TThe effect of foliar application of iron, boron, and selenium on the root morphological characteristics and the photosynthetic pigment contents of Stevia (Stevia rebaudiana Bertoni) medicinal plant under sodium chloride salinity

Salt stress is one of the factors limiting plant growth and a major obnstacles for nutrient uptake in plants. A factorial experiment with pots in the open air was implemented based on a completely randomized design with three replications to evaluate the effect of foliar application of iron (Fe: Sequestrene 138 at a concentration of 5 per thousand), boron (B: Boric acid at a concentration of 5 per thousand), and selenium (Se: Sodium selenite at a concentration of 2 per thousand) under NaCl salinity on root growth characteristics and the photosynthetic pigmentscontent of Stevia plant. The experiment factors included different levels of sodium chloride (0, 3, 6, and 9 dS m-1) and spraying composition at eight levels (no spraying, Fe, B, Se, Fe Se, Fe B, Se B, and Fe Se B). Results showed that salinity caused significant reduction in root dry weight, volume, length, diameter and density of root, percentage of dry weight to soil volume ratio, and chlorophyll a, b, and total. Among the various combinations of spraying, combined application of Fe, B, and Se had the highest mean dry weight, volume, length, area, and density of the root. The highest significant mean chlorophyll b and total chlorophyll contents were observed in spraying solution containing Fe at the non-stress level, and the combination of Fe and Se at 3 dS m-1 level of salinity. In general, decrease in root growth and leaf chlorophyll content (especially chlorophyll a) occurred under salt stress and generally, these factors led to the loss of plant growth at higher salinity levels (6 and 9 dS m-1). In order to reduce the negative effects of salinity stress, especially at lower levels (3 and 6 dS m-1), using Fe and Se elements are recommended to increase the chlorophyll content of the plant and improve root growth.