THE INFLUENCE OF UV-B RADIATION ON THE PHYSICOCHEMICAL NATURE OF TOBACCO (NICOTIANA-TABACUM-L) LEAF SURFACES

Relationships between leaf wettability and surface physicochemical cha racteristics were examined in two genotypes of tobacco (Nicotiana taba cum L. cv. Samsun) grown under controlled conditions at three differen t 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 wettabilit y, assessed by measuring leaf-water droplet contact angles, was positi vely correlated with epicuticular wax chemical composition and trichom e 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%) w ith homologues containing an odd number of carbon atoms predominating, C-28-C-32 br-alkanes (approximate to 38%), and a small quantity (appr oximate to 3%) of free C-16-C-18 fatty acids. Significant effects of U V-B radiation upon wax production and chemical composition were restri cted 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 an d changes in leaf morphology, and resulted in marked changes in wax co mposition and homologue distributions in both genotypes. UV-B-induced increases in branching, and shifts toward the synthesis of shorter-cha in homologues provided evidence for a fundamental effect of UV-B radia tion on wax biosynthesis, with the observed effects consistent with a highly specific and direct effect of UV-B radiation on microsomal-base d 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 in creases in leaf surface wettability which were particularly pronounced on the adaxial surface. The subtle, though possibly far-reaching, phy siological consequences of such UV-B-induced changes in surface wettab ility are discussed in the light of other recent findings.