A Functional Trade-Off Between Chromium Phytoremediation and Silymarin Preservation Driven by Distinct Pseudomonas Strains in Silybum marianum L.

Abstract Background and Aims Soil contamination with Cr (VI) disrupts essential plant physiological processes. This study evaluated the phytoremediation potential of milk thistle ( Silybum marianum L.) inoculated with plant growth-promoting bacteria (PGPB), Pseudomonas putida and Pseudomonas fluorescens , across varying Cr (VI) levels (0–75 mg kg⁻¹). Methods Plants were grown in Cr-contaminated soils with or without bacterial inoculation. Biomass, physiological traits, silymarin content, antioxidant enzyme activities, and Cr accumulation were evaluated to assess phytoremediation efficiency. Results Cr (VI) significantly reduced plant biomass, membrane stability, photosynthetic pigments, and silymarin content. Uninoculated plants accumulated H₂O₂, indicating insufficient endogenous defense. However, PGPB inoculation markedly alleviated oxidative stress; decreased guaiacol peroxidase activity coupled with a 4-fold increase in ascorbate peroxidase activity and reduced H₂O₂ levels suggested an optimized ROS detoxification pathway. S. marianum displayed a phytostabilization strategy (bioconcentration factor > 1, translocation factor < 1), which P. putida enhanced by reducing soil Cr concentrations by 70–89%. Crucially, a functional trade-off emerged: P. putida maximized phytoremediation efficiency, whereas P. fluorescens preserved silymarin yield across all Cr levels. Conclusions Combining efficient soil decontamination with the preservation of high-value medicinal biomass, this integrated plant–microbe system provides a sustainable strategy for managing Cr (VI)-contaminated lands.