The Role of signaling of hydrogen peroxide and 24-epibrassinosteroid on physiological traits of cumin (Cuminum cyminum L.) under drought stress

Two biochemical compounds of hydrogen peroxide and 24-epibrassinosteroid have significant biological effects on plant growth. In the present study, the effects of drought and its interaction with H2O2 and 24-epibrassinosteroid on the protein, sugars, essential oil percentage, photosynthetic pigments, phenols and flavonoids were investigated. For this purpose, a factorial experiment in a completely randomized design with three replications was conducted in a greenhouse of Agricultural Research. Plants were planted in pots. In the fifth week after germination, plants were treated under drought stress at three levels: 100% (control), 75% and 50% field capacity. The plants were sprayed with hydrogen peroxide and 24-epibrassinosteroid at concentrations of 0, 0.5 and 1 mM, sequentially in two steps: three days before stress and 15 days later. The results showed main effects of drought stress and spraying had a significant effect on the studied traits, so that with increasing stress level, the essential oil percentage, soluble sugars, carotenoids, flavonoids of the shoot increased, and the shoot protein and chlorophyll contents decreased. With increasing levels of H2O2 and 24-epibrassinosteroid, the contents of chlorophyll a and b increased, so that by spraying with 1 mM 24-epibrassinosteroid, chlorophyll a reached the highest level (10.90 mg mL-1). Spraying 24-epibrassinosteroid also increased the shoot phenol content (1.58 Obsarbance g) and decreased essential oil percentage (2.44%) under severe stress. The results showed hydrogen peroxide and 24-epibrassinosteroid acted as signal molecules at optimum concentrations and allowed the cumin plant to adapt to drought conditions by reducing membrane peroxidation and inducing physiological activities.