Combined heat and drought responses in stress-tolerant potato (Solanum tuberosum) genotypes deviate from individual-stress predictions: Implications for climate-resilient breeding

Combined drought and heat stress episodes are becoming increasingly common in tropical potato growing regions, yet most breeding programmes continue to evaluate stress tolerance under individual stress scenarios. Two commercially promoted “climate-resilient” potato genotypes (Manitou and Markies) were evaluated under control, individual and combined heat + drought stresses using temperature and soil moisture regimes informed by current and projected potato-growing conditions in Kenya. Experiments were conducted across growth room, glasshouse-polytunnel and field-polytunnel. Several physiological, yield and quality traits exhibited statistically supported departures from additive expectations under combined stress, indicating that response to co-occurring stress could not always be predicted from individual stress effects alone within tested systems. Combined heat + drought stress consistently imposed the greatest reductions in stomatal conductance (gs), maximum efficiency of photosystem II (Fv/Fm), non-photochemical quenching (NPQt), tuber yield, starch, and dry matter content, with total and marketable yield declining by 73–91%. Despite comparatively stronger drought performance, Markies, showed significant declines in starch and dry matter contents under heat and combined stress, whereas Manitou maintained stable starch content, suggesting contrasting genotype-specific stress associated development and assimilate allocation responses under combined stress. Notably, the two varieties marketed as heat or drought tolerant lost their advantages when stresses co-occur. These findings demonstrate limitations of single stress phenotyping frameworks and suggest that evaluating genotypes under isolated drought or heat conditions may overlook importance of combined stress vulnerabilities. The study supports the integration of combined stress screening approaches for developing genotypes better suited to multifactorial climate stress environment.