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

The effects of CO2 enrichment on photosynthesis and ribulose-1,5-bisphospha te carboxylase/oxygenase (Rubisco) in current year and 1-year-old needles o n 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) CO2 partial pressures. At this age trees were 3.5-4 m tall. Measurements ma de late in the growing cycle (March) showed that photosynthetic rates at th e growth CO2 concentration [(pCO(2))(a)] were lower in 1-year-old needles o f trees grown at elevated CO2 concentrations than in those of trees grown a t ambient (pCO(2))(a). At elevated CO2 concentrations V-cmax (maximum carbo xylation rate) was reduced by 13% and J(max) (RuBP regeneration capacity me diated by maximum electron transport rate) by 17%. This corresponded with p hotosynthetic rates at the growth (pCO(2))(a) of 4.68 +/- 0.41 mu mol m(-2) s(-1) and 6.15 +/- 0.46 mu mol m(-2) s(-1) at 36 and 65 Pa, respectively ( an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO(2))(a) were 6.2 +/- 0.72 mu mol m(-2) s(-1) at 36 Pa and 10.15 +/- 0.64 mu 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 accompanie d 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 n ot significantly different in the CO2 treatments. There was no evidence in biochemical parameters for down-regulation at elevated (pCO(2))(a) in fully fexpanded needles of the current year cohort. These data show that enhance ment of photosynthesis continues to occur in needles after 4 years' exposur e to elevated CO2 concentrations. Photosynthetic acclimation reduces the de gree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO2 concentration change during the lifetime of need les, and acclimation may not be apparent in current year needles. This tran sitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carboh ydrates. The implications of such age-dependent responses are that older tr ees, in which the contribution of older needles to the photosynthetic bioma ss is greater than in younger trees, may become progressively more acclimat ed to elevated CO2 concentration.