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

Abstract

Low nitrogen (N) availability could attenuate the enhancement of leaf photosynthesis under elevated CO₂ concentration (eCO₂), but there is still conflicting evidence. Moreover, the response of leaf respiration to eCO₂ and N supplementation is not well understood. To explore the effects of eCO₂ 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) CO₂ concentrations in combination with low (2 mmol L⁻¹) and high N (8 mmol L⁻¹ nitrate) treatments. We found that eCO₂ 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 eCO₂ and N treatments on A_n. eCO₂ decreased the A_n of old leaves, which may be due to the accelerated leaf senescence at eCO₂. Leaf respiration in the light (R_L) and darkness (R_Dk) decreased under eCO₂ in both N treatments for both leaf age classes, and eCO₂ amplified the increase in leaf respiration with leaf N. Our results showed that the response of photosynthesis to eCO₂ was not limited by soil N availability, and eCO₂ modulated the relationship between leaf respiration and leaf [N], providing implications for understanding and modeling CO₂ fertilization.