Responses of photosynthesis and respiration of wheat leaves to elevated CO2 are not constrained by soil nitrogen

Low nitrogen (N) availability could attenuate the enhancement of leaf photosynthesis under elevated CO 2 concentration (eCO 2 ), but there is still conflicting evidence. Moreover, the response of leaf respiration to eCO 2 and N supplementation is not well understood. To explore the effects of eCO 2 and N availability on leaf photosynthesis and respiration, wheat plants ( Triticum aestivum L.) were grown under ambient (420 μmol mol −1 ) and elevated (840 μmol mol −1 ) CO 2 concentrations in combination with low (2 mmol L⁻¹) and high N (8 mmol L⁻¹ nitrate) treatments. We found that eCO 2 decreased leaf N content but significantly increased the net photosynthesis rate ( A n ) of young leaves by c. 10 % under both N levels, and there was no significant interaction between eCO 2 and N treatments on A n . eCO 2 decreased the A n of old leaves, which may be due to the accelerated leaf senescence at eCO 2 . Leaf respiration in the light ( R L ) and darkness ( R Dk ) decreased under eCO 2 in both N treatments for both leaf age classes, and eCO 2 amplified the increase in leaf respiration with leaf N. Our results showed that the response of photosynthesis to eCO 2 was not limited by soil N availability, and eCO 2 modulated the relationship between leaf respiration and leaf [N], providing implications for understanding and modeling CO 2 fertilization. • Both low and high nitrogen treatments resulted in consistent stimulation of leaf photosynthesis by elevated CO 2 . • Leaf respiration in the light and darkness decreased under elevated CO 2 • Elevated CO 2 and low N supply increased the nitrogen sensitivity of leaf respiration