Intraspecific variation in drought response of Populus cathayana grown under ambient and enhanced UV-B radiation

? 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 (g _s ), transpiration rate (E) and photosynthetic nitrogen use efficiency (PNUE). However, instantaneous water use efficiency (WUE _i ), transpiration efficiency (WUE _T ), 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 WUE _i , WUE_T, 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.