Twenty genotypes of onion (Allium cepa L.) were studied to determine physiological responses under drought stress. Based on their physiological response to stress, the genotypes were divided into different groups. Specific leaf weight (SLW) which is an indication of leaf thickness and accumulation of photo-assimilate in the leaf varied from 53 to 74 mg cm-2 in different genotypes. The majority of genotypes showed relative water content (RWC) at 80-84%. Three genotypes, viz., SM-11, SM-39, and JNDWO-85 of varying leaf shape, SLW, and bulb size were further studied for long- and short-term stress at two different (30 and 50 days after planting) growth stages. Significant genotypic differences in leaf area, photosynthesis, and stomatal conductance were found between irrigated and stressed plants (during short- and long-term stress). The variation in photosynthetic rate was attributed to the significant changes in stomatal conductance. The narrow leaf genotype (SM-11 and SM-39) maintained high photosynthetic rate (1.7-9.20 ?mol m-2 s-1) during stress at both the stages. The reduction in bulb yield is associated to the lower leaf area recovery after the relief of stress. Genotype SM-11 response was better than that of SM-39.