The effect of soil moisture depletion on Stevia (Stevia rebaudiana Bertoni) grown in greenhouse conditions: Growth, steviol glycosides content, soluble sugars and total antioxidant capacity

The purpose of the present study was to determine threshold values of soil moisture content for Stevia (Stevia rebaudiana Bertoni) and to evaluate the effects of drought stress on the main metabolites of this species. For these purposes, a greenhouse experiment was carried out with four soil moisture levels and plant growth, steviol glycoside (SVglys) contents, soluble sugars and antioxidant capacity were investigated at variable soil moisture content. Irrigation was scheduled at 3, 6, 9 and 12-day irrigation intervals, based on soil moisture content at 90, 75, 60 and 45% of field capacity (FC) respectively. The results showed that soil water depletion up to 60% FC (9-day irrigation interval) had no negative effect on plant growth and leaf dry weight, whereas a significant growth reduction occurred at 45% FC (12-day irrigation interval). Similarly, the total SVglys content increased when soil moisture was depleted to 60% FC (9-day irrigation interval), but these metabolites contents decreased by 45% FC treatment. Although Stevia growth and SVglys content significantly decreased under severe drought stress (45% FC), the total antioxidant capacity and soluble sugars increased in the identical condition. The obtained results suggest that Stevia plants can grow well with a soil water content near to 60% FC, showing a good SVglys content. The Stevia tolerance to mild water stress is noteworthy, especially in water limited regions. In addition, it was found that soil water depleted to 45% FC was detrimental to Stevia in greenhouse conditions. The improvement of antioxidant capacity and soluble sugar content by soil water stress conditions could be considered as physiological and biochemical responses to a progressive drought stress in Stevia and may be an acclimation response to drought stress.