Introduction: Cumin (Cuminum cyminum L.)  is one of the most important medicinal plants grown in arid and semiarid environments. Cumin is suitable for rainfed cultivation due to its short growing season, leaf shape, canopy structure, and low water requirement. However, when rainfall falls short of crop requirements, rain-fed agriculture does not produce adequate economic yields. Several agronomic and physiological strategies have been implemented to mitigate the negative effects of drought stress and increase plant tolerance to it. Utilizing plant hormones is one of the realistic and promising strategies for increasing crop yields under stressful conditions. Few studies have examined the use of these substances as stress modulators in medicinal plants. This research was conducted to investigate the effect of stress modulator application on seed and essential oil yield of cumin in dryland and irrigated conditions.
Materials and Methods: The split-plot factorial experiment was done based on a randomized complete block design with 3 replications on a private farm in the city of Jaghtai (Khorasan Razavi) in 2017-18. The cultivation method at two levels (dryland and irrigated conditions) and sources of stress modulators at four levels (control, 1.5 M Glycine Betaine, 10^-7 M Brassinosteroids, and 10^-3 M Sodium nitroprusside) were studied as the main plot and subplot, respectively. The experimental field underwent wheat and barley cultivations in the previous first and second years. For soil preparation, the land was plowed in autumn. The fertilizers consisted of P₂O₅ (75 kg.ha^-1), K₂O (100 kg.ha^-1), and N (100 kg.ha^-1) represented ammonium diphosphate, potassium sulfate, and a form of urea, and were applied at sowing under irrigated conditions. In the rain-fed condition, nitrogen fertilizer application was reduced to 50 kg.ha^-1. At the end of the growing season, 10 plants were randomly selected from each plot and their cumin yield components were measured, including final plant height, number of branches, number of umbels per plant, number of seeds per plant, and 1000-seed weight. To determine grain yield and biological yield, a 2.5 m² area was harvested from each plot after removing the margins. Using the steam distillation method, the Clevenger apparatus was utilized to measure the essential oil. For this purpose, 50 grams of the seeds in each treatment were selected, crushed with a laboratory mill, and dissolved in 500 mL of water for three hours.The essential oil yield was obtained by multiplying the essential oil percentage by seed yield. ata analysis was performed using SAS software (ver. 9.4) and tables and charts were drawn using Word and Excel software. Mean data was compared with LSD method.
Results and Discussion: Slicing of moderator interaction at the planting method levels revealed that foliar application with brassinosteroids in irrigated and dryland conditions increased number of umbrellas per plant (150 and 137%), number of seeds per plant (85.5% and 41.7%) and seed yield (57.9% and 44.8%) relative to no foliar application. Under irrigated conditions, the type of stress modulators affected the number of lateral branches and essential oil yield, with foliar application of glycine betaine producing the highest number of lateral branches and foliar application of brassinosteroids producing the highest essential oil yield. Foliar application of brassinosteroids decreased the essential oil content of irrigated plants by 8.81%.
Conclusion: In general, the results of the experiment indicated that in cumin, in order to obtain proper seed and essential oil yield, cultivation should be done as irrigated and the most suitable modifiers to obtain maximum grain and essential oil yield could be classified as brassinosteroids > glycine betaine > Sodium nitroprusside.