Surface lipids and their distribution on willow ( Salix) leaves: a combined chemical, morphological and physicochemical study

Abstract

The surfaces of leaves of three willow ( Salix spp.) clones with different frost tolerance and biomass productivity were investigated in order to elucidate further the previously observed relationship between high epicuticular leaf wax load, high biomass productivity and poor overwintering survival of Salix clones. Morphological studies by scanning electron microscopy (SEM) were combined with contact angle measurements, surface overall chemical analysis by X-ray photoelectron spectroscopy (XPS) and gas chromatographic (GC) analyses of extracted waxes and cutin. The GC analyses revealed that epicuticular waxes contained n-alkanes, n-alcohols, n-aldehydes, wax esters and free fatty acids. Mono- and dihydroxy hexadecanoic and monohydroxy octadecanoic acids were major cutin acids in the leaves of the three Salix clones. Overall chemical analysis showed that the Salix leaf surfaces were composed primarily of carbon (about 90%) and oxygen (about 10%). SEM showed that the leaf surfaces of the three Salix clones were partly (15–40% of the area) covered by smooth spherical particles on or in a planar matrix. By combining these chemical and morphological analyses, it was concluded that the particles observed by SEM were composed of epicuticular waxes, and the planar matrix between the particles was cutin. The high epicuticular wax load and the high amounts of n-alkanes, high levels of dihydroxy hexadecanoic acid and total C 16 acids in cutin, and the high leaf-surface coverage by the wax spheres correlated with the poor frost tolerance and high productivity of the three Salix clones in this study. Furthermore, water contact angles on the leaf surface of the frost-susceptible high-producer clone were 93°, about 20° higher than on the leaf surfaces of the two other clones.