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 CO 2 partial pressures [ p (CO 2 ) a ] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25 ° C), but constant daytime temperatures (30-32 ° C), in a short-term whole-ecosystem environmental manipulation. "¢ Rates of night-time leaf dark respiration ( R d ) increased significantly at all growth CO 2 partial pressures when nocturnal temperatures were increased from 15 to 25 ° C. 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 25 ° C 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 (CO 2 ) a . This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.