Patterns of Physiological Parameters and Nitrogen Partitioning in Flag Leaf Explain Differential Grain Protein Content in Rice

The grain protein content (GPC) in rice is low, and more efforts with agronomic and molecular approaches were performed to increase them. However, the rice research focusing on the plant physiological behaviour that modulates the phenomenon of grain protein filling is very scarce. This work contains physiological parameters related to photosynthetic activity in the flag leaf in the grain filling period and N partitioning assays of high (Nutriar) and traditional (Camba) GPC cultivars. Results indicated a higher photosynthetic capacity, a better capacity to provide CO to the chloroplast and a healthier PSII structure in Camba relative to Nutriar. Chlorophyll fluorescence parameters decreased more steeply over time in the high protein variety, and a strong negative correlation was observed between GPC and PSII structure parameters. N content in the flag leaf at anthesis showed lower values and higher remobilisation during the grain filling period in Nutriar compared to Camba. The results of this work suggested that the inactivation of some PSII structures in higher GPC cultivars is associated with N remobilisation and would contribute to an increase in the free N available to be translocated to the grain.