The role of hydrogen peroxide and 24-epibrassinosteroid signaling 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 and development, including increased plant tolerance to environmental stresses. In the present study, the effects of drought and its interaction with H2O2 and 24-epibrassinosteroid on protein content, sugars, essential oil percentage, photosynthetic pigments, phenols, and flavonoids were investigated. A factorial pot experiment in a completely randomized design was conducted with three replications in the research greenhouse south of Kerman province and plants. In week 5 after germination, plants were treated under drought stress at three levels: 100%, 75%, and 50% field capacity. The plants were sprayed with hydrogen peroxide and 24-epibrassinosteroid at 0, 0.5, and 1 mM concentrations, sequentially at two steps: three days before stress and then, 15 days later. Results showed a significant effect of drought stress and spraying on the studied traits, so that with increasing stress level, the essential oil percentage, soluble sugars, carotenoids, phenolic compounds, and shoot flavonoids 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 spraying with 1 mM 24-epibrassinosteroid led to the highest level of chlorophyll (10.90 mg mL-1). Spraying 24-epibrassinosteroid also increased the shoot Flavonoids content (1.58 mg/g DW) and decreased essential oil percentage (2.44%) under severe stress conditions. Results showed that hydrogen peroxide and 24-epibrassinosteroid played as signal molecules at optimum concentrations and allowed the cumin plant to adapt to drought conditions by reducing membrane peroxidation and inducing physiological and biochemical activities.