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

• Goal: characterization of the relevancy of N source in photosynthetic performance in plant growth under elevated [CO2].
• Nodules increase C sink strength under elevated [CO2], avoiding leaf carbohydrate build-up.
• N2 fixing plants matched plant N requirements with the consequent increase in photosynthetic rates.
• NO3−-fed plants exposure to elevated [CO2] negatively affected leaf C sink/source balance and N assimilation with the consequent photosynthetic down-regulation.

The main goal of this study was to test the effect of [CO2] on C and N management in different plant organs (shoots, roots and nodules) and its implication in the responsiveness of exclusively N2-fixing and NO3−-fed plants. For this purpose, exclusively N2-fixing and NO3−-fed (10 mM) pea (Pisum sativum L.) plants were exposed to elevated [CO2] (1000 μmol mol−1versus 360 μmol mol−1 CO2). 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 [CO2] increased total dry mass (DM) in both N treatments, photosynthetic activity was down-regulated in NO3−-fed plants, whereas N2-fixing plants were capable of maintaining enhanced photosynthetic rates under elevated [CO2]. In the case of N2-fixing plants, the enhanced C sink strength of nodules enabled the avoidance of harmful leaf carbohydrate build up. On the other hand, in NO3−-fed plants, elevated [CO2] 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 N2 fixation matched plant N requirements with the consequent increase in photosynthetic rates, in NO3−-fed plants, exposure to elevated [CO2] negatively affected N assimilation with the consequent photosynthetic down-regulation.