Quantitative changes in secondary metabolites of dark‐leaved willow (Salix myrsinifolia) exposed to enhanced ultraviolet‐B radiation

This is a study of the impact of increased ultraviolet‐B (UV‐B) radiation on the secondary chemistry of Salix myrsinifolia (dark‐leaved willow). For nearly two decades, the loss of stratospheric ozone above the high latitudes of the Northern Hemisphere has increased UV‐B radiation (280–320 nm) over the long‐term mean. Willows (Salicaceae) are widely distributed in these northern regions. To determine the effects of increased UV‐B radiation on willows, the plantlets of three clones of S. myrsinifolia were grown under ambient (3.6 kJ m −2 day −1 ) or enhanced (7.18 kJ m −2 day −1 ) UV‐B irradiance. After the 2‐week indoor experiment, the concentrations of UV‐B‐screening phenolics (flavonoids and phenolic acids) and low‐UV‐B‐screening phenolics (salicylates and condensed tannins) in fresh leaves were investigated and the biomass of leaves, stems and roots was determined. As expected, the total amount of flavonoids in willow leaves clearly increased when plantlets were exposed to higher UV‐B irradiation. However, the degree of increase of individual compounds varied: luteolin‐7‐glucoside, monomethyl‐monocoumaryl‐luteolin‐7‐glucoside and one myricetin derivative increased significantly, while the apigenin‐7‐glucuronide increased only slightly. The enhanced UV‐B also increased the amount of p ‐hydroxycinnamic acid derivative. The UV‐B effects on other phenolic acids and tannins were minor. In contrast to the other phenolics, the amounts of two salicylates, salicin and saligenin, decreased under enhanced UV‐B irradiation. Our results indicate that the concentrations of both UV‐B‐screening and low‐UV‐B‐screening phenolic compounds in leaves of S. myrsinifolia may vary in response to elevated UV‐B radiation. However, while the UV‐B protective flavonoids and phenolic acids accumulate during UV‐B exposure, the concentrations of certain salicylates decrease.