24-Epibrassinolide Alleviates the Injurious Effects of Cr(VI) Toxicity in Tomato Plants: Insights into Growth, Physio-Biochemical Attributes, Antioxidant Activity and Regulation of Ascorbate–Glutathione and Glyoxalase Cycles

Abstract

Brassinosteroids (BRs) are well-ascertained growth regulators that play prime roles in alleviation of a stress in plants. The current investigation determined the impact of epibrassinolide (EBR) applied as seed pretreatment on growth attributes, physiological attributes and antioxidant potential of tomato plants grown under Cr(VI) metal stress. Applications of EBR were found to be effective in improving growth of control as well as Cr-stressed plants in terms of lengths of shoot and root and their dry weights. Of the physiological parameters, stomatal conductance was the most sensitive to chromium exposure exhibiting 72.15% decline, while photosynthetic rate decreased to 33.16%. Chromium stress led to decline in non-enzymatic antioxidants like GSH by 41.85% and in total flavonoid by 69.48%. Application of EBR modulated more significantly the titres of antioxidants (glutathione, ascorbic acid, total flavonoids and activities of antioxidant enzymes (ascorbate peroxidase, catalase, superoxide dismutase, glutathione-S-transferase and glutathione reductase) as well as proline and glycine betaine in Cr-stressed plants compared to those in plants receiving no supply of EB. Diminution of Cr stress by EBR was also maintained by higher values of stress indices, such as flavonoids, photosynthetic pigments and reduction in malondialdehyde and H2O2 levels in the Cr-treated plants than those in the controls. Restricted production of ROS and improved quenching of ROS were also recorded due to EBR application under Cr stress. At the same time, insignificant reduction in uptake of Cr was noticed due to application of EBR in leaves compared to that in roots of Cr-stressed plants. Overall, our results reveal that application of EBR can effectively act as a growth stimulant in plants subjected to Cr stress. Therefore, it can be conjectured that the interaction of 24-epibrassinolide remodulates the physiological, metabolic and defence system of the tomato thriving under chromium stress by equilibrating the Cr accumulation.