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Five-year-old Scots pine ( Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 μmol mol −1 ) CO 2  concentrations. Net photosynthesis ( A ), specific leaf area (SLA) and concentrations of nitrogen (N), carbon (C), soluble sugars, starch and chlorophyll were measured in current-year and 1-year-old needles during the second year of CO 2  enrichment. The elevated CO 2  treatment stimulated photosynthetic rates when measured at the growth CO 2  concentration, but decreased photosynthetic capacity compared with the ambient CO 2  treatment. Acclimation to elevated CO 2  involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO 2  treatment, elevated CO 2  reduced light-saturated photosynthesis (when measured at 350 μmol mol −1  in both treatments) by 18 and 23% (averaged over the growing season) in current-year and 1-year-old needles, respectively. We observed significant interactive effects of CO 2  treatment, needle age and time during the growing season on photosynthesis. Large seasonal variations in photosynthetic parameters were attributed to changes in needle chemistry, needle structure and feedbacks governed by whole-plant growth dynamics. Down-regulation of photosynthesis was probably a result of reduced N concentration on an area basis, although a downward shift in the relationship between photosynthetic parameters and N was also observed.

Effects of season, needle age and elevated atmospheric CO 2 on photosynthesis in Scots pine ( Pinus sylvestris )