Effect of Different Water Regimes and Potassium Fertilisation on Tuber Quality and Bioactive Compounds in Three Potato Cultivars

Abstract Studies investigating water stress on potatoes have largely focused on bioactive compounds at the leaf level, primarily within the framework of osmotic adjustment, while their contribution to tubers remains relatively underexplored. This study investigated the combined effects of irrigation regimes and potassium fertilisation on quality attributes and bioactive compounds of three potato cultivars. The experiment was conducted in a tunnel using a split–split plot design within a completely randomised design. Treatments included three irrigation regimes (40%, 70%, and 100% of crop evapotranspiration, ETc) and four potassium fertilisation levels (0, 100, 150, and 200 kg K ha⁻¹). Three potato cultivars, including ‘Mondial’, ‘Markies’, and ‘Panamera’ were used for evaluations since they are among the widely cultivated and commercially marketed potato cultivars in South Africa. Irrigation regimes were assigned to main plots, potassium levels to subplots, and cultivars to sub-subplots. Key response variables included dry matter content, soluble sugars, total phenolic content, and ascorbic acid. Results showed that dry matter content was significantly affected by both irrigation and cultivar, with higher values observed under deficit irrigation. Furthermore, sucrose concentration was influenced by both irrigation regime and potassium fertilisation, while glucose and fructose were primarily governed by cultivar. Moreover, total phenolic content was significantly ( p < 0.01) higher under full irrigation (100% ETc) with 0.15 mg of gallic acid equivalent (GAE) g⁻¹ DW (dry weight) compared to moderate (70% ETc) and deficit (40% ETc) irrigation, which accumulated 0.118 mg and 0.113 mg of GAE g⁻¹ DW, respectively. On the other hand, ascorbic acid content increased under deficit irrigation by 5 and 17% under 40 and 70 ETc, respectively, compared to full irrigation (0.41 mg GAE g − 1 ). Overall, the findings demonstrate the complex interaction between irrigation, potassium fertilisation, and cultivar in determining potato tuber composition and quality, with important implications for optimising production under water-limited conditions.