Effect of Plant Hormones On Productivity of Peppermint and Spearmint

Peppermint (Mentha X piperita L.) and spearmint (Mentha spicata L.) are major essential oil crops in the US and worldwide. The objective of this controlled environment study was to evaluate the effect of three plant hormones (salicylic acid SA, methyl jasmonate MJ, and gibberelic acid GA3, each at 10, 100 and 1000 mg/L, applied as a foliar spray), on biomass yields, essential oil content and oil yields of peppermint and spearmint. Overall, the application of SA at 1000 mg/L increased biomass yields of both mints relative to the control, while the other treatments were not significantly different from the control. The essential oil content was higher in peppermint than in spearmint, and treatments had different effect on the two mint species. The application of MJ at 100 and 1000 mg/L, GA at 10 mg/L, SA at 10 and at 100 mg/L, increased the essential oil content of peppermint, while the oil content of spearmint was not affected by treatments. None of the treatments reduced essential oil content relative to the respective controls. The overall essential oil yields (function of biomass yields and essential oil content) were also differentially affected by the treatments in the two mint species. The application of MJ at 100 mg/L and SA at 100 mg/L increased essential oil yields of peppermint, while the application of SA at 1000 mg/L increased oil yields of spearmint. Treatments did not reduce essential oil content or oil yield of either mint species. This study demonstrated that plant hormones may be used as a tool for increasing essential oil content and essential oil yields of peppermint and spearmint crops.  Further research is needed to elucidate the effect of these treatments on essential oil composition and to verify the effects under field conditions.