Genotypic Variation in Drought Stress Response and Water Use Efficiency Among Local Landraces of Common Bean (Phaseolus vulgaris L.) from the Caucasus Region

Common bean (Phaseolus vulgaris L.) is a vital source of dietary protein and micronutrients across the Eurasian corridor, yet its productivity is increasingly threatened by drought stress driven by climate variability. Local landraces conserved in diverse microclimates of the Caucasus region represent underexplored genetic reservoirs of climate resilience. This study evaluated the physiological, biochemical, and agronomic responses of twenty-four common bean landraces collected across Georgia, Armenia, and Azerbaijan, alongside two commercial cultivars, under well-watered (100% field capacity) and water-stressed (40% field capacity imposed at the early flowering stage) conditions. Significant genotypic variation (p < 0.001) was observed for relative water content (RWC), membrane stability index (MSI), intrinsic water use efficiency (WUEi), leaf proline accumulation, and maximum quantum efficiency of photosystem II (F_v/F_m). Drought stress reduced grain yield by an average of 46.8% across all genotypes; however, landraces 'GEO-09', 'ARM-04', and 'AZE-14' maintained over 68% of their yield potential under stress, displaying low yield reduction indices and superior stress tolerance indices (STI > 0.85). Principal component and cluster analyses categorized the germplasm into distinct drought-tolerant, intermediate, and susceptible groups, closely corresponding to their collection elevation and ecological provenance. These findings highlight the potential of Caucasian common bean landraces as donors for introgressing drought-adaptive traits into modern breeding lines.