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

Relationships between leaf wettability and surface physicochemicalcharacteristics were examined in two genotypes of tobacco (Nicotianatabacum L. cv. Samsun) grown under controlled conditions atthree different levels of biologically effective ultraviolet-B(UV-B_BE; 280–320 nm) radiation; 0 (control), 4.54 and5.66 kJ m^–2d^–1. Leaf wettability, assessed by measuringleaf-water droplet contact angles, was positively correlatedwith epicuticular wax chemical composition and trichome density,but not the amount of wax on the surface of leaves. Tobaccowax comprised a mixture of C₁₉–C₃₃ n-alkanes (59%) withhomologues containing an odd number of carbon atoms predominating,C₂₈–C₃₂ br-alkanes (38%), and a small quantity (3%) offree C_l6–C₁₈ fatty acids. Significant effects of UV-Bradiation upon wax production and chemical composition wererestricted to the adaxial surface of leaves. Enhanced UV-B radiationreduced the quantity of epicuticular wax in the more sensitivegenotype GR32-3], assessed from effects on dry matter accumulation,partitioning and changes in leaf morphology, and resulted inmarked changes in wax composition and homologue distributionsin both genotypes. UV-B-induced increases in branching, andshifts toward the synthesis of shorter-chain homologues providedevidence for a fundamental effect of UV-B radiation on wax biosynthesis,with the observed effects consistent with a highly specificand direct effect of UV-B radiation on microsomal-based elongasesin the epidermis. UV-B radiation also reduced the density oftrichomes on the adaxial leaf surface, whilst increasing thenumber of trichomes on the abaxial leaf surface. Changes inwax composition and trichome density induced by UV-B radiationwere associated with increases in leaf surface wettability whichwere particularly pronounced on the adaxial surface. The subtle,though possibly far-reaching, physiological consequences ofsuch UV-B-induced changes in surface wettability are discussedin the light of other recent findings. Key words: Epicuticular wax chemistry, wax quantity, leaf wettability, trichome density, ultraviolet-B radiation