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

Abstract

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 μmol 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 (α_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, α_A.max, F_v/F_m, and the effective photochemical efficiency (ΔF/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.