Carbon assimilation and nitrogen in needles of fertilized and unfertilized field-grown Scots pine at natural and elevated concentrations of CO 2
AbstractEffects of elevated CO 2 concentration ((CO 2 )) 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 (CO 2 ) (360 μmol mol −1 ), a second branch was enclosed in a bag supplied with elevated (CO 2 ) (680 μmol mol −1 ) and a control branch was left unbagged. The CO 2 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 (CO 2 ) were not affected by growth (CO 2 ) or N fertilization. Elevated (CO 2 ) reduced the amount and activity of Rubisco, and the relative proportion of Rubisco to soluble proteins and N in needles of unfertilized trees. Elevated (CO 2 ) also reduced the chlorophyll concentration (fresh weight basis) of needles of unfertilized trees. Soluble protein concentration of needles was not affected by growth (CO 2 ). Elevated (CO 2 ) decreased the Rubisco:chlorophyll ratio in unfertilized and fertilized trees. Starch concentration was significantly increased at elevated (CO 2 ) only in 1-year-old needles of fertilized trees. Elevated (CO 2 ) 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 (CO 2 ) indicates reallocation of N resources away from Rubisco to nonphotosynthetic processes in other plant parts. Acclimation occurred in a single branch exposed to high (CO 2 ), despite the large sink of the tree. The responses of 1-year-old and current-year needles to elevation of growth (CO 2 ) were similar.