Interaction Effect of Different Levels of Zinc and Methyl Jasmonate on Morphological Characteristics and Photosynthetic Pigments of Peppermint in Soilless Culture Conditions

A pot experiment in soilless culture was performed as a factorial experiment based on the randomized block complete design with four replications in the research greenhouse of Faculty of Agriculture, Ferdowsi University of Mashhad, Iran. The effect of foliar application of methyl jasmonate (MeJA) [distilled water (control), ethanol 1% (as solvent of MeJA and control), 1 and 2 mM MeJA] and Zn application (0, 0.025, 0.05 and 0.1 mgL-1) in Hoagland nutrient solution on the growth characteristics, photosynthetic pigments, and essential oil production of Mentha piperita L. was evaluated. According to the results, the highest growth was observed in 0.025 mg L-1 Zn and foliar application of 1 and 2 mM MeJA. The highest photosynthetic pigments of chlorophyll a and total chlorophyll were obtained in 0.025 mg L-1 Zn and foliar application of 2 mM MeJA. The highest root fresh and dry weights, chlorophyll b and carotenoid were observed in 0.05 mg L-1 Zn and foliar application of 2 mM MeJA and the treatment without Zn and foliar application with distilled water. In addition, the highest amount of essential oil production was observed in 0.1 mg L-1 Zn and foliar application with 1 mM MeJA. In general, 0.025 mg L-1 Zn and foliar application of 2 mM MeJA was the best treatment and improved most studied traits. The results of this experiment showed that Zn in low concentrations stimulate the peppermint growth; whereas Zn omission or application of high concentrations created some stressful effects in this plant increasing its essential oil production. Therefore, application of proper concentrations of Zn and MeJA can be a suitable way to improve the growth and essential oil production of peppermint.