Abstract
The main objective of the current study was to improve the essential oil contents of Thymus vulgaris L. using bio-inoculation with bacterial endophytes. Therefore, out of fourteen endophytic bacterial isolates obtained from roots of T. vulgaris, five isolates were selected based on the highest nitrogen-fixation and phosphate solubilization activity and identified as: Bacillus haynesii T9r, Citrobacter farmeri T10r, Bacillus licheniformis T11r, Bacillus velezensis T12r, and Bacillus velezensis T13r. These five strains have been recorded as ammonia, hydrogen cyanide (HCN), siderophores, and indole-3-acetic acid (IAA) producers. These strains have the efficacy to fix-nitrogen by reduction of acetylene with values of 82.133±1.4-346.6±1.4 n-mole-C2H4/ml/24 h. The IAA, gibberellic acid, abscisic acid, benzyl, kinten, and ziaten production were confirmed using HPLC. Two strains of T11r and T13r showed the highest plant growth-promoting properties and were selected for bio-inoculation of T. vulgaris individually or in a consortium with different mineral fertilization doses (0, 50, 75, and 100%) under field conditions. The highest growth performance was attained with the endophytic consortium (T11r+T13r) in the presence of 100% mineral fertilization. The GC-MS analysis of thyme oil contents showed the presence of 23 various compounds with varying percentages and the thymol fraction represented the highest percentages (39.1%) in the presence of the bacterial consortium.
Highlights
Thymus vulgaris L. or thyme (Family: Lamiaceae) is a flowering plant and is considered indigenous in Northern Africa, the Mediterranean, and various parts of Asia
Five endophytic strains identified as Bacillus haynesii T9r, Citrobacter farmeri T10r, Bacillus licheniformis T11r, Bacillus velezensis T12r, and Bacillus velezensis T13r were selected as the most potent strains
The strains Bacillus licheniformis T11r and Bacillus velezensis T13r were further selected based on the highest plant growth-promoting activity to be used as bioinoculant for T. vulgaris under different mineral fertilization doses (0, 50, 75, and 100%)
Summary
Thymus vulgaris L. or thyme (Family: Lamiaceae) is a flowering plant and is considered indigenous in Northern Africa, the Mediterranean, and various parts of Asia. The thyme plant is a common flavoring agent which has various importance in herbal medicine for the treatment of fever, cough, cold, diabetes, and chest infections. The importance of thyme plants is mainly related to their essential oils, which have antiviral, antimicrobial, and antioxidant activities along with their carminative actions [4]. According to European Pharmacopoeia, the minimum essential oil contents in the thyme plant is 12 ml kg–1, and it contains mainly six chemotypes, geraniol, γ-terpineol, linalool, thymol, carvacrol, and thujan-4-ol [5]. These findings raise the great necessity to improve the development and growth of the thyme plants for their essential oil contents
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