Pontederia cordata, an ornamental aquatic macrophyte with great potential in phytoremediation of heavy-metal-contaminated wetlands

Abstract

Pontederia cordata can tolerate heavy metal toxicity and possesses great potential for phytoremediation of heavy-metal-contaminated wetlands, yet how it copes with heavy metal stress has still not been determined. Hydroponic experiments were used to assess the effects of various levels of Cd²⁺ on the photosynthesis and activity of redox-regulatory systems in the plant leaves, and we also sought to elucidate the tolerance mechanism of the plant to Cd²⁺ by investigating Cd²⁺ enrichment characteristics and chemical forms. The plant can manage a low cadmium concentration (≤0.04 mM) with relatively stable biomass and photosynthetic performance. Cd²⁺ at the highest concentration (0.44 mM) decreased superoxide dismutase and peroxidase activities by 37.17% and 93.29%, respectively. Similar trends were demonstrated in the contents of ascorbic acid, carotenoids, lutein, glutathione, and non-protein thiol, as well as phytochelation in the leaves, exacerbating membrane peroxidation despite the significantly increased catalase activity observed. Moreover, the highest Cd²⁺ concentration disturbed the biosynthesis of chlorophyll precursors in the leaves, reduced chlorophyll a and b, as well as total chlorophyll contents by 60.47%, 67.47%, and 68.12%, respectively, which inhibited photosynthesis, leading to a decline in biomass. Compared with maximum quantum efficiency (F_v/F_m) and the potential activity (F_v/F_o) of photosystem II, the performance index for energy conservation from photons absorbed by PSII to the reduction of intersystem electron acceptors (PI_abs), and of PSI end acceptors (PI_total), can indicate Cd²⁺ toxicity to the photosynthetic apparatus in the leaves. 49.95%-76.90% of the Cd²⁺ was sequestered in the plant roots, restraining translocation from roots to shoots, which is considered a tolerance mechanism, probably resulting from disturbed transpiration in leaves and increased Cd²⁺ content with low activity. Pontederia cordata is a candidate plant for phytoremediation of heavy-metal -contaminated wetlands.

Keywords: Antioxidative activity; Cadmium; Chemical forms; Photosynthesis; Pontederia cordata; Translocation.