Carbon partitioning in N₂ fixing Medicago sativa plants exposed to different CO₂ and temperature conditions

Many of the studies analysing the CO₂ effect on plant development have been conducted in optimal growth conditions. Furthermore, although some of those studies suggest that legumes might show a steady productivity increase with rising CO₂, the role of nodule activity on the plant responsiveness to predicted atmospheric CO₂ enhancement is not well understood. In this study, C (metabolism and allocation) and N (nodule activity) interaction between the plant and the bacterial symbiont during the photosynthetic acclimation of N₂-fixing alfalfa (Medicago sativa L. cv. Aragón) plants exposed to elevated CO₂ and temperature conditions was analysed. The plants were grown in temperature gradient greenhouses (TGG) where, in the case of elevated CO₂ treatments, the isotopic ¹³C/¹²C composition (δ¹³C) inside the TGG was modified. Compared with the corresponding temperature treatment, exposure to 700 μmol mol^-1 CO₂ enhanced dry mass (DM) of plants in elevated temperature treatments (26%), whereas no significant effect was detected in ambient temperature treatments. The δ¹³C data revealed that although all the carbon corresponding to leaf total organic matter (TOM) came from newly assimilated C, plants exposed to elevated CO₂ did not develop strong sink activity (especially in ambient temperature conditions). Leaf carbohydrate build-up induced reduction in the Rubisco (E.C. 4.1.1.39) carboxylation capacity of plants. Despite this reduction in Rubisco content, plants exposed to elevated CO₂ conditions maintained (at ambient temperature) or increased (at elevated temperature) photosynthetic rates (measured at growth conditions) by increasing N use efficiency. The larger C sink strength of nodules in plants grown at elevated CO₂ and temperature conditions did not contribute towards overcoming photosynthetic acclimation. Further, the inhibitory effect of CO₂ on nodule total activity was caused by a large depletion in total soluble protein (TSP) of nodules. Depletion of leaf N demand, together with the reduction in nodule carbohydrate availability (as reflected by the nodule starch concentration), negatively affected the nodule TSP content and enzymatic activity.