PHOTOSYNTHETIC RESPONSES TO NEEDLE WATER POTENTIALS IN SCOTS PINE AFTER A 4-YEAR EXPOSURE TO ELEVATED CO2 AND TEMPERATURE

Effects of needle water potential (psi(1)) on gas exchange of Scots pi ne (Pinus sylvestris L.) grown for 4 years in open-top chambers with e levated temperature (ET), elevated CO2 (EC) or a combination of elevat ed temperature and CO2 (EC + ET) were examined at a high photon flux d ensity (PPFD), saturated leaf to air water vapor pressure deficit (VPD ) and optimal temperature (T). We used the Farquhar model of photosynt hesis to estimate the separate effects of psi 1 and the treatments on maximum carboxylation efficiency (V-c,V-max), ribulose-1 ,5-bisphospha te regeneration capacity (J), rate of respiration in the light (R(d)), intercellular partial pressure of CO2 (Ci) and stomatal conductance ( G(s)). Depression of CO2 assimilation rate at low psi(1) was the resul t of both stomatal and non-stomatal limitations on photosynthetic proc esses; however, stomatal limitations dominated during short-term water stress (psi(1)<-1.2 MPa), whereas nonstomatal limitations dominated d uring severe water stress. Among the nonstomatal components, the decre ase in J contributed more to the decline in photosynthesis than the de crease in Long-term elevation of CO2 and temperature led to difference s in the maximum values of the parameters, the threshold values of psi (1) and the sensitivity of the parameters to decreasing psi(1). The CO 2 treatment decreased the maximum values of V-c,V-max J and R(d) but s ignificantly increased the sensitivity of V-c,V-max J and Rd to decrea sing psi(1) (P < 0.05). The effects of the ET and EC + ET treatments o n V-c,V-max J and R(d) were opposite to the effects of the EC treatmen t on these parameters. The values of G(s), which were measured simulta neously with maximum net rate of assimilation (A(max)), declined in a curvilinear fashion as psi(1) decreased. Both the EC + ET and ET treat ments significantly decreased the sensitivity of G(s) to decreasing ps i(1-). We conclude that, in the future, acclimation to increased atmos pheric CO2 and temperature could increase the tolerance of Scots pine to water stress.