Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). II. Photosynthetic capacity and nitrogen use efficiency

Four clones of Sitka spruce (Picea sitchensis (Bong.) Carr.) from two prove nances, at 53.2 degrees N (Skidegate a and Skidegate b) and at 41.3 degrees N (North Bend a and North Bend b, were grown for three growing seasons in ambient (similar to 350 mu mol mol(-1)) and elevated (similar to 700 mu mol mol(-1)) CO2 concentrations. The clones were grown in stress-free conditio ns (adequate nutrition and water) to assess the effect of elevated [CO2] on tree physiology. Growth in elevated [CO2] significantly increased instanta neous photosynthetic rates of the clonal Sitka spruce saplings by about 62% . Downward acclimation of photosynthesis (A) was found in all four clones g rown in elevated [CO2]. Rubisco activity and total chlorophyll concentratio n were also significantly reduced in elevated [CO2]. Provenance did not inf luence photosynthetic capacity. Best-fit estimates of J(max) (maximum rate of electron transport), V-cmax (RuBP-saturated rate of Rubisco) and A(max) (maximum rate of assimilation) were derived from responses of A to intercel lular [CO2] by using the model of Farquharetal.(1980). At any leaf N concen tration, the photosynthetic parameters were reduced by growth in elevated [ CO2]. However, the ratio between J(max) and V-cmax was unaffected by CO2 gr owth concentration, indicating a tight coordination in the allocation of N between thylakoid and soluble proteins. In elevated [CO2] the more southerl y clones had a higher initial N use efficiency (more carbon assimilated per unit of leaf N) than the more northerly clones, so that they had more N av ailable for those processes or organs that were most limiting to growth at a particular time. This may explain the initial higher growth stimulation b y elevated [CO2] in the North Bend clones than in the Skidegate clones.