Carbon assimilation and nitrogen in needles of fertilized and unfertilized field-grown Scots pine at natural and elevated concentrations of CO2

Effects of elevated CO2 concentration ([CO2]) on carbon assimilation and needle biochemistry of fertilized and unfertilized 25-30-year-old Scots pine (Pinus sylvestris L.) trees were studied in a branch bag experiment set up in a naturally regenerated stand. In each tree, one branch was enclosed in a bag supplied with ambient [CO2] (360 micromol mol(-1)), a second branch was enclosed in a bag supplied with elevated [CO2] (680 micromol(-1)) and a control branch was left unbagged. The CO2 treatments were applied from April 15 to September 15, starting in 1993 for unfertilized trees and in 1994 for fertilized trees, which were treated with N in June 1994. Net photosynthesis, amount and activity of Rubisco, N, starch, C:N ratio and SLA of needles were measured during the growing season of 1995. Light-saturated net photosynthetic rates of 1-year-old and current-year shoots measured at ambient [CO2] were not affected by growth [CO2] or N fertilization. Elevated [CO2] reduced the amount and activity of Rubisco, and the relative proportion of Rubisco to soluble proteins and N in needles of unfertilized trees. Elevated [CO2] also reduced the chlorophyll concentration (fresh weight basis) of needles of unfertilized trees. Soluble protein concentration of needles was not affected by growth [CO2]. Elevated [CO2] decreased the Rubisco:chlorophyll ratio in unfertilized and fertilized trees. Starch concentration was significantly increased at elevated [CO2] only in 1-year-old needles of fertilized trees. Elevated [CO2] reduced needle N concentration on a dry weight or structural basis (dry weight minus starch) in unfertilized trees, resulting in an increase in needle C:N ratio. Fertilization had no effect on soluble protein, chlorophyll, Rubisco or N concentration of needles. The decrease in the relative proportions of Rubisco and N concentration in needles of unfertilized trees at elevated [CO2] indicates reallocation of N resources away from Rubisco to nonphotosynthetic processes in other plant parts. Acclimation occurred in a single branch exposed to high [CO2], despite the large sink of the tree. The responses of 1-year-old and current-year needles to elevation of growth [CO2] were similar.