Analysis on composition and difference of volatile organic compounds in thyme from different types of grassland in Ningxia

To study the composition and differences of volatile organic compounds (VOCs) in fresh and dry materials of Thymus monogolicus from grasslands under different climatic conditions of Ningxia. The aboveground samples of T. monogolicus were collected from the desert grassland of Mahuang Mountain in Yanchi, the typical grassland of Yunwu Mountain in Guyuan, and the meadow grassland of Nanhua Mountain in Haiyuan in early October. The volatile components were separated and identified using gas chromatography-mass spectrometry (GC-MS), and the relative content of each component was determined using the area normalization method. The results showed that 339 VOCs were detected in fresh and dry samples of different parts of thyme, which belonged to hydrocarbons (125 types), alcohols (70 types), esters (29 types), and other compounds (115 types). The amount of VOCs types in thyme from meadow grassland was greater than that from desert grassland and typical grassland, and fresh samples had more VOCs than dry samples. The relative content of hydrocarbons in volatile components was the highest, mainly including terpenoids p-cymene and γ-terpinene, which were 14.24%～42.24% and 3.30%～37.35%, respectively. The relative content of p-cymene from meadow grassland was higher than that from typical grassland and desert grassland, which in tender stems was higher than that in lignified stems and leaves, which in fresh samples and dry samples was simila. Phenols, alcohols and other volatile organic compounds mainly included carvacrol, eucalyptol, linalool and 2-isopropyl-4-methylanisole. The relative content of carvacrol from desert grassland was the highest, reaching 20.61%. The relative content of eucalyptol and linalool of thyme from meadow grassland was the highest, reaching 9.68% and 12.99%, respectively. In summary, climatic conditions, sample processing, and different organs have an impact on the types and contents of VOCs in thyme, whereas dry samples can still maintain the main components of thyme. These results provide a scientific basis for the development and application of wild thyme resources in Ningxia.