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

Abstract

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.