Trichomes micromorphology and essential oil variation at different developmental stages of Mentha pulegium L.

Mentha pulegium is an aromatic plant in the Lamiaceae family. Plants of this family estimated to contain 200 genera with over 4,000 species. M. pulegium is an aromatic plant with high industrial and commercial values.In this study, morphological and anatomical analysis of the trichomes on leaves, sepals, and petals of M. pulegium were carried out using light microscopy and scanning microscopy. The essential oils were obtained using hydrodistillation method and analyzed by Gas Chromatography (GC) and Gas Chromatography Mass Spectrometry (GC/MS). Morphological investigation revealed that these species had three types of non-glandular trichomes and two types of glandular trichomes. Histochemical test demonstrated both of trichomes have a positive reaction to neutral lipids, pectin, alkaloid compounds, phenolic compounds, and terpenoids. The results of oil analysis in different development stages show the presence of several compounds. The composition analysis of the essential oils using GC and GC/MS in vegetative and flowering phases indicated that pulegone and menthone were the major compounds with more than 90% of total oil composition. In mentha pulegium, pulegone amount is reduced from 76.58% to 68.19% and menthone amount is increased from 19.94% to 25.40%.Regarding to the reduction of pulegone and menthone amount from 96.52% to 93.59% and also appearance of new chemical compounds in flowering phase and increasing in trichomes density at this phase, it can be concluded that the plant exchange some amount of energy to produce new compounds with special volatility and fragrance as well as changing the synthesis pathway in trichomes cells.