Optimized Blends of Polyhalite and Muriate of Potash Under Reduced Potassium Rate Are Associated with Enhanced Tuber Starch Content and Up-Regulation of Starch Biosynthetic Genes in Potato

Excessive potassium (K) application in potato production impairs tuber quality. To develop a rational K fertilization strategy using polyhalite (POLY4), we conducted two field trials (2022, 2023) with potato cultivar ‘Dafeng No. 10’. The 2022 trial included a no-K control, two full-rate K treatments (300 kg K2O ha−1) with low (13%) or high (37%) POLY4, and two reduced-K treatments (200 kg K2O ha−1) with medium (18.5%) or high (31.5%) POLY4. The 2023 trial comprised a no-K control, a full-rate farmer’s practice, and two reduced-K treatments (240 and 200 kg K2O ha−1 with 17% and 18.5% POLY4, respectively). Moderate K reduction combined with a low-to-medium POLY4 proportion (18.5%) caused no yield loss and achieved yield increases of 35.6% in 2022 and 20.5% in 2023 compared to the no-K control and significantly enhanced tuber starch content compared to controls. These optimal treatments were associated with elevated activities of key starch-synthesis enzymes (AGPase, SBE, SSS, GBSS) and increased transcript levels of their corresponding genes (AGPase, GBSSI, SSSIII, SBEI). Correlation analyses revealed significant positive associations between these molecular parameters and tuber starch content. We conclude that a tailored K-POLY4 blend under reduced total K input (200 kg K2O ha−1 with 18.5% POLY4) represents an optimal strategy for synergistically improving potato yield, starch quality, and K use efficiency.