Nocturnal warming increases photosynthesis at elevated CO2 partial pressure in Populus deltoides

We measured night-time respiration and daytime photosynthesis of leaves in canopies of 4 m tall cottonwood (Populus deltoides) trees to investigate the link between leaf respiration and photosynthetic capacity. Trees were grown at three CO2 partial pressures [p(CO2)(a)] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25degreesC), but constant daytime temperatures (30-32degreesC), in a short-term whole-ecosystem environmental manipulation. Rates of night-time leaf dark respiration (R-d) increased significantly at all growth CO2 partial pressures when nocturnal temperatures were increased from 15 to 25degreesC. Predawn leaf nonstructural carbohydrate (soluble sugars and starch) content was significantly lower at the higher night temperatures. Photosynthetic capacity (A(max)) during the day increased significantly between 15 and 25degreesC at 42 and 80 Pa, but not at 120 Pa. These findings indicate that the previously determined relationships between elevated night-time temperature, dark respiration and increased photosynthetic capacity may also hold at elevated p(CO2)(a). This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.