Light stress is one of the important environmental stresses which at high levels could destroy plants. However, in medicinal plants using ultraviolet-induced stress could activate secondary metabolite biosynthesis systems and could increases the values of medicinal plants. Current research was aimed to investigate the effect of UV-A radiation on growth and physiology of peppermint. Potted peppermint plantlets were cultured in glasshouse condition and were radiated by UV-A using light-emitting diode light at 365 nm at four treatments of 0, 1, 2 and 4 hours UV-A radiation. Treated plants were harvested after 60 days and were analyzed for their shoot length, leaf area, fresh weight, dry weight, dry matter, chlorophyll concentration, total protein concentration, total phenolic concentration and essential oil. Result indicated lower growth and yield in UV-A radiated plants. Although there were no significant differences between 1 and 2 hour UV-A radiation on chlorophyll in leaves, its concentration reduced in those of 4 hour radiated plants. In contrary, total phenolic concentration and essential oil concentration increased in UV-A radiated plants. In conclusion, it seems that peppermint is sensitive to UV-A radiation which for facing up to its stress, increases phenolic compounds production. Using modest stress of UV-A in peppermint by reducing growth and yield could increase its secondary metabolites.
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