Hydrogen peroxide (H₂O₂) in minute quantity serves as a signalling molecule. However, the role of H₂O₂ in combination with brassinosteroids (stress regulators) in plants under toxic levels of copper, is poorly understood. With an aim to explore and elaborate their role in plants subjected to abiotic stress, the surface sterilized seeds of mung bean (Vigna radiata) were sown in earthen pots filled with soil and manure enriched with different levels of Cu²⁺ (50 or 100 mg kg⁻¹ of soil) and allowed to grow under natural environmental conditions. At 15 and 20 days stage, the plants were sprayed with H₂O₂ (2.5 mM) and/or 28-homobrassinolide (HBL, 10⁻⁵ mM), respectively. At 45 days stage, the analysis of the plants revealed that the presence of copper in the soil caused a significant decrease in growth characteristics, activity of carbonic anhydrase and nitrate reductase, relative water content, chlorophyll content and the rate of photosynthesis whereas, the activity of antioxidant enzymes (catalase, peroxidase and superoxide dismutase) and the proline accumulation in leaves increased in Cu stressed plants. However, the exogenously applied HBL and/or H₂O₂, in the absence of Cu-stress strongly favoured the growth, photosynthetic parameters and also improved the activity of antioxidant enzymes and the proline content. Furthermore, the combined application of HBL and H₂O₂ to the foliage of the stressed plants neutralized the toxic impact of all copper regimes. Therefore, we are of the opinion that these chemicals somehow maintained the homeostasis of the metal in the plants that exhibit healthy growth.
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