Influences of Drought Stress on Photosynthetic Characteristics and Water Use Efficiency of 4 Tree Species under Elevated CO2 Concentration

Five-year-old Pinus tabulaeformis and Platycladus orientalis and three-year-old Acer truncatum and Robinia pseudoacacia saplings were exposed to 720 μmol·mol-1 CO2 for 13 months. The leaf photosynthetic ratio (Pn), internal CO2 concentration (Ci), transpiration ratio (Tr) and stomatal conductance (Cond) were measured by Li-6400. The leaf carbon isotope ratio (δ13C) was measured by Isotope Ratio Mass Spectrometer. Under normal water and heavy drought conditions, the Pn, Ci and WUEi of the 4 tree species increased, while the Tr and Cond reduced along with the CO2 concentration. Under mild drought and moderate drought conditions, the Pn, Ci, Tr, Cond and WUEi increased along with CO2 concentration. The WUEi of R. pseudoacacia under 720 μmol·mol-1 CO2 concentration reduced along with CO2 concentration under mild drought, moderate drought and heavy drought conditions. In the same drought condition, the leaf δ13C values increased along with CO2 concentration. The Pn, Tr and Cond reduced along with drought condition. The leaf WUEi and δ13C values of the 4 tree species increased under 720 μmol·mol-1 CO2 concentration, while A. truncatum and R. pseudoacacia under 380 μmol·mol-1 CO2 concentration increased under mild drought and moderate drought conditions and then reduced under heavy drought condition. The interaction of elevated CO2 concentration and drought stress decreased the sensitivity of stomata, which changed quickly under elevated CO2 concentration or drought stress, and postponed the occurrence of drought stress.