Objectives We analyzed the filling capacities and the starch synthesis activities at different positions of rice grains under different N and P supply levels, so as to provide a theoretical basis for balanced of nitrogen and phosphorus fertilization in japonica rice cultivation in northern China.

Methods In 2021 and 2022, field trials were conducted at the Kalima Rice Experimental Station of Shenyang Agricultural University using the japonica Beigeng 3. Three N and P application levels were used to compose 9 treatments, conventional level N 210 kg/hm² and P₂O₅ 105 kg/hm² (N3, P3), 15% reduction (N2, P2), and 30% reduction (N1, P1). From flowering (flower appeared on the top spikelets of rice) to maturing stage, the superior, medium and inferior grains were separately picked, and part of them were dried to determine the biomass weight, then using Richards equation to fit the filling process and calculated the grain filling parameters. The other part were used for determination of starch synthesis enzyme activities. At harvest, rice yield, effective panicles per unit area, and spikelets per panicle, seed setting rate, 1000-grain weight of superior, medium and inferior grains were investigated.

Results The P levels required to achieve high yields varied among the N levels. Among N3P3, N3P2, and N2P1 treatments, similar yields were recorded, which were significantly higher than the other treatments. Under N3 level, P2 achieved similar yield components, but P1 achieved significantly lower effective panicles per unit area, spikelets per panicle and seed setting rate. Under the N2 level, P3 significantly decreased the effective panicles per unit area and spikelets per panicle, P2 significantly decreased the spikelets per panicle, while P1 increased the spikelets per panicle, and maintained similar 1000-grain weight. Compared with the N3 level, N1 significantly decreased the yield and its components of rice. Among the three high yield treatments, N2P1 exhibited higher spikelets per panicle of superior, medium, and inferior grains, showed similar seed-setting rate for superior and medium grains, but had a significantly lower seed setting rate for inferior grains compared to N3P3 and N3P2. The 1000-grain weight was most affected by the medium grains, followed by the inferior grains. P levels were beneficial to increase the 1000-grain weight of all the superior, medium and inferior grains. Within the tested nitrogen and phosphorus fertilizer range, reducing N and P reduced the maximum grain filling rate, mean grain filling rate and growth weight reaching maximum grain filling rate, prolonged the time reaching maximum grain filling rate and active growth period. Specifically, active growth period for the N3P2 and N2P1 treatments of superior, medium and inferior grains were extended by 3.6, 2.3, 2.3 days, and 3.8, 3.3, 2.3 days, respectively. Compared with N3, N2 suppressed ADP glucose pyrophosphorylase (AGPP) activity in superior and inferior grains, but the difference in the activities of soluble starch synthase (SSS), starch branching enzyme (SBE) and granule-bound starch synthase (GBSS) were minimal. N1 reduced the activity of these enzymes. Reducing P inhibited the activities of AGPP and GBSS in superior grains and the GBSS activity in inferior grains. Correlation analysis showed that the mean grain filling rate of superior grains was mainly regulated by the activities of AGPP, SSS and GBSS, and the 1000-grain weight was mainly positively affected by the activities of SSS. Notably, the 1000-grain weight of inferior grains exhibited a strong positive relationship with AGPP and SSS activities.

Conclusions P level significantly influences the grain filling rate and growth weight reaching maximum grain filling rate in superior grains, and the time reaching maximum grain filling rate in medium grains, while N level significantly affects the growth weight reaching maximum grain filling rate in medium grains and the time reaching maximum grain filling rate in inferior grains. Balancing the N and P levels can harmonize the key enzyme activities of starch synthesis in superior and inferior grains during the filling period, affect the comprehensive advantages of grain filling characteristics and yield components of rice, ultimately achieving enhanced and stabilized rice productivity.