Photosynthetic acclimation to long‐term exposure to elevated CO₂ concentration in Pinus radiata D. Don. is related to age of needles

The effects of CO 2 enrichment on photosynthesis and ribulose‐1,5‐bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1‐year‐old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open‐top chambers at ambient (36 Pa) and elevated (65 Pa) CO 2 partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO 2 concentration [( p CO 2 ) a ] were lower in 1‐year‐old needles of trees grown at elevated CO 2 concentrations than in those of trees grown at ambient ( p CO 2 ) a . At elevated CO 2 concentrations V cmax (maximum carboxylation rate) was reduced by 13% and J max (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth ( p CO 2 ) a of 4·68 ± 0·41 μ mol m –2 s –1 and 6·15 ± 0·46 μ mol m –2 s –1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth ( p CO 2 ) a were 6·2 ± 0·72 μ mol m –2 s –1 at 36 Pa and 10·15 ± 0·64 μ mol m –2 s –1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1‐year‐old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1‐year‐old needles were not significantly different in the CO 2 treatments. There was no evidence in biochemical parameters for down‐regulation at elevated ( p CO 2 ) a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO 2 concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO 2 concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age‐dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO 2 concentration.