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

Abstract

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 nodulatedalfalfa 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 nodulatedalfalfa 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 Vcmax. Photosyntheticdown-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 underelevated CO2 maintained high photosynthetic rates owing to their superior C/N adjustment. The intermediate N treatment, 10 mM NH4NO3, also showed photosyntheticdown-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.