24-epibrassinolide in association with iron enhances the photosynthetic efficiency and upregulates the antioxidant system of the Brassica juncea

24-epibrassinolide (EBL) and iron (Fe) individually are well recognized for their significant role in various biological functions of plants but their interactive effect on iron transport, stress biomarkers, photosynthetic and biochemical parameters in Brassica juncea is poorly investigated. Therefore, this study was conducted to reveal the effect of Fe in the presence of 24-epibrassinolide (EBL), an analogue of brassinosteroids (BRs) on Brassica juncea plants. Fe in the form Fe-EDTA (1, 5 or 50 µM) was supplied with nutrient solution for 5 days followed by the foliar spray with deionised water (control) or EBL (0.01 µM) on 16-day-old plants for 5 days. Plant samples of 30-day-old plants were collected for the assessment of growth and photosynthetic performance and antioxidant parameters. Results clearly indicated that Fe altered plant metabolism. Lower concentration of Fe (5 µM) increased the growth biomarkers, SPAD (soil plant analysis development) chlorophyll level and photosynthetic rate, whereas higher concentration of Fe (50 µM) proved toxic to Brassica juncea and showed reduced growth, biomass of plant, SPAD chlorophyll level, and photosynthetic traits. However, the application of EBL to Fe-stressed plants reversed the deleterious effects caused by the Fe (50 µM) by improving the activities of antioxidant enzymes and increasing the proline accumulation in plant cells that are reflected in increased plant growth and other physiological parameters. It is concluded that lower concentration of Fe could be exploited as micronutrients whereas combined application of Fe with EBL acts as a stress alleviator for the toxic effects caused by Fe alone.