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, 15 panicles samples were collected in a 5-days frequency, and the superior, medium and inferior grains were separately picked, and part of them were dried to weight biomass, then using Richards formula to fit the filling process and calculated the initial growth potential (R₀), maximum grain filling rate (Gmax), the time reaching maximum filling rate (Tmax.G) and the biomass (Wmax.G), and calculate the mean filling rate (Gmean) and vigital filling days (D). The other part were used for determination of starch synthesis enzyme activities. At harvest, rice yield, effective panicles, 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, N3P2 N3P2, and N2P1 treatments were recorded similar yields, which were significantly higher than the other treatments. Under N3 level, P2 achieved similar yield components, but P1 achieved significantly lower effective panicles, spikelets per panicle and seed setting rate. Under N2 level, P3 significantly decreased the effective panicles 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 the N3 level, N1 significantly decreased the yield and yield components of rice. Among the three high yield treatments, N2P1 resulted higher spikelets per panicle of superior, medium, and inferior grains, similar seed-setting rate of superior and medium grains, but significantly lower seed setting rate of inferior grains, compared to N3P3 and N3P2 treatments. The 1000-grain weight was most affected by the medium grains, followed by the inferior grains. P level was beneficial to increase the 1000-grain weight of all the superior, medium and inferior grains. Within the range of nitrogen and phosphorus fertilizers tested, reducing N and P reduced the Gmax, Gmean and Wmax.G, prolonged the Tmax.G and D. The D for the N3P2 and N2P1 treatments of superior, medium and inferior grains were extended by 3.6, 2.3, 2.3 d, and 3.8, 3.3, 2.3 d, respectively. Compared with N3, N2 suppressed AGPP activity in superior and inferior grains, but the difference in the activities of SSS, SBE and 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 Gmean of superior grains was mainly regulated by the activities of AGPP, SSS and GBSS, and the grain weight was mainly positively affected by the activities of SSS. There was a significant positive correlation between the grain weight of inferior grain and its AGPP and SSS activities.

Conclusions P level significantly influences the Gmax, Gmean, and Wmax in superior grains and the Tmax.G in medium grains, and N level significantly affect the Wmax in medium grains and Tmax.G 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, affecting the comprehensive advantages of grain filling characteristics and yield components of rice, and achieving increased and stable rice yield.