Unravelling the mechanisms that improve photosynthetic performance of N2-fixing pea plants exposed to elevated [CO2]

Although the predicted enhanced photosynthetic rates of plants exposed to elevated [CO2] are expected to increase carbohydrate and plant growth, recent findings have shown a complex regulation of these processes. The aim of this study was to determine the effect of elevated [CO2] on pathways leading to the main forms of leaf C storage (starch) and export (sucrose) and the implications of this increased [CO2] on photosynthetic performance of exclusively N2 fixing plants. For this purpose, exclusively N2-fixing pea plants were exposed to elevated [CO2] (1000 μmol mol−1 versus 360 μmol mol−1 CO2). The data obtained highlighted that plants exposed to elevated [CO2] were capable of maintaining hexose levels (involved in Rubisco down regulation) at control levels with the consequent avoidance of photosynthetic acclimation. More specifically, in plants exposed to elevated [CO2] there was an increase in the activity of pathways involved in the main forms of leaf C storage (starch) and export (sucrose). Furthermore, the study highlighted that although starch content increased by up to 40% under elevated [CO2], there was also an increase in the proteins and compounds involved in starch degradation. Such a finding, together with an increase in the activity of proteins involved in sucrose synthesis revealed that these plants up-regulated the sucrose synthesis pathway in order to meet the large nodule photoassimilate requirements. As a consequence, the study highlighted the relevance of controlling the activity of pathways that determine leaf cellular carbohydrate availability and how this is linked with C-demanding organs such as nodules.