Effect of Drought Stress and Different Concentrations of Salicylic Acid on Yield, Yield Components and Essential Oil of Fennel (Foeniculum vulgare Mill.)

Medicinal plants are of the most valuable reserves of natural resources in Iran, and if there is correct recognition of them, these plants can play an important role in the health of the community, employment, and non-oil exports. Fennel (Foeniculum vulgare L.) is a perennial herb that is one of the most important and is widely used as medicinal plants that is mainly cultivated for the purpose of using essential oils in various industries of pharmaceutical, food, cosmetic and sanitary. Therefore, this research was carried out with the aim of investigating the effects of different concentrations of salicylic acid on the qualitative and quantitative characteristics of fennel under drought stress conditions at phonological stages.

The experiment was conducted as a split-plot based on a randomized complete block design with three replications. The main factor included five levels of irrigation regimes: full irrigation, irrigation at stemming stage, irrigation at a budding stage, irrigation at flowering stage, and irrigation at seed filling stage. The subfactor included four different concentrations of salicylic acid: zero, 3, 6, and 8 mM. In the treatment of irrigation regimes, irrigation proceeded until the end of the growth period after irrigation was discontinued at the growth stage. At the end of the growing season, in order to evaluate the yield components, ten plants per plot were selected randomly, and the yield components were measured. In the southern half of each plot, seed yield was measured after collecting plants and seed separation. The essence of the seeds was extracted by the water distillation method, and the essential oil content was calculated.

 The results showed that two treatments for non-application of salicylic acid in irrigation in the grain filling stage and application of six mM of salicylic acid at irrigation at flowering stage obtained the highest number of sub-branches per plant with 21.3 and 20.3 sub-branches in the plant, respectively. Treatment of application of 8 mM salicylic acid in irrigation at seed filling stage revealed the largest number of umbrellas per plant and weight of 1000 seeds. The largest number of umbellet per umbrellas belonged to six and eight mM salicylic acid applications in the treatment of irrigation at the seed filling stage. The lowest amount of seeds in the umbellet was observed at different levels of salicylic acid in irrigation at the flowering stage. Treatments of application of 8 mM salicylic acid in irrigation at budding stage and application of zero and three mM salicylic acid incomplete irrigation conditions obtained the highest seed yield by producing more than 790 kg per hectare. Application of zero and three mM salicylic acid at irrigation in seed filling stage showed the highest amount of essential oil by 3.83 and 3.73% essential oils, respectively, and after those, two treatments of application of 8 mM salicylic acid at irrigation in the flowering stage and at the time of seed filling were placed at the next rank.

In general, it seems that the occurrence of drought stress at the flowering stage compared to stress in other stages of development, had the greatest effect on reducing the yield components and grain yield of fennel. Also, the application of salicylic acid in conditions of irrigation removal in the budding stage improved seed yield, as well as at stemming and flowering stage, increased the amount of essential oil of fennel seed. However, its effectiveness did not follow a particular process. In addition, the highest amounts of essential oil were observed at the time of drought stress in the grain filling stage. The results showed that application of zero and three milliliters of salicylic acid produced the highest amount of essential oil at irrigation interruptions in the grain filling stage.