Effects of elevated CO2 and soil-nitrogen supply on chlorophyll fluorescence and gas exchange in Scots pine, based on a branch-in-bag experiment

Applying the branch-in-bag method, naturally seeded Scots pine (Pinus sylvestris L.) trees, 25-30 yr old, were subjected to two CO2 concentrations (350 and 700 mu mol mol(-1)) and two soil-nitrogen-supply regimes for three growing seasons (1994-96). Gas exchange and chlorophyll a fluorescence in detached shoots were measured simultaneously in a diffuse radiation field. Elevated CO2 did not lead to a significant 'downward regulation' in the light-saturated rate of net photosynthesis (P-n.max), the maximum apparent quantum yield (alpha(A.max)) or the maximum photochemical efficiency (F-v/F-m) of photosystem II (PS II). However, the elevated CO2 significantly decreased the light-saturated stomatal conductance and increased the sensitivity of stomatal conductance to change in low photon-flux densities. The high soil-nitrogen supply significantly increased photosynthetic capacity, as manifested by increases in P-n.max, alpha(A.max), F-v/F-m, and the effective photochemical efficiency (Delta F/F'(m)) at low photon-flux densities, did not, on the other hand, enhance the magnitude of photosynthetic response to elevated CO2 concentration. In addition, the treatment-induced modifications in fluorescence parameters are discussed in detail.