The influence of UV-B radiation on the physicochemical nature of tobacco (Nicotiana tabacum L) leaf surfaces

Relationships between leaf wettability and surface physicochemical characteristics were examined in two genotypes of tobacco (Nicotiana tabacum L. cv. Samsun) grown under controlled conditions at three different levels of biologically effective ultraviolet-B (UV-B-BE; 280-320 nm) radiation; 0 (control), 4.54 and 5.66 kJ m(-2) d(-1). Leaf wettability, assessed by measuring leaf-water droplet contact angles, was positively correlated with epicuticular wax chemical composition and trichome density, but not the amount of wax on the surface of leaves. Tobacco wax comprised a mixture of C-19-C-33 n-alkanes (approximate to 59%) with homologues containing an odd number of carbon atoms predominating, C-28-C-32 br-alkanes (approximate to 38%), and a small quantity (approximate to 3%) of free C-16-C-18 fatty acids. Significant effects of UV-B radiation upon wax production and chemical composition were restricted to the adaxial surface of leaves, Enhanced UV-B radiation reduced the quantity of epicuticular wax in the more sensitive genotype [GR32-3], assessed from effects on dry matter accumulation, partitioning and changes in leaf morphology, and resulted in marked changes in wax composition and homologue distributions in both genotypes. UV-B-induced increases in branching, and shifts toward the synthesis of shorter-chain homologues provided evidence for a fundamental effect of UV-B radiation on wax biosynthesis, with the observed effects consistent with a highly specific and direct effect of UV-B radiation on microsomal-based elongases in the epidermis. UV-B radiation also reduced the density of trichomes on the adaxial leaf surface, whilst increasing the number of trichomes on the abaxial leaf surface. Changes in wax composition and trichome density induced by UV-B radiation were associated with increases in leaf surface wettability which were particularly pronounced on the adaxial surface. The subtle, though possibly far-reaching, physiological consequences of such UV-B-induced changes in surface wettability are discussed in the light of other recent findings.