Investigating the effect of water stress on yield, essential oil content and physiological characteristics of Thymbra spicata L

Thymbra spicata L. is a genus of Thymbra plant in the family Lamiaceae (Kizi, 2010). The essential oil of the plant is widely used in the industries cosmetics and hygiene. It is also used as an antiseptic for pathogenic pathogens. The decoction (hot aqueous extract) of the plant is used as an anti-cold and antimicrobial drug (Sarac et al, 2009; Inan et al, 2011; Kizil et al, 2015). The results of some studies have demonstrated that the stress caused by water shortages decreases the growth of different parts of the plant, including reduction in leaf area, height, dry weight, stomatal closure, photosynthesis reduction, amino acid accumulation, chlorophyll depletion, degradation of enzymes and proteins, and changes in protein structure (Gorgini Shabankareh et al, 2016). Moreover, in a study of the effect of water stress on morpho-physiological and phytochemical characteristics of Thymus kotschyanus Boiss, it was reported that when water stress increased, the morphological traits (height, leaf area, root length and dry weight of the plant), essential oil content and composition, and water consumption efficiency were reduced (Bahreininejad & Razmjoo, 2014). The experiment was conducted as a split-plot based on randomized complete block design with three replications at Research Farm of the Faculty of Agriculture, Ilam University, during 2016 - 2017. The main plots consisted of three levels of normal irrigation, mild stress, and severe stress (irrigation after 40, 80 and 120 mm cumulative evaporation from a class A pan evaporation), and subplots included two ecotypes of Thymbra (Maleshahi and Sumar). Thymbra was first cultivated in greenhouse conditions in February 2015. Then the seedlings were transferred to the field in May 2016. During the first year of the experiment, irrigation was done regularly every 7 days by Flooding irrigation method until the proper placement of the plants in soil. Then, drought stress was applied according to the defined level of evaporation from pan evaporation.
Measurement of plant height: 10 plants per plot were measured randomly and the average values were recorded.
Measurement of leaf area index: In order to measure the leaf area index of samples, a digital Leaf area meter (CI-203CA conveyor attachment) was used.
Dry leaf and plant yield: The samples were taken from an area of one square meter of plant and placed in an oven for 48 hours at 75 ° C, then dry weight was measured.
Percentage and yield of essential oil: To extract the essential oil, the samples were dried in shade and at 40 ° C. Next, their dried leaves were ground. Then, using a precision digital scale, 40 g of leaf powder of each sample was separated and the essential oil was extracted using water distillation method by Clevenger apparatus.
Photosynthetic pigments: Photosynthetic pigments including chlorophyll a and b, total chlorophyll, and carotenoids were measured by using Lichtenthaler method (1987). Dry leaf and plant yield: The interaction effects of year and water stress, water stress and ecotype, and year and ecotype, were significant on dry leaf yield at 1 and 5%, respectively. The highest dry leaf yield was obtained from mild water stress levels and normal irrigation in the second year of planting. The highest dry leaf yield was obtained from Maleshahi treatment. The Interaction effects of year, water stress, and ecotype on plant dry yield were significant at 5% probability level. The highest measure was obtained from the second year at normal irrigation and mild stress levels and in Malekshahi ecotype.
The yield of essential oil: The results of combined analysis of variance showed that the essential oil yield was affected by simple effects of year, water stress and ecotype at the probability level of 1%, and the interaction effects of water stress and ecotype at 5% probability level. It was also observed that the simple effect of water stress and ecotype on essential oil percentage was significant at 1% probability level. The highest essential oil yield was obtained from mild water stress and Malekshahi ecotype, and the lowest was observed in severe water stress and Sumar ecotype. In terms of essential oil percentage, mild water stress with an average production of 3.17% was superior to normal irrigation and severe stress. Between the studied ecotypes, Malekshahi with an average production of 3.38% produced a higher than Sumar.
Photosynthetic pigments: Water stress, ecotype and year had a significant effect on chlorophyll a, b, total, and carotid. As the water stress level increased, the amount of these pigments decreased. Chlorophyll a was more sensitive to dry stress than other pigments. Increase in water stress, due to the reduction of Turgor pressure and cell growth, resulted in a decrease of plant height, leaf area index of plants. Also, because of the decline of leaf area and photosynthetic pigments, the amount of photosynthesis and thus the yield of a dry plant, leaf and the yield of essential oil in the plant were decreased.