Abstract
Introduction
Anethum graveolens L is used to treat colds, coughs, urinary problems, bloating, and spasms. Water deficit may cause significant changes in the yield and composition of essential oils of medicinal plants. In order to improve water stress tolerance of plants, various mechanisms have been proposed. The use of materials such as vermicompost, zeolite, superabsorbent and methanol can partially compensate for the lowering of yield from drought stress. Therefore, it is important to use methods that can tolerate plants under drought stress conditions.
 
Materials and Methods
To evaluate the effect of organic and chemical growth improvers application on yield and essential oil percentage of dill (Anethum graveolens L.) under water deficit stress conditions, a field experiment was conducted as split plot in randomized complete block design with three replications in Birjand, Iran, during 2017 growing season. The main factor included three levels of water stress (60, 120, 180 mm evaporation form class A pan) and the sub-factor included five levels of organic and chemical growth improvers; control (no application), zeolite (5 t.ha^-1), vermi compost (6.5 t.ha^-1), super absorbent polymer (100 kg.ha^-1) and methanol (20 v/v).
 
 
 
Results and Discussion
The results showed that water deficit significantly reduced all studied traits except harvest index and essential oil percentage. Also, organic and chemical improvement agents had a positive and significant effect on plant height, number of umbrellas per plant, grain yield, biological yield and essential oil yield. Interaction of water deficit stress and organic and chemical improvement agents was significant on number of umbrellas per plant, number of seeds per umbrella, grain yield, biological yield and essential oil yield. By increasing the intensity of water deficit stress, the use of organic and chemical improvers increased the yield of seeds and essential oils. Under severe stress conditions and in comparison, with control treatment, super absorbent application increased the yield of seeds and essential oil to 34.43 and 37.27%, respectively, and methanol spraying increased the yield of seeds and essential oils by 34.17% and 42.85. The results of this experiment showed that the use of organic and chemical improvers reduced the damage to seed yield and essential oil yield under water deficit stress conditions. Also, the results revealed that when the plant is exposed to intensive stress, substances such as superabsorbent and methanol can be used to modify the stress. Using these materials under severe stress conditions increased the grain yield by 34% compared to the control. In order to achieve maximum yield of essential oil, use of organic and chemical improvers has been beneficial. Under severe stress conditions, methanol spraying and super absorbent application increased the essential oil yield of dill (Anethum graveolens L.) in Birjand region.
 
Conclusion
In order to achieve the maximum yield of essential oil, use of organic and chemical improvers has been beneficial. Under severe stress conditions, methanol spraying and super absorbent application increased the essential oil yield of dill (Anethum graveolens L.) in Birjand region.