Investigating Effect‏ ‏‎of Harvest Time and Drying ‎Methods in Compounds and Essential oil Yeild of Mentha Piperita Accessions from Iran

Introduction
Preserving plant biodiversity is important for creating structural diversity and main compounds in the future for the sustainable development of human civilization. The basic requirement for plant breeding programs is a germplasm diversity that provides necessary facilities for breeding species with desirable features. Therefore, accurate identification of genotypes is considered as a prerequisite in this manner. Medicinal plants, as one of the important sources for the treatment of diseases, have been used from thousands years ago. These plants produce a major and diverse group of secondary metabolites. Secondary metabolites are the compounds derived from the primary metabolites (metabolites associated with the plant nutrition and survival) essential to sustain plant life. Peppermint (Mentha piperita L.) from Lamiaceae family is one of the most widely used medicinal plants. Is a medicinal plant that its extracted essential oil represents a viable source for therapeutic goals, perfume industry, and a flavoring or olfactory substance. The medicinal significance of this aromatic plant is owing to the presence of compounds in its essential oil.
 
Materials and methods
In this research the influence of harvesting time and drying method on the peppermint essential oil yield was evaluated. This experiment was conducted in research field Meshginshahr in 2021, year Factorial experiment based on randomized complete block design with three replications. In this research, different accessions of this plant were collected from different regions of Iran (Noorabad, Zanjan, Tabriz, Orumieh, Shiraz 1 and Shiraz 2). In this study, the impact of pick up and drying method of the aerial part of accessions were evaluated. Their essential oils were extracted by using of Clevenger apparatus and Chemical compositions of essential oil were identified with a GC-MS system.
 
Results and discussion
The aerial part of peppermint contains essential oil, phenolic and flavonoid compounds, fatty acids, vitamins, minerals, and salicylic acid. Menthol is the most important constituents of peppermint oil which synthesized and accumulated in glandular trichomes on the leaf surface. Analysis of variance showed that interaction effect of accession type with harvesting time, accession type with drying method and also harvesting time with drying method were significant at 1% probability level. The mean comparison of traits in total showed the Noorabad accession that were in the full flowering stage and dried in the shade had the highest amount of essential oil also, the lowest amount of essential oil was related to Tabriz mass in 50% flowering and shade drying method. In total, 30 compounds were identified, and the largest amount of compounds was found in the Orumieh accession and the highest number of compounds was observed in the Noorabad accession. Correlation analysis showed Propazulene and Oxalic acid r= 0.98 had the highest correlation and lowest correlation was showed between. Eucalyptol and Cadinol r= 0.01. The most important components of peppermint essential oil were: Menthol (52.20%), Eucalyptol (31.08%), Menthyl acetate (27.89%) and Caryophyllene (12.62%).
 
Conclusions
 
Identifying the compounds in the Iranian native plants, can provide a suitable field for the practical use of its plant resources and may be the basis of breeding programs. The Cultivation of medicinal plants such as Lamiaceae family has been increasing throughout the world considerably. Peppermint known as an important species in this family which it’s important is due to menthol in the essential oil. The various studies conducted on different species and populations of Mentha genus, have mentioned the valuable use of their extracts and essential oils in various fields such as pharmaceutical industries (in order to standardizing medicinal products), food, health cosmetics, etc. In general, it can be concluded that in this research peppermint plants had the highest amount of essential oil and compounds in the stage of reaching full flowering and with the shade drying method. Overall, according to the results, the accessions Orumieh and Noorabad could be recommended in terms of dry matter yield and phytochemical characteristics, respectively. The considerable variations were observed among populations in viewpoint of number, type and amount of essential oil compounds. Therefore, it is possible to select the desired accessions and identify superior traits and use them in breeding programs.