Dynamic remobilization of leaf nitrogen components in relation to photosynthetic rate during grain filling in maize

Remobilization of leaf nitrogen (N) contributes greatly to grain N in maize, but leads to low photosynthetic rate (P_n). P_n is determined by various N components involving in light harvest and CO₂ reduction. However, it is less clear which N component is the major contributor for the reduction of photosynthesis in modern stay-green maize hybrids. In this study, we analyzed the relationship between remobilization of different N components and P_n during grain filling stage under low N (no N application) and high N (180 kg N ha^-1) in a field experiment. The remobilization efficiency of photosynthetic enzymes (PEPc, PPDK and Rubisco) in the leaf was much higher than that of thylakoid N and other N components. Low N supply increased the remobilization efficiency of all the leaf N components. During grain filling stage, the amount of all the N components decreased together with P_n. The ratio of P_n to the N in the PEPc, PPDK and Rubisco kept increase in the whole grain filling stage, while the ratio of P_n to chlorophyll and thylakoid-N decreased. Correlation analysis indicated that P_n was more related to the content of photosynthetic enzymes than to chlorophyll and thylakoid N. It is concluded that photosynthetic enzymes serve as an N storage reservoir at early grain filling stage and their degradation is critical in the reduction of P_n during later grain filling stage. Future breeding targets may be focused on enhancing the efficiency of photosynthetic enzymes during late grain filling stage.