Physiological and biochemical responses of onion plants to deficit irrigation and humic acid application

Onion is an important crop with significant roles in human diets. The growth, yield, and quality of vegetable crops, including onions, are more vulnerable to water stress than other crops. In this study, different levels of deficit irrigation (DI) as factor A (a1: 80%, a2: 70%, and a3: 60% of soil field capacity [FC]) and humic acid (HA) as factor B (b1: without and b2: with HA application) were evaluated on onion growth characteristics in a factorial design with four replications. The results showed that the interaction of DI and HA was significant on leaf protein, peroxidase (POD), superoxide dismutase (SOD), and on bulb protein and potassium (K) concentrations. The highest record of these traits was observed in a3b2 (highest DI with HA application), and their lowest was in those at first level of DI (a1). Leaf protein and, to a lesser extent, bulb protein were increased by DI and HA applications. DI at 60% but not at 70% FC significantly reduced bulb fresh weight. There was a gradual increase in leaf proline, soluble sugars, protein, catalase (CAT), POD, SOD activity, and bulb K by application of DI; however, most of bulb traits including protein, iron (Fe), zinc (Zn), and CAT and POD activity were increased only under highest DI level (a3: 60% FC). However, application of HA further increased the soluble sugars and protein concentration as well as the POD and SOD activities of leaves, and protein, Fe, K concentrations, and CAT activity of bulbs under DI. The results indicated that HA benefitted onion growth particularly under DI conditions.