Photosynthetic down-regulation under elevated CO₂ exposure can be prevented by nitrogen supply in nodulated alfalfa

Increasing atmospheric CO₂ concentrations are expected to enhance plant photosynthesis and yield. Nevertheless, after long-term exposure, plants acclimate and show a reduction in photosynthetic activity (called down-regulation), which may cause a reduction in potential yield. Some authors suggest that down-regulation is related to nutrient availability, and more specifically, to an insufficient plant C sink strength caused by limited N supply. In this paper, we tested whether or not N availability prevents down-regulation of photosynthesis in nodulated alfalfa plants (Medicago sativa L.). To do so, we examined the effect of the addition of different levels of NH₄NO₃ (0, 10, and 15 mM) to 30-day-old nodulated alfalfa plants exposed to ambient (approximately 400 μmol mol⁻¹) or elevated CO₂ (700 μmol mol⁻¹) during a period of 1 month in growth chambers. After 2 weeks of exposure to elevated CO₂, no significant differences were observed in plant growth or photosynthesis rates. After 4 weeks of treatment, exclusively N₂ fixing alfalfa plants (0 mM NH₄NO₃) showed significant decreases in photosynthesis and Vc(max). Photosynthetic down-regulation of these plants was caused by the C/N imbalance as reflected by the carbohydrate and N data. On the other hand, plants supplied with 15 mM NH₄NO₃ grown under elevated CO₂ maintained high photosynthetic rates owing to their superior C/N adjustment. The intermediate N treatment, 10 mM NH₄NO₃, also showed photosynthetic down-regulation, but to a lesser degree than with 0 mM treatment. The present study clearly shows that external N supply can reduce or even avoid acclimation of photosynthesis to elevated CO₂ as a consequence of the increase in C sink strength associated with N availability.