Photosynthetic responses to needle water potentials in Scots pine after a four-year exposure to elevated CO2 and temperature

Abstract

Effects of needle water potential (Ψ1) on gas exchange of Scots pine (Pinus sylvestris L.) grown for 4 years in open-top chambers with elevated temperature (ET), elevated CO2 (EC) or a combination of elevated temperature and CO2 (EC + ET) were examined at a high photon flux density (PPFD), saturated leaf to air water vapor pressure deficit (VPD) and optimal temperature (T). We used the Farquhar model of photosynthesis to estimate the separate effects of (Ψ1) and the treatments on maximum carboxylation efficiency (V(X,max), ribulose-1,5-bisphosphate regeneration capacity (J), rate of respiration in the light (R(d)), intercellular partial pressure of CO2 (C(i)) and stomatal conductance (G(s)). Depression of CO2 assimilation rate at low (Ψ1) was the result of both stomatal and non-stomatal limitations on photosynthetic processes; however, stomatal limitations dominated during short-term water stress ((Ψ1) < -1.2 MPa), whereas non- stomatal limitations dominated during severe water stress. Among the non- stomatal components, the decrease in J contributed more to the decline in photosynthesis than the decrease in V(C, max). Long-term elevation of CO2 and temperature led to differences in the maximum values of the parameters, the threshold values of (Ψ1) and the sensitivity of the parameters to decreasing (Ψ1). The CO2 treatment decreased the maximum values of V(C, max), J and R(d) but significantly increased the sensitivity of V(C, max), J and R(d) to decreasing (Ψ1) (P < 0.05). The effects of the ET and EC + ET treatments on V(c,max) J and R(d) were opposite to the effects of the EC treatment on these parameters. The values of G(s), which were measured simultaneously with maximum net rate of assimilation (A(max)), declined in a curvilinear fashion as (Ψ1) decreased. Both the EC + ET and ET treatments significantly decreased the sensitivity of G(s) to decreasing (Ψ1). We conclude that, in the future, acclimation to increased atmospheric CO2 and temperature could increase the tolerance of Scots pine to water stress.