عنوان مقاله [English]
Introduction: Today, many investigations have done on effects of the elicitors on biosynthesis of secondary metabolites in the medicinal and aromatic plants, which the results of these studies indicate that the natural and synthetic elicitors such as jasmonates, salicylic acid, brassinosteroids, chitosan effect growth, yield, and active substances of the medicinal and aromatic plants. Thymbra spicata L. known "Mediterranean thyme" in English and "Avishan Zofaei" in Farsi belonging to the family Lamiaceae is an important medicinal and aromatic plant species growing wild in the Mediterranean regions. The aerial parts of the herb could be an important industrial crop because natural products in the form of volatile oil are used in pharmaceutical and food industries. The volatile oil of T. spicata as an important active substance is used in pharmaceutical, cosmetic, food, and perfumery industries.
Experimental: The seeds of Mediterranean thyme were sown in 25 cm plastic pots at a research greenhouse (polyethylene plastic) and thereafter, the pots of the T. spicata seedlings were transferred from the greenhouse to farm conditions at the research filed, Banganjab region, Western Iran (32° 20¢ N; 50° 51¢ E; altitude. 2070 m asl). In this study, the effect of the foliar applications including control and salicylic acid (Merck Co., Darmstadt, Germany) at two levels (2.5 and 5.0 mM) and chitosan (Sigma–Aldrich Co., Steineheim, Germany) at 0.5 and 1.0 g/L under different irrigation regimes including normal irrigation, slight water deficit stress (irrigation at 70% F.C) and mild water deficit stress on carvacrol content of the essential oil from T. spicata and expression of gene 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) were investigated. Experimental treatments were arranged as a factorial in a completely randomized design with three replications which each replication has 12 pots. Carvacrol as the major constituent in the essential oils from T. spicata, as reference compound (purity > 95%) was purchased from Sigma–Aldrich (Bornem, Belgium). Normalized data were analyzed based on the GLM procedure using SAS (SAS/STAT® v.9.2. SAS Institute Inc., Cary, NC). Duncan multiple range tests were used to compare the significance of differences among treatments at p values of < 0.05.
Results and Discussion: The level of DXR gene expression was determined by real time PCR and the carvacrol content was identified by GC-FID. Results indicated that the expression of studied gene and carvacrol content were strongly up-regulated with water deficit stress and the foliar application, especially salicylic acid. The results of the concentration of carvacrol of the T. spicata essential oil indicated that the foliar spraying × irrigation had significant effect on this constituent. The maximum and minimum values of carvacrol content were recorded for the foliar spraying of salicylic acid at 5.0 mM × mild water deficit stress and control (non-foliar application) × mild water deficit stress, respectively. Biosynthesis and accumulation of the secondary metabolites in the medicinal and aromatic plants is considered one of the main defense mechanisms that plants undergo to confer protection against different environmental stresses like water deficit stress. The foliar application of salicylic acid (especially at 5.0 mM) effectively has up-regulated the synthase DXR gene expression. However, the foliar spraying of chitosan had significant impact on DXR gen expression in compared with control. Generally, the foliar application of salicylic acid under water deficit stress condition can increase carvacrol content, in part, through increasing the expression level of DXR. Carvacrol is a monoterpenic phenol produced by an abundant number of the medicinal and aromatic plants, including thyme, savory, and oregano. Presently, natural carvacrol is used at low concentrations as a food flavoring ingredient and preservative, as well as a fragrance ingredient in cosmetic formulations. In recent years, considerable investigations have been undertaken in an effort to establish the biological actions of carvacrol for its potential use in clinical applications.
Extension: According to our results and reviewing of previous investigations, the foliar spraying of salicylic acid, which seems to be economically viable due to the increasing medical compound like carvacrol of Mediterranean thyme. Producers and farmers of this herb with the advice and preparation of salicylic acid solution by experts in laboratories, research centers, and university can improve the quantity and quality of this medicinal crop by the salicylic acid spraying on the aerial parts of Mediterranean thyme.