Effects of elevated CO2 on chloroplast components, gas exchang

dry matter production in oak (Quercus robur L.) seedlings and clonal cherry (Prunus avium L. × pseudocerasus Lind.) plants were measured during 19 months of growth in climate-controlled greenhouses at ambient (350 vpm) or elevated (700 vpm) CO2. In both species, the elevated CO2 treatment increased the PPFD saturated-rate of photosynthesis and dry matter production. After two months at elevated CO2, Prunus plants showed significant increases in leaf (55%) and stem (61%) dry mass but not in root dry mass. However, this initial stimulation was not sustained: treatment differences in net assimilation rate (A) and plant dry mass were less after 10 months of growth than after 2 months of growth, suggesting acclimation of A to elevated CO2 in Prunus. In contrast, after 10 months of growth at elevated CO2, leaf dry mass of Quercus increased (130%) along with shoot (356%) and root (219%) dry mass, and A was also twice that of plants grown and measured at ambient CO2. The amounts of Rubisco and the thylakoid-bound protein cytochrome f were higher in Quercus plants grown for 19 months in elevated CO2 than in control plants, whereas in Prunus there was less Rubisco in plants grown for 19 months in elevated CO2 than in control plants. Exposure to elevated CO2 for 10 months resulted in increased mean leaf area in both species and increased abaxial stomatal density in Quercus. There was no change in leaf epidermal cell size in either species in response to the elevated CO2 treatment. The lack of acclimation of photosynthesis in oak grown at elevated CO2 is discussed in relation to the production and allocation of dry matter. We propose that differences in carbohydrate utilization underlie the differing long-term CO2 responses of the two species.