Media-Dependent Growth, Stress Responses, and Metal Accumulation Patterns in Azolla imbricata (Roxb.) Nakai Exposed to As, Cd, Cu, Pb, and Zn: Individual and Combination Effects (Synergistic, Antagonistic, and Additive)

Abstract

In this study, we examined metal accumulation and biochemical responses of Azolla imbricata (Roxb.) Nakai exposed to water medium (WM) and nutrient medium (NM) under single and combined (co-exposure) treatments with five metals (As, Cd, Cu, Pb, and Zn) at 10% environmentally relevant concentrations. Marked differences between WM and NM highlighted the influence of nutrient availability on plant responses. An inverse relationship was observed, with higher biomass in NM (WM < NM) and greater metal accumulation in WM (WM > NM). Growth inhibition, reflected by reduced photosynthetic pigment contents, was accompanied by elevated stress indicators, including electrolyte leakage, proline, malondialdehyde, and anthocyanins, confirming metal-induced phytotoxicity. Metal interactions under co-exposure were predominantly antagonistic in WM and synergistic in NM. Metal accumulation patterns (mg/kg) varied across media and exposure types: under single exposure, values ranged from 83.8 (As) to 43 881 (Zn) in WM and from 11.7 (Cd) to 12 135 (Cu) in NM; under co-exposure, they ranged from 346 (Cd) to 12 688 (Cu) in WM and from 46 (Cd) to 2859 (Cu) in NM. Accumulation sequences did not correspond to exposure concentrations, indicating metal-specific and media-dependent uptake. Under co-exposure, concurrent accumulation of multiple metals was more frequent in WM than that in NM, suggesting nutrient availability constrained simultaneous uptake. Bioconcentration factor values exceeded common thresholds under certain treatments, indicating strong accumulation potential rather than definitive hyperaccumulation. Overall, these findings highlight the potential of A. imbricata for phytoremediation of metal-contaminated waters while emphasizing cautious application in agroecosystems due to potential soil–crop–food pathway transfer.