Single Scots pines (Pinus sylvestris L.) trees were subjected to elevated temperature (year-round elevation), elevated CO2 (elevation from April 15 to September 15), and a combination of elevated temperature and CO2, for 4 years in open-topped chambers. Measurements and modelling were performed to determine if long-term growth at elevated CO2 concentration and temperature altered water use efficiency (WE) and the responses of stomatal conductance (gs) to photon flux density (Qp), the leaf-to-air vapour pressure difference (Dv), leaf temperature (Tl), and intercellular concentration of CO2 (Ci). Long-term elevation of CO2 led to a significant decline in the absolute value of gs at almost all levels of Qp, Dv, Ci, and Tl. Elevated temperature treatment increased the absolute value of gs only at higher Dv and Tl. The effect of the combination of elevated CO2 and temperature did not appear as a mean of the effects of the two single factors, while there was an interaction between the two factors. The modifications in the sensitivity of stomata, resulting from different treatments, did not have the same pattern as the change in gs, but depended on levels of Qp, Dv, and Tl. Compared with the control treatment, elevated concentration of CO2 or a combination of elevated CO2 and temperature led, on average, to 50 and 30% increases in WE, respectively, which can be attributed mainly to an increase in the rate of net assimilation. In contrast, elevated temperature alone did not significantly change WE, although transpiration rate was increased. Key words: long-term CO2 and temperature elevation, stomatal conductance, transpiration, water use efficiency, Pinus sylvestris.
Water Consumption Characteristics and Water Use Efficiency of Winter Wheat under Long-Term Nitrogen Fertilization Regimes in Northwest China
