Growth, bulb yield, water productivity and quality of onion (Allium cepa L.) as affected by deficit irrigation regimes and exogenous application of plant bio-regulators

Effect of plant bio-regulators (PBRs) viz., potassium nitrate (KNO3, 15 g L−1), thio-urea (TU, 500 ppm), salicylic acid (SA, 10 μM), gibberellic acid (GA3, 25 ppm) and sodium benzoate (SB, 150 mg L−1) for two years (2015-17) under various levels of deficit irrigation created using line source sprinkler system (LSS) was evaluated in onion (Allium cepa L.). The crop could sustain little water deficits and its bulb yield declined to 0.84, 0.66, 0.48, 0.35, 0.24 and 0.16 when irrigation water (IW) applied equalled 0.85, 0.70, 0.55, 0.40, 0.25 and 0.10 times the pan evaporation (CPE) against maximum yield at full irrigation (IW:CPE 1.00). Application of PBRs helped to mitigate the water stress through maintenance of leaf water content, modulating the canopy temperature and better water usage thereby improving average bulb yields by 10.1-25%. Especially KNO3 and TU were more effective under low to medium water deficits. The water productivity ranged between 7.78 and 9.61 with PBRs against 7.36 kg m−3 under control. The overall water saving was 18.3, 25.7, 48.4 and 63.8% with PBRs namely GA3, SA, TU and KNO3, respectively. The marketable quality monitored in terms of bulb weight, geometric mean diameter and sphericity was significantly reduced with water deficits while it improved with PBRs. Among the other physicochemical and functional quality characteristics of the onion bulb, rehydration ratio, protein content, total soluble sugar, total phenolics content and pyruvic acid were lowered by water deficits. These were improved significantly with PBRs. Thus it was concluded that combining PBRs like KNO3 and TU can further facilitate to implement deficit irrigation technology for sustaining productivity and quality of onion under water scarce conditions.