Post-anthesis dry matter production and leaf nitrogen distribution are associated with root-derived cytokinins gradient in rice

Aligning leaf nitrogen (N) distribution to match the light gradient is crucial for maximizing canopy dry matter production (DMP) and improving N utilization efficiency. However, the relationship between the gradient of root-derived cytokinins and N distribution in rice leaves and its impact on DMP and the underlying mechanisms remains poorly understood. A two-year field experiment was conducted using two japonica N-efficient varieties (NEVs) and two japonica N-inefficient varieties (NIVs) under four different N rates (0, 90, 180, and 360 kg N ha⁻¹). These selected varieties exhibited similar values in the coefficient of light extinction (K_L). Results showed that at lower N rates (0–180 kg N ha⁻¹), the NEVs exhibited greater dry matter weight at maturity, higher grain yield, and improved internal N use efficiency (IE_N) compared to the NIVs, despite possessing comparable total N uptake. Compared with the NIVs, the NEVs exhibited a more pronounced nitrogen distribution gradient in leaves, as indicated by the coefficient of nitrogen extinction (K_N) values during the middle and early grain-filling stages. This enhanced gradient led to improved coordination between light and nitrogen, resulting in greater photosynthetic production, particularly at lower N rates. Furthermore, the NEVs demonstrated a larger gradient of zeatin (Z)+zeatin riboside (ZR) in leaves (i.e., higher ratios of Z+ZR levels between upper and lower leaves), enhanced expression levels of genes related to N export in lower leaves and Z+ZR loading in the root, respectively, elevated enzymes activities related to N assimilation in upper leaves, in relative to the NIVs. Correlation and random forest analyses demonstrated a strong positive correlation between the Z+ZR gradient, K_N, and DMP, and the gradient facilitated the export of N from lower leaves and its assimilation in upper leaves, contributing significantly to both K_N and DMP. This process was closely linked to root activity, including root oxidation activity, root Z+ZR content, and Z+ZR loading capacity, as confirmed by applying an inhibitor or a promoter of cytokinins biosynthesis to roots. Interestingly, at the N rate of 360 kg N ha⁻¹, both NEVs and NIVs showed indistinguishable plant traits, achieving a super high-yielding level (over 10.5 t ha⁻¹) but with remarkably low IE_N. The results suggest that increasing the Z+ZR gradient can improve K_N and DMP, where it needs to maintain higher root activity, thus leading to high yield and high IE_N. Further research is needed to explore and develop cultivation practices with reduced N to unlock the super-high-yielding potential of the NEVs.