A Novel Aspect of Essential Oils: Coating Seeds with Thyme Essential Oil induces Drought Resistance in Wheat

Coating seeds with biostimulants is among the promising approaches in crop production to increase crop tolerance to drought stress. In this study, we evaluated the potential of coating durum wheat seeds of the cultivar 'Karim' with thyme essential oil on enhancing seed germination and seedling growth, and on plant growth promotion and induction of drought resistance. Coated seeds were pre-germinated, grown in hydroponics, and grown in pots under controlled well-watered and progressive water/nutrient stress conditions. Seed coating with thyme oil increased germination rate and enhanced seedling growth development in hydroponics. In the pot experiment, thyme oil increased, when well watered, root and shoot development, chlorophyll, nitrogen balance index (NBI), abscisic acid (ABA), anthocyanins and flavonoids in leaves, decreased nitrogen isotope composition (delta N-15) and increased carbon isotope composition (delta C-13) of shoots. Increasing water/nutrient stress in control plants induced higher accumulation of ABA and anthocyanins coupled with a transient decrease in chlorophyll and NBI, a decrease in shoot and root development, the Normalized Difference Vegetation Index (NDVI), shoot C content, delta N-15, and an increase in delta C-13, revealing the avoidance strategy adopted by the cultivar. Thyme oil had the potential to enhance the avoidance strategy by inducing roots elongation, reducing the loss of shoot and roots dry matter and chlorophyll, maintaining balanced NBI, an decreasing anthocyanins, flavonoids, and delta C-13 via maintaining lower ABA-mediated-stomatal closure. Thyme oil increased shoot N content and delta N-15 indicating preferential uptake of the N-15 enriched NH4+. Coating seeds with thyme oil is suggested as a promising alternative approach to improve plant's water and nutrient status and to enhance drought resistance.