Influence of zinc and salicylic acid foliar application on total chlorophyll, phenolic components, yield and essential oil composition of peppermint ( Mentha piperita L.) under drought stress condition

A two-year field experiment was conducted at the Islamic Azad University, Iran, in 2018 and 2019 growing season to examine the influence of foliar application of zinc and salicylic acid on chlorophyll, phenolic and flavonoid content, essential oil percentage, oil yield, total dry matter and oil composition of peppermint (Mentha piperita L) under drought stressed conditions. Irrigation regimes (no stress, moderate, and severe drought stress), zinc (0, 2 and 4 mg L−1), and salicylic acid (0 and 1 mM) foliar application were included in the treatment group. Drought stress reduced the chlorophyll content and oil percentage of peppermint, but moderate drought stress increased oil percentage. The application of zinc increased total phenolic compound content. Total phenolic and flavonoid contents increased with increasing stress levels. Comparisons of the drought and zinc treatments showed that the highest DPPH (2,2-diphenyl-1-picrylhydrazyl) was in severe drought stress plus 2 mg L−1 zinc treatment. Drought stress and salicylic acid comparisons showed that the highest oil yield was observed in well-watered plants and salicylic acid. The application of zinc + salicylic acid resulted in higher oil percentage and oil yield compared with the non-treated control. Menthol and menthone concentrations increased significantly in response to moderate drought but were reduced under severe drought. Moderate drought stress increased synthesis of oil compounds while foliar applications of zinc and salicylic acid alleviated its effect.