Effects of using humic acid and selenium on some morphophysiological characteristics of Calendula officinalis L. under salinity stress

Salinity stress is one of the most important factors in reducing the crop yield in the world. The application of humic acid along with selenium can improve the plant yield under salinity stress conditions by increasing the antioxidant capacity. To evaluate the effects of humic acid and selenium application under salinity stress conditions in Calendula officinalis L., a factorial greenhouse experiment was conducted in a completely randomized design with three replications in Islamshahr city in 2018. The experimental treatments included sodium chloride (0, 50, and 100 mg l-1), humic acid (0, 100, and 200 mg l-1), and selenium (0, 5, and 10 mg l-1) and their combined effects. The results showed that increasing the salinity significantly reduced the fresh and dry weight of shoots and roots and number of flowers per plant. The foliar application of 200 mg l-1 humic acid and 10 mg l-1 selenium improved carotenoids content of the petals and total chlorophyll of the leaves at all salinity levels. Also, with increasing the sodium chloride level up to 100 mg l-1, the amount of proline increased significantly. Meanwhile, the interaction treatments of humic acid and selenium had significant effects (P≤0.0.1) on improving the activity of superoxide dismutase and peroxidase enzymes at different salinity levels. The highest amount of essential oil (1.37%) was obtained in the interaction treatment of 200 mg l-1 humic acid and 10 mg l-1 selenium under salinity stress of 50 mg l-1 sodium chloride. In general, the results of this study showed that the foliar application of 200 mg l-1 humic acid and 10 mg l-1 selenium could be effective in reducing the negative effects of salinity stress on  C. officinalis.