Seasonal patterns of photosynthetic light response in Douglas-fir seedlings subjected to elevated atmospheric CO(2) and temperature.
Increases in atmospheric CO(2) concentration and temperature are predicted to increase the light response of photosynthesis by increasing light-saturated photosynthetic rates and apparent quantum yields. We examined the interactive effects of elevated atmospheric CO(2) concentration and temperature on the light response of photosynthesis in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings. Seedlings were grown in sunlit chambers controlled to track either ambient (~400 ppm) CO(2) or ambient + 200 ppm CO(2), at ambient temperature or ambient + 4 degrees C. Photosynthetic light response curves were measured over an 18-month period beginning 32 months after treatments were initiated. Light-response curves were measured at the growth CO(2) concentration, and were used to calculate the light-saturated rate of photosynthesis, light compensation point, quantum yield and respiration rate. Elevated CO(2) increased apparent quantum yields during two of five measurement periods, but did not significantly affect light-saturated net photosynthetic rates, light compensation points or respiration rates. Elevated temperature increased all parameters. There were no significant interactions between CO(2) concentration and temperature. We conclude that down-regulation of photosynthesis occurred in the elevated CO(2) treatments such that carbon uptake at a given irradiance was similar across CO(2) treatments. In contrast, increasing temperature may substantially increase carbon uptake rates in Douglas-fir, assuming other environmental factors do not limit photosynthesis; however, it is not clear whether the increased carbon uptake will increase growth rates or be offset by increased carbon efflux through respiration.
Lewis
J. D.
JD
National Research Council, 200 S.W. 35th Street, Corvallis, OR 97333, USA.
Olszyk
D.
D
Tingey
D. T.
DT