Optimizing cadmium phytoremediation with phytohormone combinations

Heavy metal contamination, particularly cadmium (Cd), poses a significant global environmental threat. Phytoremediation, which utilizes plants for heavy metal removal, offers a sustainable remediation strategy. However, its efficiency is often limited by plant growth and metal uptake capacity. This study investigates whether combined applications of indole-3-acetic acid (IAA), brassinosteroid (BR), and kinetin (KT) can enhance Cd phytoremediation in Bidens pilosa, a known Cd-hyperaccumulator. A pot experiment was conducted using a completely randomized design with nine treatments, including individual and combined hormone applications under controlled high Cd stress (40 mg/kg). Here, we show that a combined IAA + BR treatment synergistically increased the Cd bioconcentration factor by 1.41-fold compared to the control. This was accompanied by enhanced biomass, photosynthetic activity, and antioxidant enzyme activity (catalase [CAT] and peroxidase [POD]). The triple combination (IAA + KT + BR) synergistically increased the translocation factor by 1.38-fold, indicating enhanced Cd movement to shoots, while other combinations exhibited additive or antagonistic effects on Cd accumulation, specifically in leaf and root tissues. Individual hormone applications generally maintained photosynthetic efficiency, whereas combinations often reduced it. These findings demonstrate that tailored phytohormone combinations can significantly enhance Cd phytoremediation efficacy in B. pilosa through modulation of key physiological responses. By optimizing plant growth and Cd partitioning, this hormone application approach offers an effective strategy for remediating Cd-contaminated soils. Further research should focus on addressing cost limitations, particularly for BR, and evaluating the long-term efficacy and ecological impacts of phytohormone treatments under field conditions.