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

Effects of elevated CO2 concentration ([CO2]) on carbon assimilation and ne edle 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 mu mol mol(-1)), a second branch was enclosed in a bag supplied with elevated [CO2] (680 mu mol mol(-1)) and a control branch was left unbagged. The CO2 treatments were applied from Apri l 15 to September 15, starting in 1993 for unfertilized trees and in 1994 f or fertilized trees, which were treated with N in June 1994. Net photosynth esis, amount and activity of Rubisco, N, starch, C:N ratio and SLA of needl es were measured during the growing season of 1995. Light-saturated net photosynthetic rates of 1-year-old and current-year sho ots measured at ambient [CO2] were not affected by growth [CO2] or N fertil ization. Elevated [CO2] reduced the amount and activity of Rubisco, and the relative proportion of Rubisco to soluble proteins and N in needles of unf ertilized trees. Elevated [CO2] also reduced the chlorophyll concentration (fresh weight basis) of needles of unfertilized trees. Soluble protein conc entration of needles was not affected by growth [CO2]. Elevated [CO2] decre ased the Rubisco:chlorophyll ratio in unfertilized and fertilized trees. St arch concentration was significantly increased at elevated [CO2] only in 1- year-old needles of fertilized trees. Elevated [CO2] reduced needle N conce ntration on a dry weight or structural basis (dry weight minus starch) in u nfertilized trees, resulting in an increase in needle C:N ratio. Fertilizat ion had no effect on soluble protein, chlorophyll, Rubisco or N concentrati on of needles. The decrease in the relative proportions of Rubisco and N co ncentration in needles of unfertilized trees at elevated [CO2] indicates re allocation of N resources away from Rubisco to nonphotosynthetic processes in other plant parts. Acclimation occurred in a single branch exposed to hi gh [CO2], despite the large sink of the tree. The responses 1-year-old and current-year needles to elevation of growth [CO2] were similar.