Effects of growth stage on essential oils and gene expression of terpene synthases in Mentha aquatica L.

In this work, the impact of different stages of plant growth on the composition of essential oils, the density of glandular trichomes, and gene expression of enzymes involved in terpenes production in Mentha aquatica were investigated. Our experiment was performed on 15-day-old plants (the initial stage of vegetative growth) and 45-day-old plants (the late stage of vegetative growth). Based on our investigation, the leaves of the M. aquatica plant were covered by non-glandular and glandular trichomes in the two growth stages. The density of glandular trichomes as the location of storage and biosynthesis of essential oils was higher in the late growth stage than in the early growth stage. The maximum harvest of essential oils was achieved at the late vegetative stage, representing that the generation of essential oils was boosted as the age of plants increased. Moreover, the growth stage influenced the essential oils composition in the M. aquatica plant. The main compounds of essential oils from M. aquatica plants in the early growth stage were menthofuran, limonene, germacrene D, pinene, viridiflorol, 1-terpinene, ledene, cymene, 3-carvon, terpinene, and cis-ocimene. The top compounds exit in the essential oils obtained from M. aquatica plants in the late vegetative stage were as follows: caryophyllene, cubebene, camphene, gurjunene, humulene, bicyclogermacrene, sabinene, 1-pinene, D-nerolidol, farnesene, 1,8-cineole, and cadinene. The ratio of sesquiterpenes to monoterpenes was enhanced as the plants developed. The expression level of the gene encoding enzymes that contributed to terpenoid production includes 1-Deoxy d-xylulose-5-phosphate synthase, geranyl diphosphate synthase, limonene synthase, isopentenyl diphosphate isomerase, and menthofuran synthase, which were also enhanced in the late growth stage. Gene expression studies supported our findings and demonstrated that the increased production of essential oils might be due to the stimulation of enzyme activity, contributing to their biosynthesis pathway. Overall, to obtain the maximum amount of essential oils, the late vegetative stage of M. aquatic is recommended.