Photosynthetic Activity of Leaves of Pinus radiata and Nothofagus fusca After 1 Year of Growth at Elevated CO₂

Abstract

Radiata pine (Pinus radiata D.Don) and red beech (Nothofagus fusca (Hook. f.) Oerst.) were grown for over 1 year at elevated (ELEV, 64 Pa) and ambient (AMB, 38 Pa) CO₂ partial pressure in open-top chambers. Springtime measurements of overwintering leaves showed that light- and CO₂-saturated photosynthetic rates (A_max) of pine leaves were similar for the two treatments (AMB: 6.7 ± 1.08 μmol m^-2 s^-1, mean ± 1 s.e.; ELEV: 6.6 ± 0.47) but, for beech leaves, A_max was greater for AMB plants (8.8 ± 0.90 μmol m^-2 s^-1) than for ELEV plants (6.10 ± 0.71). Summertime measurements of leaves grown that spring showed that for pine, A_max was similar in the two CO₂ treatments (AMB 14.9 μmol m^-2 s^-1 ± 0.80; ELEV: 13.5 ± 1.9) while, for beech, A_max was higher in AMB plants (21.0 ± 1.1) than in ELEV plants (17.2 ± 1.9), although the difference was not statistically significant. These results indicate downregulation of photosynthetic capacity of beech but not pine. V_cmax did not differ between treatments within species, suggesting that there was no acclimation of rubisco activity. Triose phosphate utilisation limitation may have contributed to the downregulation of A_max in beech. For pine, photosynthesis at treatment CO₂ partial pressures was greater in ELEV plants in both spring and summer. For beech measured at treatment CO₂ partial pressures, photosynthesis was greater in ELEV plants in summer, but was similar between treatments in the springtime.