The effect of foliar application of humic acid on increasing Stevia rebaudiana tolerance under different irrigation regimes

In order to study the effect of foliar application of humic acid on the yield, yield components, and some physiological and phytochemical properties of Stevia rebaudiana under different irrigation regimes, a factorial experiment was conducted based on a randomized complete block design with three replications in the research campus of University of Agricultural Sciences and Natural Resources, Gorgan, Iran. The treatments included four levels of irrigation regimes (70, 85, 100 (control), and 115% of field capacity) and humic acid foliar application at four concentrations (zero, 250, 500, and 750 mg/L). Irrigation regimes and humic acid treatments started from the 10-leaf stage and continued until the flowering stage. At full flowering stage, data collected for shoot and root length, root diameter, shoot and biomass fresh and dry weights, number of sub branches, flowers number, internodes number, leaves number, leaf area, root antioxidant activities, total phenol, total flavonoid, proline, ion linkage, and leaf relative water content. According to the results, the effect of different irrigation regimes on shoot height, shoot and biomass fresh and dry weights, sub-branches number, internodes number, was significant (p<0.01). By flooding, the plant height, shoot and biomass fresh and dry weights were increased. The interaction effect of drought stress by humic acid was significant for plant height, root antioxidant activity, and proline. Low irrigation stress had negative effect on the growth traits but humic acid foliar application had moderated the negative effects caused by low irrigation stress. Therefore, based on the findings, it was concluded that stevia is a resistant plant to both flooding and drought stress and its resistance mechanisms to drought stress for maintaining vegetative yield components, are to increase osmolytes and phenolic compounds.