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 provenances, 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 conditions (adequate nutrition and water) to assess the effect of elevated [CO2] on tree physiology. Growth in elevated [CO2] significantly increased instantaneous photosynthetic rates of the clonal Sitka spruce saplings by about 62%. Downward acclimation of photosynthesis (A) was found in all four clones grown in elevated [CO2]. Rubisco activity and total chlorophyll concentration were also significantly reduced in elevated [CO2]. Provenance did not influence 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 intercellular [CO2] by using the model of Farquharetal.(1980). At any leaf N concentration, the photosynthetic parameters were reduced by growth in elevated [CO2]. However, the ratio between J(max) and V-cmax was unaffected by CO2 growth concentration, indicating a tight coordination in the allocation of N between thylakoid and soluble proteins. In elevated [CO2] the more southerly 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 available for those processes or organs that were most limiting to growth at a particular time. This may explain the initial higher growth stimulation by elevated [CO2] in the North Bend clones than in the Skidegate clones.