Adaptive responses of Brassica juncea vs. Sorghum bicolor to increasing toxicity of potentially toxic elements in soil

Understanding species-specific responses to potentially toxic elements (PTEs) is essential for phytoremediation and ecological risk assessment. This study evaluated the tolerance and stress response of two phytoremediation species, Brassica juncea and Sorghum bicolor, grown under a pollution gradient created by mixing mine-contaminated soil with uncontaminated soil. Soil DTPA-extractable metal concentrations, and plant biomass yield, PTEs bioaccumulation, and biochemical indicators of oxidative stress (chlorophylls, proline and phenolic compounds) were evaluated. Soil DTPA-extractable Cd and Zn concentrations increased markedly with the pollution gradient, reaching up to 3.3 and 1400 mg kg^-1, respectively. Brassica juncea showed an exponential accumulation of Cd and Zn with increasing contamination, whereas S. bicolor displayed linear uptake patterns, indicating distinct PTE-uptake dynamics. Brassica juncea exhibited consistent tolerance with limited changes in its biomass and biochemical response across the pollution gradient; however, a strong decline in flavonols, particularly kaempferol-3-O-(6″-malonyl-glucoside), which correlated negatively with Cd availability and foliar accumulation (r = -0.73 to -0.88, P < 0.01), was observed. In contrast, S. bicolor activated a stronger stress response, with marked increases in chlorophylls (15 - 111 %), proline (5 - 84 %) and total phenolics (22 - 66 %) with increasing pollution, where flavone compounds, particularly luteolin-7-glucoside, increased linearly and were positively correlated with Cd and Zn availability and foliar accumulation (r = 0.84 - 0.89, P < 0.001). These contrasting patterns highlight two adaptive strategies in the face of increasing PTEs exposure: S. bicolor acting as a sensitive indicator species and B. juncea as a tolerant metal accumulator, both being of interest for contaminated soils phytoremediation.