaf extracts in the UV spectral region. In all sun leaves tested, UV-A shielding by the adaxial epidermis was high, usually above 90%, whereas in shade leaves the epidermal UV-A shielding was markedly lower and varied widely between species. In most cases UV-A shielding by the abaxial epidermis was lower than by the adaxial epidermis. UV absorbance of the leaf extracts was generally higher in sun than in shade leaves, and the absorbance was much higher in the UV-B spectral region at 305 nm than in the UV-A region at 375 nm. The data demonstrate that sun leaves of tropical plants are well protected against solar UV-A and UV-B radiation. However, UV-induced damage may occur when shade leaves become exposed to full solar radiation.