Effects of UV-B Radiation on Wax Biosynthesis

Abstract

Two genotypes of tobacco (Nicotiana tabacum L.) were exposed in controlled environment chambers to three levels of biologically effective ultraviolet-B radiation (UV-BBE; 280-320nm): 0,4.54 (ambient) and 5.66 (≈ 25% enhancement) kJ m-2 d-1. After 28 days, the quantity of wax deposited on leaf surfaces was determined gravimetrically; epicuticular wax chemical composition was determined by capillary gas chromatography with homologue assignments confirmed by gas chromatography-mass spectrometry. Leaf wettability was assessed by measuring the contact angle of water droplets placed on leaf surfaces. Tobacco wax consisted of three major hydrocarbon classes: straight-chain alkanes (C27-C33) which comprised ≈ 59% of the hydrocarbon fraction, containing a predominance of odd-chain alkanes with C31 as the most abundant homologue; branched-chain alkanes (C25-C32) which comprised ≈38% of the hydrocarbon fraction with anteiso 3-methyltriacontane (C30) as the predominant homologue; and fatty acids (C14-C18) which comprised ≈ 3% of the wax. Exposure to enhanced UV-B radiation reduced the quantity of wax on the adaxial surface of the transgenic mutant, and resulted in marked changes in the chemical composition of the wax on the exposed leaf surface. Enhanced UV-B decreased the quantity of straight-chain alkanes, increased the quantity of branched-chain alkanes and fatty acids, and resulted in shifts toward shorter straight-chain lengths. Furthermore, UV-B-induced changes in wax composition were associated with increased wettability of tobacco leaf surfaces. Overall, the data are consistent with the view that UV-B radiation has a direct and fundamental effect on wax biosynthesis. Relationships between the physico-chemical nature of the leaf surface and sensitivity to UV-B radiation are discussed.