Bulb productivity and quality of monsoon onion (Allium cepa L.) as affected by transient waterlogging at different growth stages and its alleviation with plant growth regulators

Rainfall induced hypoxia with transient waterlogging is a major constraint limiting the productivity and quality of monsoon season onion (Allium cepa L.). Therefore, the effects of water stagnation at different growth stages and possibilities of its alleviation with plant growth regulators (PGRs) namely; potassium nitrate (PN, 20 g L-1), thio-urea (TU, 600 mg L-1), salicylic acid (SA, 2.07 mg L-1), sodium benzoate (SB, 200 mg L-1) were evaluated during 2018-20. Bulb yield loss ranged between 7 and 60 per cent. Bulb initiation stage was the most sensitive (20 %) to waterlogging followed by vegetative growth (15 %) and bulb development (7 %). Effects became additive when imposed at vegetative growth and bulb initiation (44 %), vegetative growth and bulb development (40 %), bulb initiation and bulb development (35%) and all the three stages (60 %). Spraying PGRs improved marketable bulb yields by 10.4-23.3 %, water productivity from 4.0 to 4.4-4.9 kg m- 3 and helped to mitigate waterlogging stress through maintenance higher leaf water contents, greenness and plant vigour, lower canopy temperatures and modulating stomatal openings. Especially the crop was more receptive to PN and TU sprays at the most sensitive bulb initiation stage. Waterlogging also impacted the quality traits like bulb weight, geometric mean diameter, sphericity and firmness of bulbs but these got largely recovered with PGRs. PGRs exhibited considerably higher rehydration ratio (RR), pH, total soluble solids (TSS), protein contents, accumulation of total phenols and pyruvic acid. Thus, spraying PGRs like PN and TU emerged as a viable option to off-shoot the impacts of transient waterlogging.