The effect of humic acid application on Stevia (Stevia rebaudiana) growth and metabolites under drought stress

The soil water limitation is a relevant abiotic stress that affects the plants life. In this respect, adoption of an efficient strategy may improve the plant growth during this stress. The present study was conducted in order to find the likely useful effect of the humic acid (HA) on stevia (Stevia rebaudiana) in a pot trial, especially under drought stress condition. Therefore, five levels of humic acid (0, 1, 1.5, 2, and 2.5 mg.kg^-1 soil) were applied to the soil planted with Stevia seedling under normal (well-watered) and drought stress condition (resulted from 10-day irrigation intervals), based on a factorial experiment with three replications. Drought stress significantly reduced the stevia leaf production and steviol glycosides (SVglys) content and compositions. Stevioside (Stev) and Rebaudioside A (Reb A) contents significantly decreased under drought stress and all concentrations of HA. On the other hand, HA (in concentrations more than 1.5 mg.kg-1 soil) strongly increased leaf production in stevia and resulted in a significant increase in SVglys yield (44% in comparison with no HA). Results clearly demonstrated that although HA showed an inhibitory effect for SVglys content in stevia leaf, it can increase the SVglys yield through a remarkable increase in leaf growth in stevia, especially under drought stress condition.