Potassium (K) has a significant effect on wheat yield and quality. Owing to the limitations of irrigation and production costs, soil-based applications of potassium fertilizer are not performed in wheat production on the Loess Plateau of China. In the late growth stage of wheat, potassium deficiency occurs even under sufficient nitrogen/phosphorus (N/P) levels, so it is necessary to supplement potassium through foliar spraying. However, there are few studies on the effect of the foliar application of potassium fertilizer (KFA) on wheat quality. Field experiments were conducted at two experimental sites for 2 years to study the effects of different potassium fertilizer application levels and periods on wheat yield and quality. The results showed that KFA had no significant effect on the yield of the wheat variety Xinong 20 (XN20) but increased the yield of the wheat variety Xiaoyan 22 (XY22). The improvement effect of KFA on the wet gluten content and stabilization time (ST) of XN20 was better than that on these parameters of XY22, while the sedimentation value (SV) and formation time (FT) showed the opposite trend. KFA significantly reduced the albumin content of the two varieties but had no significant effect on the globulin content. Compared with that at the other two stages, the potassium application in the form of potash fertilizer spray at a concentration of 60 mmol L−1 (K2) at the flowering stage (BBCH 65) significantly increased the protein content, wet gluten content, SV and gluten protein content in XN20 grains, whereas the application at 10 days after flowering (AA10, BBCH 71) at the K2 concentration was more beneficial to prolonging the dough FT. For XY22, the application of potassium fertilizer at the K2 concentration at the flowering stage increased the wet gluten and gluten protein levels and dough development time. There were significant genotypic differences in the composition and content of HMW-GS between the two varieties. KFA significantly increased the levels of the 1, 7 + 8, and 4 + 12 subunits in XN20 and the 1 subunit in XY22, but had no significant effect on the 2 + 12 subunit in XY22. Partial least squares path modelling (PLS-PM) analysis showed that the processing quality indexes (SV, FT, ST) and gluten protein and HMW-GS levels were regulated by the potassium fertilizer foliar spraying stage and concentration and revealed in part that KFA affected the processing quality by affecting the HMW-GS content.
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