Effects of season, needle age and elevated atmospheric CO2 on photosynthesis in Scots pine (Pinus sylvestris)

Five-year-old Scots pine (Pinus sylvestris L.) seedlings were grown in open-top chambers at ambient and elevated (ambient + 400 mu mol mol(-1)) CO2 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 CO2 enrichment. The elevated CO2 treatment stimulated photosynthetic rates when measured at the growth CO2 concentration, but decreased photosynthetic capacity compared with the ambient CO2 treatment. Acclimation to elevated CO2 involved decreases in carboxylation efficiency and RuBP regeneration capacity. Compared with the ambient CO2 treatment, elevated CO2 reduced light-saturated photosynthesis (when measured at 350 mu mol 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 CO2 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.