Optimized leaf storage and photosynthetic nitrogen trade‐off promote synergistic increases in photosynthetic rate and photosynthetic nitrogen use efficiency

Abstract A coordinated increase in the photosynthetic rate ( A ) and photosynthetic nitrogen use efficiency (PNUE) is an effective strategy for improving crop yield and nitrogen (N) utilization efficiency. PNUE tends to decrease with increasing N levels, but there are natural variations. Consequently, leaf functional N partitioning in Brassica napus genotypes under different N rates was measured to explore the optimized N allocation model for synchronously increasing A and PNUE values. The results showed that genotypes whose PNUE increased with increasing N supply (PNUE‐I) produced an approximate A value with a relatively low leaf N content, owing to reduced storage N (N store ) and close photosynthetic N (N psn ) content. Partial least squares path modeling showed that A was dominated by the N psn content, and PNUE was directly influenced by A and N store . The A value increased with the N psn content until the N psn content exceeded the threshold value. The boundary line of PNUE varied with the N psn and N store proportions, indicating that the optimum N psn and N store proportions were 51.6% and 40.3%, respectively. The N store proportion of PNUE‐I was closer to the thresholds and benefited from lower increments in Rubisco content and nonprotein form storage N content with improved N supply. Optimized N store and N psn trade‐off by regulating increments in N store content with increased N supply, thereby promoting coordinated increases in A and PNUE.