Combined Application of Citric Acid and Cr Resistant Microbes Improved Castor Bean Growth and Photosynthesis while it Alleviated Cr Toxicity by Reducing Cr+6 to Cr3+

Chromium is highly harmful to plants because of its detrimental effects on the availability of vital nutrients and secondary metabolites required for proper plant growth and development. A hydroponic experiment was carried out to analyze the effect of citric acid on castor bean plants under chromium stress. Furthermore, the role of two chromium-resistant microorganisms, Bacillus subtilis and Staphylococcus aureus, in reducing Cr toxicity was investigated. Different amounts of chromium (0 µM, 100 µM, 200 µM) and citric acid (0 mM, 2.5 mM, and 5 mM) were used both alone and in combination to analyze the remediation potential. Results showed that elevated amounts of chromium (specifically 200 µM) minimized the growth and biomass because the high concentration of Cr induced the oxidative markers. Exogenous citric acid treatment boosted plant growth and development by improving photosynthesis via enzymes such as superoxide dismutase, guaiacol peroxidase, catalase, and ascorbate peroxidase, which decreased Cr toxicity. The application of citric acid helped the plants to produce a high concentration of antioxidants which countered the oxidants produced due to chromium stress. It revealed that castor bean plants treated with citric acid could offset the stress injuries by decreasing the H2O2, electrolyte leakage, and malondialdehyde levels. The inoculation of plants with bacteria further boosted the plant growth parameters by improving photosynthesis and reducing the chromium-induced toxicity in the plants. The findings demonstrated that the combination of citric acid and metal-resistant bacteria could be a valuable technique for heavy metal remediation and mediating the adverse effects of metal toxicity on plants.