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

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.