STOMATAL CONDUCTANCE AND TRANSPIRATION IN SHOOTS OF SCOTS PINE AFTER 4-YEAR EXPOSURE TO ELEVATED CO2 AND TEMPERATURE

Single Scots pines (Pinus sylvestris L.) trees were subjected to eleva ted temperature (year-round elevation), elevated CO2 (elevation from A pril 15 to September 15), and a combination of elevated temperature an d CO2 for 4 years in open-topped chambers. Measurements and modelling were performed to determine if long-term growth at elevated CO2 concen tration and temperature altered water use efficiency (W-e) and the res ponses of stomatal conductance (g(s)) to photon flux density (Q(p)), t he leaf-to-air vapour pressure difference (D-v), leaf temperature (T-l ), and intercellular concentration of CO2 (C-i). Long-term elevation o f CO2 led to a significant decline in the absolute value of g(s) at al most all levels of Q(p), D-v, C-i and T-l. Elevated temperature treatm ent increased the absolute value of g(s) only at higher D-v and T-l. T he effect of the combination of elevated CO2 and temperature did not a ppear as a mean of the effects of the two single factors, while there was an interaction between the two factors. The modifications in the s ensitivity of stomata, resulting from different treatments, did not ha ve the same pattern as the change in g(s), but depended on levels of Q (p), D-v, and T-l. Compared with the control treatment, elevated conce ntration of CO2 or a combination of elevated CO2 and temperature led, on average, to 50 and 30% increases in W-E, respectively, which can be attributed mainly to an increase in the rate of net assimilation. In contrast, elevated temperature alone did not significantly change W-E, although transpiration rate was increased.