Differential response of floating and submerged leaves of longleaf pondweed to silver ions

In this study, we have investigated variations in the potential of floating and submerged leaves of longleaf pondweed (Potamogeton nodosus) to withstand silver ion (Ag⁺)-toxicity. Both floating and submerged leaves changed clear colorless AgNO₃ solutions to colloidal brown in the presence of light. Transmission electron microscopy revealed the presence of distinct crystalline Ag-nanoparticles (Ag-NPs) in these brown solutions. Powder X-ray diffraction pattern showed that Ag-NPs were composed of Ag⁰ and Ag₂ O. Photosystem (PS) II efficiency of leaves declined upon exposure to Ag⁺ with a significantly higher decline in the submerged leaves than in the floating leaves. Similarly, Ag⁺ treatment caused a significant reduction in the carboxylase activity of the ribulose bisphosphate carboxylase/oxygenase in leaves. The reduction in this carboxylase activity was significantly higher in the submerged than in the floating leaves. Ag⁺ treatment also resulted in a significant decline in the levels of nonenzymatic and enzymatic antioxidants; the decline was significantly lower in the floating than in submerged leaves. X-ray photoelectron spectroscopy revealed the presence of Ag₂ O in these leaves. Inductively coupled plasma mass spectrometry analysis revealed a three-fold higher Ag content in the submerged than in floating leaves. Our study demonstrates that floating leaves of longleaf pondweed have a superior potential to counter Ag⁺ -toxicity compared with submerged leaves, which could be due to superior potential of floating leaves to reduce Ag⁺ to less/non-toxic Ag⁰/Ag₂ O-nanoparticles/nanocomplexes. We suggest that modulating the genotype of longleaf pondweed to bear higher proportion of floating leaves would help in cleaning fresh water bodies contaminated with ionic forms of heavy metals.