Antioxidant enzyme responses and physiological thresholds of metal stress in Lemna minor in dose-dependent phytoremediation of chromium and lead

<title>Abstract</title> <p> The presence of chromium (Cr) and lead (Pb) in aquatic environments is a major environmental concern because they are toxic and pose ecological risks. Despite the extensive research on the application of <italic>Lemna minor</italic> for phytoremediation, physiological mechanisms that link antioxidant defense and metal removal efficiency are not well understood. In this study, <italic>L. minor</italic> was exposed to different concentrations of Cr and Pb (1, 2 and 5 mg L ^-1 ) in a dose-dependent manner and evaluated for its phytoremediation efficacy, physiological response, antioxidant defense, oxidative stress biomarkers and statistical modeling to elucidate the defense mechanisms over a period of 7 days. Removal efficiency (RE) and bioconcentration factor (BCF) were used to assess the phytoremediation performance, and the relative growth rate, root growth inhibition, chlorophyll, catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), malondialdehyde, electrolyte leakage, proline, and Enzyme Inhibition Index (EII) were used to evaluate the physiological and biochemical responses. One-way ANOVA, Tukey's HSD test, Pearson correlation and multiple linear regression were used to analyze the data. Removal of chromium was high and constant at 96.32–98.89%, while that of lead was reduced from 92.10% to 86.10% as the concentration increased. EII was considered an enzymatic indicator of Cr stress, whereas APX and POD were the most important regulators of chromium and lead removal by multiple regression, respectively. The results showed that coordinated antioxidant regulation is important for the efficiency of phytoremediation and showed the possibilities of using some parameters as mechanistic biomarkers for optimization of phytoremediation by <italic>L. minor</italic> . </p>