Influence of humic acid application on onion growth characteristics under water deficit conditions

Abstract

In this study, effect of water shortage on onion growth and quality was evaluated under soil humic acid (HA) application. Three levels of irrigation namely: a1; 80%, a2; 70% and a3; 60% of field capacity (FC) were supplied to plants in combination with two levels of HA namely: b1; 0 and b2; 100 mg/kg soil, in a factorial and completely randomized design with four replications. The interaction of irrigation and HA was significant on leaf dry weight, vitamin C, calcium, total chlorophyll and bulb flavonoids, while the onion fresh yield was not affected by treatments. Many plant growth traits showed a reducing trend with deficit irrigation levels. The leaf biomass as well as the leaf concentrations of potassium, calcium and zinc were significantly decreased under highest water deficit treatment (60% FC; a3b1 and a3b2) compared to control (a1b1) treatment. Leaf vitamin C and calcium concentration showed almost a similar response in which their highest levels were observed in a1b2 treatment, while the lowest amounts were recorded under higher water shortage (a3b1), and application of HA (a3b2) significantly increased these traits under higher water shortage. The amounts of bulb flavonoids and phenols were increased by deficit irrigation and application of HA further increased their content under highest deficit irrigation. The results indicate that water deficit particularly at highest level reduced onion plant growth, while soil application of HA significantly increased the growth and quality traits including leaf biomass, vitamin C, and minerals like Ca, as well as bulb flavonoids under water deficit conditions.