Assessment of agro-physiological and biochemical markers for drought tolerance in onion (Allium cepa L.) genotypes during bulb development

Drought is a major constraint that limits onion productivity. This current study explored the physio-biochemical, morphological, and yield attributes of 14 onion genotypes during the bulb development stage under drought conditions. A greenhouse experiment was conducted using a Completely Randomized Design (CRD) with four replications and two treatments. Drought stress was applied for a duration of 30 days. Drought reduced all morpho-physiological parameters of onion genotypes compared to the control. Drought significantly increased (p < 0.001) hydrogen peroxide (H₂O₂) levels in onion plants. The genotypes Goudami and Prema showed higher levels of catalase and ascorbate peroxidase. Additionally, Prema and Goudami exhibited the highest proline content, while Red_Creole had the lowest. Yields were significantly reduced (p < 0.001), resulting in a 49.58% loss compared to the control. The yield of onion genotypes was strongly positively associated with several factors, including stress tolerance index, bulb diameter, proline, drought tolerance efficiency, ascorbate peroxidase, catalase, relative water content, total chlorophyll, and stomata number. Prema, Goudami, and Red_Jewel recorded higher stress tolerance index, antioxidant activity, proline, chlorophyll, drought tolerance efficiency, stomatal aperture, and better yield; therefore, they were categorized as tolerant. The local variety, Rouge_Tama ARES, IDOL, AVON1317, AVON1074, Synthetique, Violet_Galmi, Dayo, and SAFARI were considered intermediate, displaying lower chlorophyll, stress tolerance index, potential activity of PS II, and proline. In contrast, Red_Creole was identified as a sensitive genotype, showing low morphological traits, drought tolerance efficiency, and a high percentage of yield reduction. Drought-tolerant onion genotypes identified could be essential for farmers and future improvement programs in drought-prone regions. This study emphasizes the significance of these tolerant onions for effectively responding to drought while maintaining bulb quality and productivity.