Clonal differences in growth and phenolics of willows exposed to elevated ultraviolet-B radiation

In this study, the effects of elevated ultraviolet-B (UV-B, 280-320 nm) radiation on growth and leaf phenolics were evaluated in clones of dark-leaved willow (Salix myrsinifolia Salisb.) and tea-leaved willow (Salix phylicifolia L.). Willows were raised for one growing season in an irradiation field, where they were exposed either to a constant 50% increase in UV-B-CIE radiation simulating 20-25% ozone depletion or to a small increase in UV-A radiation (320-400 nm). Control willows were grown in the irradiation field under solar radiation (ambient control). Despite the high constitutive concentrations of a UV-absorbing leaf flavonoid, dihydromyricetin, UV-treatments clearly reduced the biomass and height growth of the shoots of one tea-leaved willow clone. In contrast, the growth of three other tea-leaved willow clones showed no significant sensitivity to UV-radiation. Under elevated UV-B radiation, the leaves of these clones were able to accumulate some of the UV-B-absorbing quercetins, myricetins or luteolins. In dark-leaved willows, biomass production and growth were not decreased by UV-exposures, although the concentrations of leaf flavonoids were clearly lower than those in tea-leaved willows; however, in all the dark-leaved willow clones, elevated UV-radiation increased the concentrations of certain quercetins, dihydromyricetin and phenolic acids. Other willow leaf phenolics, i.e. salicylates, condensed tannins and gallic acid derivatives, were either decreased by the UV-treatments or were unaffected. The results indicate that (1) the constitutive level and quality of secondary chemicals in native willow species or clones does not predict their sensitivity to elevated UV-radiation, (2) secondary chemical responses to UV-radiation in willows are more clone-specific than species-specific and (3) the leaves of field-grown willows treated with UV-B radiation accumulate only those phenolics that screen UV-B efficiently.