Comparative zinc uptake and metabolic responses of Ipomoea aquatica to hydroponic exposure of zinc sulphate, zinc oxide, and zinc oxide nanoparticles

This research aimed to investigate zinc (Zn) accumulation patterns and the resulting metabolite-profile alterations in Ipomoea aquatica (I. aquatica) (water spinach) when hydroponically exposed to 50.0 mg/L concentrations of zinc sulphate (ZnSO₄), zinc oxide (ZnO), and zinc oxide nanoparticles (ZnO NPs). Roots consistently exhibited the highest Zn accumulation across all treatments. ZnSO₄ exposure led to the maximum concentration of 4620 ± 458 mg/kg DW in roots, while ZnO NPs resulted in 2830 ± 321 mg/kg DW and ZnO in 957 ± 40.5 mg/kg DW in roots. Stems generally showed the lowest Zn accumulation, with the control at 43.2 mg/kg DW, though ZnSO₄-treated stems had the highest accumulation among treated groups at 962 ± 19.2 mg/kg DW. The overall Zn uptake in all biological parts (leaves, stems, and roots) followed the order: ZnSO₄ > ZnO NPs > ZnO > Control. GC-MS analysis revealed that all Zn treatments significantly altered I. aquatica's metabolite profile, resulting in a drastically reduced diversity of detectable metabolites compared to the control. For instance, the number of consistently detected metabolites plummeted from 45 in the control to just 6 in ZnSO₄, 9 in ZnO, and 7 in ZnO NPs treatments. Moreover, the mean peak height, an indicator of overall metabolite abundance, sharply declined from 486000 ± 36000 in the control to 4600 ± 462 in ZnO NPs exposure. While cyclooctasiloxane (hexadecamethyl) and cyclononasiloxane (octadecamethyl) were consistently detected in all treatments, cyclohexasiloxane (dodecamethyl) and cyclopentasiloxane (decamethyl) were exclusive to Zn-enriched experimental sets, and cyclotetrasiloxane (octamethyl) was specifically associated with ZnO and ZnO NPs exposure. Notably, silicic acid (diethyl bis(trimethylsilyl) ester) and pyridine-3-carboxamide (1,2-dihydro-4,6-dime) were uniquely detected in the leaves of plants exposed to ZnO NPs. This suggests potential roles in toxicity mediation, possibly through a silica coating effect reducing Zn²⁺ dissolution, or altered metabolic pathways, given pyridine-3-carboxamide's diverse biological activities, which include antiviral, antibacterial, and anticancer properties. These findings collectively underscore how the form of Zn critically influences its bioavailability, accumulation patterns, phytotoxicity, and distinct metabolite profile alterations in I. aquatica, providing crucial insights for optimizing agricultural practices and environmental management strategies, particularly regarding phytoremediation in heavy metal-contaminated environments and micronutrient management.