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 meas ured at various temperatures and at either ambient (360 [mu mol mol(-1), AM B) 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 wer e 200 mu mol mol(-1) (AMB) and 450 mu mol mol(-1) (EL), respectively. Needl e N status and temperature strongly affected A(max) The response to N incre ased with temperature, and the photosynthetic temperature optimum increased with N status. This was assumed to be a result of reduced mesophyll CO2 co nductance. The relative increase of A(max) in the EL treatment compared to the AMB treatment varied from 15 to 90%, and increased with temperature, bu t decreased with N status. Nevertheless, the absolute photosynthetic respon se to EL increased with shoot N status. The relative increase in the instan taneous 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 respo nse to elevated [CO2]. The photosynthetic temperature optimum increased 4 d egreesC with either a short- or a long-term increase in [CO2]. The bag trea tment itself increased A(max) by approximately 16% and the temperature opti mum of A(max) by approximately 3 degreesC.