The effect of chitosan and salicylic acid elicitors on morphological and phytochemical properties of Thymus daenensis Celak.

Thymus daenensis Celak. is an aromatic species with medicinal properties that is native to the northwest, center, and southwest of Iran. In Iranian traditional medicine, the infusion and decoction of the aerial parts of Thymus species are used to produce tonic, carminative, digestive, antispasmodic, anti-inflammatory, and expectorant substances, as well as to treat colds. This plant's essential oil contains several active compounds, the most important of which are thymol, paracemen, beta-caryophyllen, carvacrol, and gamma-terpinene. Among the numerous techniques used to increase the production of active compounds in essential oil, bioelicitors are the most acceptable. This study examined the impact of biological elicitors, chitosan and salicylic acid, on the quantitative and qualitative characteristics of Thymus daenensis. This research was designed in factorial experiment based on a completely randomized design with three replications during the 2016-2017 crop year at the institute of agricultural research in University of Zabol.  The first experimental factor was chitosan (0, 0.5, 1, and 1.5 mM) and the second factor was salicylic acid (0, 0.5, 1, and 1.5 mM). Thyme seeds were collected from the natural areas of Hamadan province in autumn 2015 and then was cultivated in December 2016 in 3 Kg pots containing equal parts garden soil, sand, and cow manure. Foliar elicitation was performed in two stages, every seven days, 4 months  after planting (10 cm plant height), at the stage of full bloom. The pigments of photosynthetic organisms were measured using spectrophotometry. The Folin–Ciocalteu and Aluminum chloride methods were used to measure the total phenolic and flavonoid contents, respectively. Using the Diphenyl Picryl Hydrazyl (DPPH) assay, antioxidant activity of extracts was determined. The essential oil was extracted using a clevenger-type apparatus and water distillation for 3 hours. The data were analyzed using SAS statistical software (Ver. 9.1), and the mean of the treatments was compared using Duncan's multiple range test. The effects of chitosan, salicylic acid, and their interactions on morphological and phytochemical parameters were significant. Salicylic acid and chitosan treatment produced the highest levels of fresh weight, dry weight, inflorescence length, chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids. The highest total phenol content was found in chitosan and salicylic acid (67.91 mg GA/g DW) (with concentrations of 1 and 1.5 mM, respectively). The greatest amount of total flavonoids (4.79 mg Q/g DW) was obtained when chitosan and salicylic acid were applied together (1.5 and 1 mM, respectively). The combination of chitosan and salicylic acid exhibited the highest antioxidant activity (98.13%) (both of them in 1 mM concentration). In addition, elicitation with salicylic acid (1.5 mM) produced the highest amount of essential oil (1.33 % V/W), a 47% increase compared to the control. The use of these elicitors can improve the yield and quality of Thymus daenensis. According to the findings, foliar application of salicylic acid and chitosan at a concentration 1.5 mM higher than other treatments had a positive effect on the quantitative and qualitative characteristics of this plant by stimulating the plant's physiological cycle and photosynthetic pigment synthesis. By increasing vegetative growth, it increased photosynthetic efficiency and the synthesis of secondary metabolites. Given the positive role of biostimulants of salicylic acid and chitosan in achieving a higher quantitative and qualitative yield in thyme, it is recommended that these compounds be used as the best option for achieving sustainable agricultural goals and reducing environmental risks.