In a previous study, we noticed a significant reduction in leaf nitrogen concentration in the phase that precedes the maturation of sugarcane in field conditions. Although we know that there is a strong relationship between nitrogen and photosynthesis in higher plants, the processes underlying the assimilation of CO2 affected by nitrogen are poorly understood in C4 plants, such as sugarcane. Low photosynthesis associated with low leaf nitrogen content could be a consequence of the inefficiency of the CO2 concentration mechanism in the bundle-sheath cells, where the carboxylation by the enzyme Rubisco occurs. In C4 metabolism, not only the enzymes PEPC and Rubisco have a key role in assimilation of CO2, but also the resistances that determine the CO2 concentration at the carboxylation sites. The importance of stomatal conductance for photosynthesis has been extensively studied, while mesophyll conductance is a current research topic on C4 photosynthesis. However, the role of bundle-sheath conductance has been underestimated to such an extent that we do not know the importance of this variable in photosynthesis regulation of C4 plants such as sugarcane. Considering the current literature, this proposal has as hypothesis that low leaf nitrogen content reduces sugarcane photosynthesis due to decreases in activities of both carboxylation enzymes PEPC and Rubisco, without affecting the mechanism of CO2 concentration in bundle-sheath cells. The proposal also aims to reveal the relative importance of stomatal, mesophyll and bundle-sheath conductances for regulation of sugarcane photosynthesis under varying nitrogen availability. The major photosynthetic variables linked to C4 metabolism in mesophyll cells and C3 metabolism in bundle-sheath cells will be estimated in vivo using a biochemical model of C4 photosynthesis based on combined measurements of leaf gas exchange and chlorophyll fluorescence. These evaluations will be carried out in sugarcane plants grown under five nitrogen doses, with leaf nitrogen content and accumulation of biomass being also evaluated. With this study, we will reveal the importance of nitrogen to the mechanism of CO2 concentration and sugarcane photosynthesis, contributing for the understanding of the physiological bases of reduced light conversion efficiency in field-grown sugarcane plants. In addition to the scientific advance in relation to the regulation of sugarcane photosynthesis, the proposal will allow the development of competence and the creation of a research group on modeling of photosynthesis in higher plants, still absent in our country.
News published in Agência FAPESP Newsletter about the scholarship: