Phytoremediation potential of pot marigold (<i>Calendula officinalis</i> L.) under plant growth-promoting rhizobacteria (PGPR) treatments for sustainable urban green infrastructure development

This study evaluated the phytoremediation potential of pot marigolds on green wall systems for removing soil cadmium. The experiment was a factorial arrangement in a randomized complete block design. The first factor was cadmium (0, 1, 2, and 3 mg/kg soil), and the second was seven single or mixed plant growth-promoting rhizobacteria (PGPR) and the control treatments. All root and flower-related traits significantly increased after bacterial treatments (<i>p</i> ≤ 0.05). Substrates inoculated with <i>Thiobacillus thioparus</i> strain 300, Mix2 (<i>Aztobactor chorococcum</i> strain D0941 + <i>Azosporillum liposferum</i> strain So131), and Mix3 (<i>Thiobacillus thioparus</i> strain 300+ <i>Pseudomonas putida</i> strain G0951+ <i>Aztobactor chorococcum</i> strain D0941+ <i>Azosporillum liposferum</i> strain So131) increased phytoremediation of pot marigold in roots and shoots up to 3 mg/kg soil cadmium. The transfer factor (TF) was under one in flowers, showing the low potential of the flowers for phytoremediation. TF was above one in aerial vegetative plant sections, indicating the species as a cadmium accumulator and extractor for managing cadmium contaminant sites. Based on purification ratios and transfer factors, <i>Thiobacillus thiolates</i> (alone or combined) is recommended for enhancing pot marigold esthetics in landscapes, while Pseudomonas strains (single or mixed) improve its phytoremediation potential in urban green infrastructure. This study introduces an innovative approach to sustainable green infrastructure by developing multifunctional green walls capable of remediating cadmium, a common urban heavy metal pollutant. Utilizing <i>Calendula officinalis</i> (pot marigold) and plant growth-promoting bacteria, the system leverages nature-based solutions for urban sustainability. Unlike conventional wastewater treatment, which faces high costs and technical challenges, this approach integrates phytoremediation with esthetic and functional benefits. Using low-quality irrigation water, the study demonstrates a practical strategy for enhancing urban landscaping while mitigating pollution, offering a cost-effective and scalable solution for sustainable city environments. The findings provide practical insights for urban, industrial, and agricultural sectors, demonstrating how natural systems can be leveraged for heavy metal remediation and sustainable urban development.