Phenological rhythms of volatile substances in Thymus mongolicus leaves

Thymus mongolicus is an aromatic herb that performs the functions of aromatization, greening, and beautification. This study aimed to provide data support and a theoretical basis for volatile substances in T. mongolicus by exploring the relationship between the types and amounts of volatile substances released in T. mongolicus leaves and the changes occurring with phenological period. Headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) combined with an external standard method was used to detect the volatile substances released by T. mongolicus leaves during different phenological periods. The volatile substances release rhythms of T. mongolicus were explored by analyzing the composition and content of the volatile substances. A total of 69 volatile compounds, including olefins, alcohols, esters, aromatic hydrocarbons, and ketones, were detected in T. mongolicus leaves at four phenological stages. The types and amounts of volatile compounds released from T. mongolicus leaves at different phenological stages were different. The types of volatile species varied in the order of the vigorous growth (53 species) > flowering (50 species) > fruiting (43 species) > decay (28 species) periods. Volatile substances content varied in the order of flowering (163.90 μg·g⁻¹) > fruiting (157.21 μg·g⁻¹) > vigorous growth (104.47 μg·g⁻¹) > decay (75.42 μg·g⁻¹) periods, showing a trend of increasing first and then decreasing. Eleven common high-content volatile substances γ-terpinene, camphene, bicyclo3.1.0hexagon-2-ene, laurylene, sabinene, β-pinene, trans-Caryophyllene, eucalyptol, (-)-Borneol, o-isopropylbenzene, and 2-methoxy-4-methyl-1-(1-methylethyl) benzene were detected in the leaves at different phenological stages. Among these, the levels of γ-terpinene (44.96 μg·g⁻¹), trans-Caryophyllene (9.50 μg·g⁻¹), and 2-methoxy-4-methyl-1-(1-methylethyl) benzene (7.02 μg·g⁻¹) were high at the flowering stage; camphene (14.94 μg·g⁻¹), bicyclo3.1.0hexagon-2-ene (13.13 μg·g⁻¹), laurylene (9.09 μg·g⁻¹), β-pinene (4.09 μg·g⁻¹), sabinene (3.13 μg·g⁻¹), and cineole (18.90 μg·g⁻¹) were high at the fruiting stage. The levels of (-)-Borneol (6.41 μg·g⁻¹) and o-isopropylbenzene (52.93 μg·g⁻¹) were the highest at the vigorous growth stage and decay stage. In summary, the phenological period influenced the types and amounts of volatile substances in T. mongolicus leaves.