Pea plant responsiveness under elevated [CO2] is conditioned by the N source (N2 fixation versus NO3− fertilization)

The main goal of this study was to test the effect of [CO 2 ] on C and N management in different plant organs (shoots, roots and nodules) and its implication in the responsiveness of exclusively N 2 -fixing and NO 3 − -fed plants. For this purpose, exclusively N 2 -fixing and NO 3 − -fed (10 mM) pea ( Pisum sativum L.) plants were exposed to elevated [CO 2 ] (1000 μmol mol −1 versus 360 μmol mol −1 CO 2 ). Gas exchange analyses, together with carbohydrate, nitrogen, total soluble proteins and amino acids were determined in leaves, roots and nodules. The data obtained revealed that although exposure to elevated [CO 2 ] increased total dry mass (DM) in both N treatments, photosynthetic activity was down-regulated in NO 3 − -fed plants, whereas N 2 -fixing plants were capable of maintaining enhanced photosynthetic rates under elevated [CO 2 ]. In the case of N 2 -fixing plants, the enhanced C sink strength of nodules enabled the avoidance of harmful leaf carbohydrate build up. On the other hand, in NO 3 − -fed plants, elevated [CO 2 ] caused a large increase in sucrose and starch. The increase in root DM did not contribute to stimulation of C sinks in these plants. Although N 2 fixation matched plant N requirements with the consequent increase in photosynthetic rates, in NO 3 − -fed plants, exposure to elevated [CO 2 ] negatively affected N assimilation with the consequent photosynthetic down-regulation.