Abstract: We explored the effects of nitrogen application rates on starch particle size distribution and pasting properties in superior and inferior grains of two soft wheat cultivars, Longke 1109 and Huacheng 1688. We analyzed starch particle size distribution and viscosity parameters in superior and inferior grains of soft wheat and the relationship between them across four nitrogen application levels, N0 (0 kg·hm^-2), N1 (120 kg·hm^-2), N2 (240 kg·hm^-2) and N3 (360 kg·hm^-2). The results showed that the volume and surface area percentage of B-type (≤10 μm) starch granules in superior and inferior grains decreased with increasing nitrogen application levels. In B-type starch granules, nitrogen fertilization had stronger influence on the volume and surface area percentage of starch granules with particle size of 2.8-10 μm. The volume and surface area percentage of A-type starch granules showed an upward trend. In A-type (>10 μm) starch granules, nitrogen fertilization had a greater effect on the volume and surface area percentage of starch granules with particle size >22 μm. There was no significant effect on the percentage of starch granules number. The peak viscosity, trough viscosity, final viscosity and other viscosity parameters of the superior and inferior grains of both cultivars showed an upward trend. Under the same nitrogen treatment, the peak viscosity, trough viscosity, final viscosity, breakdown and setback of the superior grains of both cultivars were significantly higher than those of the inferior grains. With increasing nitrogen application levels, the protein and wet gluten content of wheat grain significantly increased, while the starch content significantly decreased. The grain yield of wheat was the highest when nitrogen application level was 240 kg·hm^-2. Peak viscosity, trough viscosity, final viscosity and setback of Longke 1109 starch granules were positively correlated with the volume ratio of B-type starch granules, and negatively correlated with A-type starch granules. In summary, increasing nitrogen application levels can change the distribution characteristics of starch granules in wheat grains, that is, increasing the volume and surface area ratio of A-type starch granules but reducing that of B-type starch granules, thereby improving starch viscosity parameters and processing quality.

Key words: soft wheat, starch granule, pasting property, nitrogen fertilizer, superior grain, inferior grain