Effects of iron and zinc foliar spray on growth factors and essential oil constituents of Thymus vulgaris L. under nitrogen deficiency

The deficiency of nutrients such as nitrogen disrupts growth and changes the quantity and quality of essential oil. Foliar application of micronutrients improves plant growth. Therefore, due to this issue and its importance, the present study aimed to evaluate the effects of iron and zinc foliar spray on growth factors, percentage and essential oil components of thyme (Thymus vulgaris L.) under soil nitrogen deficiency. This study was conducted as a factorial experiment in a completely randomized design with three replications. Experimental treatments consisted of nitrogen levels (control and deficiency) and iron and zinc foliar spray (1% and 2%). The results showed that the essential oil yield of thyme decreased under nitrogen deficiency; however, the foliar application of micronutrients, especially zinc, significantly increased the essential oil yield. Also, thymol, γ-terpinene and p-cymene formed the most components of the essential oil. Thymol content increased in control plants (full strength Hoagland) with foliar spray and its highest amount (54.4%) was obtained with foliar spray at zinc-2%. The p-cymene content increased from 17% at nitrogen control level to 24.6% under nitrogen deficiency. In control plants (full strength Hoagland), foliar spray increased shoot length, and iron-2% increased shoot dry weight. The foliar application of micronutrients caused changes in the content of essential oil compounds and the synthesis of some new compounds such as 1,8-cineole in the essential oil. The foliar application also improved some growth factors such as shoot length and dry weight in the control plants.