Effects of different humic substances concentrations on root anatomy and Cd accumulation in seedlings of Avicennia germinans (black mangrove)

Highlights: • Heavy metals pollution is a major degradation factor in mangrove forests. • Cd can be accumulated even when present at low concentration in sediments. • Both humic substances and Cd influence Avicennia germinans root anatomy in vitro. • Humic substances can be exploited in degraded mangroves and dredged sediment recovery. • Different humic substances doses depending on remediation target have been identified. - Abstract: Mangrove areas are among most threatened tropical ecosystems worldwide. Among polluting agents Cadmium is often found in high concentrations in mangrove sediments. Humic substances, complex biomolecules formed in soil and sediments during animal and plant residuals decomposition, have a known biostimulant activity and can be adopted to counteract various plant stresses. This study explores, in controlled conditions, the effect of humic substances on Avicennia germinans seedlings, with or without cadmium contamination. Humic compounds significantly changed plant root architecture, and, when coupled with cadmium, root anatomy and Cortex to Vascular Cylinder diameter ratio. These modifications led to lower Cd uptake by humic substances-treated plants. Humic substances amendment could be effective, depending on their concentrations, on improving plant health in mangrove areas, for forest recuperation and/or dredged sediments phytoremediation purposes.