The effects of drought, enhanced UV-B radiation and their combination on plant growth and physiological traits were investigated in a greenhouse experiment in two populations of Populus cathayana Rehder originating from high and low altitude in south-west China. In both populations, drought significantly decreased biomass accumulation and gas exchange parameters, including net CO2 assimilation rate (A), stomatal conductance (gs), transpiration rate (E) and photosynthetic nitrogen use efficiency (PNUE). However, instantaneous water use efficiency (WUEi), transpiration efficiency (WUET), carbon isotope composition (δ13C) and nitrogen (N) content, as well as the accumulation of soluble protein, UV-absorbing compounds and abscisic acid (ABA) significantly increased in response to drought. On the other hand, cuttings from both populations, when kept under enhanced UV-B radiation, showed very similar changes, as under drought, in all above-mentioned parameters.  Compared with the low altitude population, the high altitude population was more tolerant to drought and enhanced UV-B, as indicated by the higher level of biomass accumulation, gas exchange, water-use efficiency, ABA concentration and UV-absorbing compounds.  After one growing season of exposure to different UV-B levels and watering regimes, the decrease in biomass accumulation and gas exchange, induced by drought, was more pronounced under the combination of UV-B and drought. Significant interactions between drought and UV-B were observed in WUEi, WUET, δ13C, soluble protein, UV-absorbing compounds, ABA and in the leaf and stem N, as well as in the leaf and stem C:N ratio.  Our results showed that UV-B acts as an important signal allowing P. cathayana seedlings to respond to drought and that the combination of drought and UV-B may cause synergistically detrimental effects on plant growth.

In a natural environment, plants are typically exposed to several stress factors simultaneously. Alexieva et al. (2001) have shown that UV-B stress exceeds the effects of drought stress on the superoxide dismutase (SOD) and catalase (CAT) activities in pea (Pisum sativum L., cv. Citrina) and wheat (Triticum aestivum L., cv. Centauro). However, UV-B radiation has been reported to alleviate drought stress of plants due to changes in epidermal anatomy (Petropoulou et al., 1995) and increases in carbon assimilation (Sullivan and Teramura, 1989). Meanwhile, interactive responses of Quercus suber L. seedlings to light and moderate water stress have been reported by Puértolas et al. (2008). Studies on the combined effects of UV-B radiation and drought stress on trees have been conducted in some species, e.g., Mediterranean pines (Pinus pinea L.) (Petropoulou et al., 1995), olive (Olea europaea L.) (Nogu˙es and Baker, 2000), Douglas fir (Pseudotsugamenziesii Franco) (Poulson et al., 2002) and willows (Salix myrsinifolia L.) (Turtola et al., 2006), and it appears that species differ in their responses to multiple stress factors. The interaction between UV-B radiation and moderate drought is still not well-understood. Furthermore, evidence from plant research indicates that there can be altitudinal variation in response to UV-B (Hubner and Ziegler, 1998; Rau and Hofmann, 1996). However, few studies have examined the same species from both high and low altitude sites (Yang et al., 2005). Such limited numbers of studies do not allow us to establish any general conclusion, whether high altitude populations differ from low elevation populations in response to UV-B.