Temperature effects on the photosynthetic response of C-3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO2 and temperature on plants possessing the C-3 photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 degrees C mean day temperature and a mean ambient partial pressure of CO2 equal to 350 mu bar; (2) 34 degrees C and 350 mu bar CO2; and (3) 34 degrees C and 750 mu bar CO2. No effect of the growth treatments was observed on the CO2 response of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO2 used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO2 on photosynthetic activity. At low CO2 (< 300 mu bar), an increase in temperature from 25 to 35 degrees C was inhibitory for photosynthesis, while at elevated CO2 (> 400 mu bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO2 enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO2 (195 mu bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO2 levels below its k(m) for CO2. At elevated CO2 (750 mu bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 degrees C. These results indicate that increasing CO2 and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO2 and temperature.