Effect of foliar application of selenium, boron and iron on some physiological traits and glycosides of stevia (Stevia rebaudiana Bertoni) under salinity stress

This research was aimed to investigate the foliar application of selenium, boron and iron on leaf yield, the photosynthetic pigments rate, antioxidant enzymes activity and the steviol glycosides content under sodium chloride stress conditions. The study was conducted in a factorial experiment in Anzali of Guilan Province based on a completely randomized design in three replications in 2105-16. The salinity stress levels were used by sodium chloride at four levels: control (less than 10), 30, 60 and 90 mM, and the foliar application of iron, boron, and selenium was applied. The results showed that the effect of salinity stress and foliar application was significant on the leaf yield, the chlorophyll pigments rate (a, b and total), carotenoid, protein content, antioxidant activity catalase (CAT), peroxidase (POX), superoxide dismutase (SOD) and the steviol glycosides composition. The highest total chlorophyll content, chlorophyll a and chlorophyll b were obtained in the integrated application of iron, boron and selenium in free-salinity conditions (with means of 39.48, 24.86 and 14.63 mg/g fresh weight, respectively). The highest protein content was observed in the foliar application of iron and selenium at 90 mM NaCl, resulted in an increase of 5.3 times compared to the control treatment. The highest CAT and SOD activities were in the integrated foliar application of three elements and selenium and iron composition at salinity level of 90 mM, increasing the activity of these enzymes by approximately 130 and 11 times, respectively. The highest and lowest rates of rebaudioside A, stevioside, and rebaudioside C were obtained in the integrated application of three elements (iron, boron, and selenium) at salinity levels of 30 mM and non-spraying in 90 mM salinity, respectively. Generally, to achieve a higher amount of steviol glycosides, the integrated application of three elements iron, boron, and selenium and salinity stress up to 30 mM is recommended.