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Lewis, J.D., David Olszyk, and David.T. Tingey. 1999. Seasonal patterns of photosynthetic light response in Douglas-fir seedlings subjected to elevated atmospheric CO2 and temperature. Tree Physiol.19:243-252.

Increases in atmospheric CO2 and temperature are predicted to increase the light response of photosynthesis by increasing light-saturated photosynthetic rates and apparent quantum yields. In this study, we examined the interactive effects of elevated atmospheric CO2 and temperature on the light response of photosynthesis in Douglas-fir (Psuedotsuga menziesi(Mirb.) France) seedlings. Seedlings were grown in sunlit chambers controlled to track either ambient (-400 ppm) CO2 or ambient +200 ppm CO2 at ambient temperature of ambient +4C. Photosynthetic light response curves were measured during the winter and during the growing season over an 18 -month period beginning 32 months after treatments were initiated. Light response curves were measured at the growth CO2 concentration, and were used to calculate the light-saturated rate of photosynthesis, light compensation point, quantum yield and respiration rate. Elevated CO2 increased apparent quantum yields at 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 CO2 concentration or temperature. These results suggest that downregulation of photosynthesis occurred in the elevated CO2 treatments such as carbon uptake at a given light level was similar across CO2 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 will be offset by increased carbon efflux through respiration.

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