Introduction: Drought stress is one of the most important challenges in agriculture, causing significant economic losses by reducing plant growth, yield loss, and changes in effective compounds. Reducing the adverse effects of water deficit stress on plants is one of the most important challenges in agriculture in arid and semi-arid regions, which requires scientific, managerial, and technological solutions. Cumin (Cuminum cyminum L.), as a valuable medicinal plant, is highly sensitive to water deficit, and drought stress can affect both the quantity and quality of its essential oil. Therefore, finding effective solutions to reduce the negative effects of drought through the use of stress modifiers (such as jasmonic acid, paclobutrazol, salicylic acid, chitosan, abscisic acid micronutrients such as silica and selenium, root growth-promoting bacteria (PGPR), mycorrhizal fungi and organic compounds such as seaweed extract and humic acid) is of great importance. Stress modulators in crop plants are substances that help plants to resist environmental stresses such as drought, salinity, extreme temperatures and diseases. These modulators maintain plant performance and health under stress conditions by strengthening the plant's defense system, improving water and nutrient absorption and reducing oxidative damage.
Materials and Methods: In order to investigate the effect of foliar spraying of stress modifiers on yield and yield components as well as water use efficiency of cumin, an experiment was conducted in the 2019-2020 crop year at the Birjand Islamic Azad University Research Farm. The experiment was conducted in split plots in a randomized complete block design with three replications. Drought stress at three levels (40, 70 and 100% of plant water requirement in terms of the percentage of evapotranspiration of the reference plant defined as control, moderate water deficit and severe water deficit) was considered as the main factor and foliar spraying at five levels (control "no foliar spraying", foliar spraying with salicylic acid (300 mg/L), jasmonic acid (60 μmol/L), paclobutrazol (100 mg/L) and chitosan (50 mg/L) was considered as the secondary factor. Due to the possibility of leaching, the amount of nitrogen fertilizer was added to the soil in two stages (half of the fertilizer at planting and the other half after planting as a slurry at the four-leaf stage from the urea fertilizer source), and phosphorus fertilizer (pure P₂O₅ from the triple superphosphate source) and potash fertilizer (from the potassium sulfate source) based on soil tests and the recommended rates of P=60, N=30, and K=60 pure kg/ha. The green cumin seeds required for the experiment were obtained from the native seed mass of Birjand, at a rate of 20 kg/ha from Sistan Bazr Zabol Company (registration number 1974). To implement irrigation regimes at three levels of 100, 70, and 40 percent of plant water requirement, first the reference evapotranspiration rate (ET₀) was calculated using the standard Penman-Monteit equation based on FAO-56 guidelines and daily meteorological data (including temperature, relative humidity, solar radiation, and wind speed). Then, by multiplying ET₀ by the cumin plant coefficient (Kc), the actual plant water requirement (ETc) was determined. For foliar spraying, the compounds used were mixed with appropriate solvents and applied as foliar sprays twice during the growing season, the first time after one irrigation before stress application and the second time at the beginning of the reproductive stage of the plants according to the design plan. Cumin was harvested in the first half of May 2020, when the plants turned yellow and the umbels turned brown. Water use efficiency was determined by dividing the seed yield obtained to the irrigation water applied. Data 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 Duncan multiple range test method.
Results and Discussion:  The results indicated that drought stress significantly reduced growth traits such as plant height (52.3%) and the number of umbels per plant (29.4%). The greatest number of lateral branches was recorded under non-stress conditions with paclobutrazol foliar application. Drought stress also decreased the number of seeds per umbel (34.02%) and water use efficiency (22.2%). The maximum seed yield (533 kg ha⁻¹) was obtained under non-stress conditions with paclobutrazol foliar application. Under severe drought stress, jasmonic acid and paclobutrazol resulted in the highest seed yields. Although the essential oil percentage increased under severe drought stress, foliar application of jasmonic acid reduced it. The highest essential oil yield and water use efficiency were observed under moderate drought stress with jasmonic acid foliar application
Conclusion: In general, the use of growth modifiers, especially jasmonic acid and salicylic acid, can help reduce the negative effects of drought stress and improve the yield of cumin.