Responses to Solar UV-B Exclusion and Drought Stress in Two Cultivars of Chestnut Rose with Different Leaf Thickness

Plants adopt a series of strategies to tolerate solar UV-B radiation (with the side effects of growth reduction), but the positive effects of solar UV-B radiation have not been yet clarified. In this study, two cultivars with different leaf thickness for Chestnut rose (Rosa roxburghii Tratt), a pioneering species for ecological restoration in Karst region of Southwest China, were employed to investigate its responses to ultraviolet-B (UV-B) exclusion, moderate drought, and their combination in an outdoor experiment. Thin-leaf cultivars (Gui 2) adopt an opportunistic growth strategy, growing better than Gui 7 under UV-B exclusion combined with well-watered conditions. To avoid the penetration of solar UV-B wavelengths into the underlying leaf tissues, Gui 2 exhibited higher enhancements in leaf thickness, palisade/spongy tissue ratio, antioxidant responses such as the leaf concentration of flavonols compounds catalyse activity under solar UV-B exposure. Moreover, ambient solar UV-B radiation alleviated the adverse impact caused by drought in both cultivars, improving total biomass and reducing membrane penetration. This alleviation may be related with two potential explanations. First, solar UV-B radiation primes chestnut rose plants with increased antioxidative capacity against drought stress, shown in both antioxidative enzyme activity and non-enzyme antioxidants (in particular, with total flavonols and flavonol ratio [(quercetin+myricetin)/kaempferol]). Second, to avoid nutrition insufficiency, solar UV-B radiation and drought endows chestnut rose plants with complementary effects on nutrition balance. Overall, solar UV-B radiation helps the chestnut tolerate drought stress that occurs frequently in the Karst region by modulating its antioxidative capacity and nutrition balance.