Differential responses of onion genotypes in growth, physiological and biochemical traits, and bulb yield under waterlogging stress

Waterlogging significantly affects the growth, yield, and quality of onion due to its shallow root system. Eight onion genotypes-four tolerant (Accession 1666, Bhima Dark Red (BDR) Selection, Accession 1630, W-355) and four sensitive (Bhima Super, Bhima Shubra, Bhima Red, Bhima Raj)-identified in a previous pot experiment were evaluated under field conditions for growth, physiological and biochemical traits, and bulb yield to validate their performance. The field experiment followed a split-plot design with three replications, with waterlogging treatments as main plots and eight onion genotypes as subplots, on clay loam soils. Accession 1666 exhibited minimal growth reduction, while BDR Selection showed moderate declines of 17.1%, 10.6%, and 11.7% in plant height, leaf number, and total leaf area, respectively, compared with control plots. Both tolerant genotypes maintained higher membrane stability index (MSI), relative water content (RWC), antioxidant enzyme activities (catalase, guaiacol peroxidase, and ascorbate peroxidase), and concentrations of pyruvic acid, and chlorophyll under stress. Accession 1666 and BDR Selection exhibited reductions in bulb yield of 29.7% and 28.8%, respectively, compared with the respective controls. Conversely, sensitive genotypes experienced greater declines in MSI, RWC, and biochemical traits, with total bulb yield reductions ranging from 46.2% (Bhima Raj) to 53.3% (Bhima Shubra). Field performance revealed that Accession 1630 and W-355, previously classified as tolerant, showed higher mortality and lower yields than Accession 1666 and BDR Selection. The findings confirmed that Accession 1666 and BDR Selection exhibit waterlogging tolerance, making them well suited for cultivation during the monsoon season. Furthermore, adopting raised bed and furrow planting methods can enhance the drainage of excess water, thereby contributing to increased onion yield.