Photosynthetic responses of Scots pine to elevated CO2 and nitrogen supply: Results of a branch-in-bag experiment

Naturally seeded Scots pine (Pinus sylvestris L.) trees, age 25-30 years, were subjected to two soil-nitrogen-supply regimes and to elevated atmospheric CO2 concentrations by the branch-in-bag method from April 15 to September 15 for two or three years. Gas exchange in detached shoots was measured in a diffuse radiation field. Seven parameters associated with photosynthetic performance and two describing stomatal conductance were determined to assess the effects of treatments on photosynthetic components. An elevated concentration of CO2 did not lead to a significant downward regulation in maximum carboxylation rate (V-cmax) or maximum electron transport rate (J(max)), but it significantly decreased light-saturated stomatal conductance (g(sat)) and increased minimum stomatal conductance (g(min)). Light-saturated rates of CO2 assimilation were higher (24-31 %) in shoots grown and measured at elevated CO2 concentration than in shoots grown and measured ured at ambient CO2 concentration, regardless of treatment time or nitrogen-supply regime. High soil-nitrogen supply significantly increased photosynthetic capacity, corresponding to significant increases in V-cmax and J(max). However, the combined elevated CO2 + high nitrogen-supply treatment did not enhance the photosynthetic response above that observed in the elevated CO2 treatment alone.