Evaluation of the effect of foliar application of brassinosteroid and melatonin on some physiological, grain yield and cumin (Cuminum cyminum L.) essential oil under drought stress conditions

Cumin (Cuminum cyminum L.) is one of the oldest aromatic plants in the world, known medicinally as the second domesticated medicinal plant in Iran. Drought stress, as oxidative stress, affects plant biochemical, physiological, and morphological responses. One of the ways to increase plants' resistance to environmental stress is to use plant growth regulators such as brassinosteroids and melatonin. These hormones increase plant performance in both stress and non-stress conditions. Plant growth regulators, by affecting plant metabolites, stimulating phytohormone biosynthesis, enhancing nutrient absorption, improving defense mechanisms, stimulating root growth, and improving gas exchange, contribute to plants' quantitative and qualitative performance in stress and non-stress conditions. Although some research shows that the use of plant growth regulators such as brassinosteroid and melatonin, which are of natural origin and compatible with the environment, has a significant effect on reducing adverse effects caused by environmental stresses; however, the combined use of these plant hormones is less reported. Therefore, in this research, the influence of the foliar application of brassinosteroid and melatonin on some physiological traits, including amount of malondialdehyde, hydrogen peroxide, electrolyte leakage, activity of antioxidant enzymes, accumulation of carbohydrates and proline, malondialdehyde, hydrogen peroxide as well as quantitative and qualitative yield of cumin was examined under water deficit conditions.

This research was conducted in the research greenhouse of the Faculty of Agriculture, Shahid Bahonar University, Kerman, Iran. A factorial experiment in a randomized complete block design with six replications was carried out in 2022. The experimental treatments included different levels of drought stress (100 % and 50 % field capacity; F.C.), melatonin (0 and 100 μM), and brassinosteroid (0 and 0.75 μM).

The results showed that water deficit increased the amount of malondialdehyde, hydrogen peroxide, electrolyte leakage, activity of antioxidant enzymes (peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) and superoxide dismutase (SOD)), as well as raised the accumulation of carbohydrates and proline in cumin plants. Indeed, brassinosteroids and melatonin significantly increased antioxidant enzyme activities and the concentrations of osmolytes. It also decreased the levels of malondialdehyde, hydrogen peroxide as well and electrolyte leakage in drought-stressed plants. Drought stress resulted in reduced cumin grain yield. In contrast, under these conditions, the percentage of cumin seed essential oil was increased. Foliar application of brassinosteroid and melatonin in stress and non-stress conditions improved cumin yield.

Overall, it was concluded that the use of brassinosteroid and melatonin increased the resistance of cumin plants against drought stress by increasing the activity of antioxidant enzymes, increasing the accumulation of compatible osmolytes such as proline and carbohydrate, and maintaining the stability of the membrane and ultimately enhancing the yield of cumin seeds. The application of melatonin and brassinosteroid also affected the quality of the product and increased the percentage of cumin essential oil; however, the positive effects of brassinosteroid and melatonin were greater than those of brassinosteroid or melatonin applied separately.