Exogenous application of selenium and nano-selenium alleviates salt stress and improves secondary metabolites in lemon verbena under salinity stress

Salinity stress is one of the most serious abiotic factors that affects the growth, performance, and secondary compounds of medicinal plants. The aim of this study was to determine the effect of foliar application of selenium and nano-selenium separately on growth, essential oils, physiological parameters, and some secondary metabolites of Lemon verbena under salinity stress. The results showed that selenium and nano-selenium significantly increased growth parameters, photosynthetic pigments, and relative water content. Compared to the control, a higher accumulation of osmolytes (i.e., proline, soluble sugars, and total protein) and higher antioxidant activity were observed in the selenium-treated plants. In addition, selenium alleviated the adversary effect of oxidative stress, resulting from salinity, by reducing leaf electrolyte leakage, malondialdehyde, and H2O2 accumulation. Furthermore, selenium and nano-selenium enhanced the biosynthesis of secondary metabolites such as essential oils, total phenolic content, and flavonoid compounds under non-stress and salinity conditions. They also reduced Na+ accumulation in the root and shoot of the salinity-treated plants. Hence it can be concluded that exogenous application of selenium and nano-selenium separately can mitigate the negative effects of salinity by improving the quantitative and qualitative performance of lemon verbena plants under salinity stress.