Atmospheric carbon dioxide concentration, nitrogen availability, temperature and the photosynthetic capacity of current-year Norway spruce shoots

Branches of field-grown Norway spruce (Picea abies (L.) Karst.) trees were exposed to either long-term ambient or to elevated CO2 concentrations ([CO2]) using the branch bag technique. The light-saturated photosynthetic rates (A(max)) of current-year shoots differing in nitrogen (N) status were measured at various temperatures and at either ambient (360 [mu mol mol(-1), AMB) or elevated (ambient + 350 mu mol mol(-1), EL) [CO2]. The value of A(max) was determined at various intercellular [CO2]s (A/C-i curves) and used to normalize photosynthetic rates to the mean treatment C-i values, which were 200 mu mol mol(-1) (AMB) and 450 mu mol mol(-1) (EL), respectively. Needle N status and temperature strongly affected A(max) The response to N increased with temperature, and the photosynthetic temperature optimum increased with N status. This was assumed to be a result of reduced mesophyll CO2 conductance. The relative increase of A(max) in the EL treatment compared to the AMB treatment varied from 15 to 90%, and increased with temperature, but decreased with N status. Nevertheless, the absolute photosynthetic response to EL increased with shoot N status. The relative increase in the instantaneous response of A(max) to elevated [CO2] was about 20% higher than the long-term response, i.e., there was downward acclimation in A(max) in response to elevated [CO2]. The photosynthetic temperature optimum increased 4 degreesC with either a short- or a long-term increase in [CO2]. The bag treatment itself increased A(max) by approximately 16% and the temperature optimum of A(max) by approximately 3 degreesC.