The study of responses of garden thyme (Thymus vulgaris L.) to the elicitors of coronatine, methyl jasmonate and cyclodextrin in different levels of supplying moisture under farm conditions

Medicinal plants contain rich storages of secondary metabolites or active compounds which are strongly affected by environmental factors, especially drought stress, and also by hormones and their derivatives. Thyme genus has more than 215 species in the world, of which 18 species have been identified in Iran. Thymus vulgaris is one of the main medicinal plants that is cultivated in the most countries. Its main components include Thymol and Carvacrol which have antibacterial, antifungal and antioxidant activity and these compounds can be effective in reducing the symptoms of COVID-19. The levels of moisture supply and elicitors like Coronatine (COR), Methyl Jasmonate (MJ) and Cyclodextrin (CYC) can be effective on the quantitative and qualitative yield.

This investigation was conducted as split plot based on a randomized complete block design (RCBD) with three replications in the farm of Research Station of Khorasan Razavi Agricultural and Natural Resources Education and Research Center which is placed at 5th km Southeast of Mashhad in Iran. The main plots included three levels of moisture supply (at 40%, 65% and 90% field capacity (FC)) and the sub plots were elicitor levels included (1) control without elicitor, (2) 150 µM cyclodextrin (CYC) (3) 75 nM coronatine (COR) + 150 µM CYC (4) 150 nM COR + 150 µM CYC (5) 150 µM methyl jasmonate (MJ) + 150 µM CYC (6) 300 µM MJ + 150 µM CYC. The seeds were sowed in pro-trays which were filled from peat moss on 24 March 2019. After the germination, two seedlings were held in each cell of portray for transplanting to the field. The seedlings were transplanted to each point of cultivation on 24 May 2019. The treatments of the moisture supply were applied 45 days after transplanting the seedlings. In order to apply moisture treatments, at the first, FC was determined by farm and pressure plate method. With sampling the soil of plots (mean of two sample 0-15 cm and 15-30 cm) for several times, was determined the irrigation date. Elicitor treatments were applied two times; the first was 45 days after transplanting and the second was two months later. The plants were harvested two weeks after the last elicitor spraying.

The largest crown diameter was observed in interaction of elicitor treatments and the levels of moisture supply at 90% FC.The highest leaf proline content (from 5.5 to 5.8 µmol g-1 fresh leaves) was observed in interaction of 4 levels with elicitor treatment (levels 3, 4, 5 and 6) at the level of moisture supply 40% FC that placed in one group. The highest and lowest electrolyte leakage (47.6% and 22.1%, respectively) were related to interaction of the control treatment at the level of moisture supply of 40% FC and the treatment of 150 μM MJ + 150 μM CYC at the level of moisture supply 90% FC. The highest yield of dry herb and leaves and essential oil (5.64 and 3.2 t ha-1 and 65.9 kg ha-1 respectively) was obtained under moisture supply at 90% FC. Except the percentage of essential oil, there was a significant correlation between all measured traits including crown diameter, height, electrolyte leakage, proline content, relative water content of leaf, fresh and dry yield of herb, dry leaf and stem yield and essential oil yield. Although the interaction of the treatments affected some physiological traits, but the quantitative and qualitative yield of the plants were affected by moisture levels.

As a result, drought stress and elicitors can effect on plant performance; morphological, physiological and chemical dynamics. In this study, suppling moisture levels had an impotent effect on biological yield and this indicator caused to increase essential oil yield. Elicitors and interaction of treatments had a significant effect on some physiological characteristics.