Genotypic Differences in Potato Yield and Starch Content Response to Potassium Fertilization Under Drought Stress

Drought stress constrains potato (Solanum tuberosum L.) production, and potassium (K) fertilization may alleviate it, but genotype-dependent responses remain unclear. Six cultivars (‘V7’, ‘DXY’, ‘JZS-3’, ‘SY-2’, ‘JZS-12’, ‘ZJ-7’) were grown under four treatments: low K with normal irrigation (T1) or drought (T2), and K application with normal irrigation (T3) or drought (T4). Dry matter in roots, stems, leaves, and tubers was measured at 50–110 days after planting, starch content at 70–110 days, and final yield recorded. Drought (T2) reduced yield by 28.6–52.3% versus T1; K under drought (T4) recovered yield by 12.8–41.5%. Starch decreased by 18.5–35.2% under drought but was restored to 82.6–94.3% of T1 levels with K. SY-2 showed the highest biomass and yield across treatments, while V7 was most affected. Based on yield responses, cultivars fell into four categories: K-efficient/drought-tolerant (SY-2), K-responsive/drought-tolerant (DXY), K-non-responsive/drought-tolerant (JZS-12), and K-inefficient/drought-sensitive (V7). K application mitigates drought-induced losses, but benefits are genotype-dependent. This classification supports genotype-specific K recommendations in water-limited systems, and SY-2 is a valuable genetic resource for breeding for K-use efficiency and drought tolerance. Physiological assessments of leaf relative water content and stomatal conductance confirmed that drought treatment effectively induced water stress and revealed that genotype-specific stomatal regulation underpinned the differential yield responses to K fertilization under drought.