Effect of terminal season water stress and foliar spraying of humic acid on morphology, productivity and yield of essential oil of three fennel (Foeniculum vulgare L.) ecotypes

Introduction Water is one of the most important environmental factors that has a significant effect on the growth, development and active ingredients of medicinal plants. Developing plant that has minimum water requirements and maximum production in dehydration conditions is the one of ways to cope with the problem of water scarcity. Medicinal plants and especially their native species due to genetics, morphological characteristics and having some secondary metabolites have higher stress tolerance potential than other herbs and are suitable alternatives for cropping under stressful conditions. Today, the use of various types of organic acids has been used to improve the quality and quantity of crops and horticulture and tolerance to drought stress. Humic acid is a natural organic material that is obtained from the final degradation of soil organic matter by microorganisms. Humic acid stimulates the growth of aerial parts and root plants. Application of Humic acid increases the amount of roots and improves the root system. Fennel is a herbaceous, aromatic and perennial grasses of umbrella and grown in warm areas of the country. In Khuzestan, due to planting in autumn, the plant reproductive stage begins with the onset of heat, precipitation discontinuity (mid-March) and dryness, so we must determine the levels of irrigation and provide solutions to reduce the effect of drought stress on the production. The comparison of genetic material and introduction of superior ecotypes are the solution to obtain adaption ecotypes. Therefore, the present study was carried out with the aim of studying the growth and yield characteristics, as well as the percentage and essential oil yield and irrigation efficiency of essential oil production in fennel ecotypes under irrigation levels and foliar spraying of humic acid in Ahwaz weather condition. Materials and methods This experiment was carried out on factorial split plot based on randomized complete block design with three replications in 2016-2017 years at research farm of Ramin Agriculture and Natural Resources University of Khuzestan. Irrigation levels (complete irrigation, irrigation stop from flowering stage, irrigation stop from grain filling stage) and foliar spraying of humicacid (no spraying and foliar spraying 400 mg.l-1) in the form of factorial in main plots and ecotype (Isfahan, Hamadan, Bushehr) were also investigated in sub plots. Number of nodes, stem diameter, second internode length, number of umbrellas in main stem, umbrella number in sub branch, seed yield, essential oil percentage, essential oil yield and water use efficiency were evaluated after harvesting. Statistical analysis including analysis of variance, comparison of averages, using statistical software system 2 / 9SAS, and comparison of mean of data did with LSD test at 5% error probability level. Results and discussion The results showed that irrigation stop, ecotype and foliar spraying of humic acid had a significant effect on the traits. Foliar spraying of humic acid increased the length of the internode and the number of umbrella in the sub branch. The Isfahan ecotype with 6.22 nodes was the highest and the Hamadan ecotype with 4.75 nodes had the lowest number of nodes. The highest stem diameter was obtained under complete irrigation and foliar spraying of humic acid (1.93 cm). Increasing drought stress decreased the length of the second internode, while it was the highest in complete irrigation conditions (11.36 cm). Foliar spraying of humic acid increased by 20% in umbrella number in the sub branche, and Bushehr ecotype with 61.25 in complete irrigation conditions had the largest number of umbrella in the sub branch. Also, the least umbeller in the main stem was obtained in Bushehr ecotype under foliar spraying of humic acid conditions (4.82) and the lowest in the Hamedan ecotype with no spraying. The results showed that the highest grain yield was related to Bushehr ecotype with 1720.12 kg / ha. The highest yield and essential oil efficiency were obtained in Bushehr ecotype under complete irrigation and foliar spraying of humic acid. Conclusions Based on the results of this study, water shortage tension due to irrigation stop in each stage of reproductive development of fennel can have adverse effect on grain yield, but the effect of stress in flowering stage is much higher than grain filling stage. The results of the experiment showed that irrigation stop at the reproductive stage of fennel had a decreasing effect on all studied traits. The use of organic fertilizer such as humi cacid reduced lightly the effect of water stress. Also, Bushehr ecotype is recommended for cultivation in the studied area due to having more umbrellas in main stem and sub branch, higher grain yield, higher water use efficiency along with foliar spraying of humic acid