Effects of the addition of phytohormone and plant growth-promoting bacteria on the health and development of Polygonum hydropiperoides cultivated in constructed wetlands treating chromium-contaminated wastewater

Abstract

Constructed wetland (CW) systems are used for treating different types of wastewaters, including tannery effluent, which are rich in heavy metals. The purpose of this study was to assess the potential use of Polygonum hydropiperoides and its chromium removal efficiency in CWs treating synthetic tannery wastewater (STW), after the addition of phytohormone auxin and plant growth-promoting bacteria (PGPB) Azospirillum brasiliense. We installed and operated nine pilot-scale CWs named as: CWP+B – unit with phytohormone and PGPB; CWP – unit with phytohormone; CWB – unit with PGPB; CWV – vegetated unit, without PGPB and auxin, and CWUV – unvegetated unit. Also, each vegetated CW had a parallel control unit to evaluate plant phytotoxicity. According to the results, all CWs were efficient in chromium removal and there were no significant differences among them. The visual symptoms observed in the aerial parts of the individuals exposed to chromium-rich wastewater were curling, necrosis, chlorosis and senescence of the leaves. The most intense positive detection of cell death occurred in the leaves and roots of the individuals exposed to chromium. In the vegetated wetlands with added phytohormone and PGPB, there were fewer visual damages, lower detection of cell death in the leaves and roots, and an increase in growth and productivity of P. hydropiperoides.