Stomatal conductance, not biochemistry, drives low temperature acclimation of photosynthesis in Populus balsamifera, regardless of nitrogen availability

Low-temperature thermal acclimation may require adjustments to N and water use to sustain photosynthesis because of slow enzyme functioning and high water viscosity. However, understanding of photosynthetic acclimation to temperatures below 11 degrees C is limited. We acclimated Populus balsamifera to 6 degrees C and 10 degrees C (6A and 10A, respectively) and provided the trees with either high or low N fertilizer. We measured net CO2 assimilation (A(net)), stomatal conductance (g(s)), maximum rates of Rubisco carboxylation (V-cmax), electron transport (J(max)) and dark respiration (R-d) at leaf temperatures of 2, 6, 10, 14 and 18 degrees C, along with leaf N concentrations. The 10A trees had higher A(net) than the 6A trees at warmer leaf temperatures, which was correlated with higher g(s) in the 10A trees. The instantaneous temperature responses of V-cmax, J(max) and R-d were similar for trees from both acclimation temperatures. While soil N availability increased leaf N concentrations, this had no effect on acclimation of photosynthesis or respiration. Our results indicate that acclimation below 11 degrees C occurred primarily through changes in stomatal conductance, not photosynthetic biochemistry, and was unaffected by short-term N supply. Thermal acclimation of stomatal conductance should therefore be a priority for future carbon cycle model development.