Response of Bambara groundnut ( Vigna subterranean L. ) and Maize ( Zea mays L. ) to heavy metal stress

Plants are usually the target of environmental pollution. This study, therefore, investigates the effects of Zinc (Zn) and lead Pb on Bambara nut (Vigna subterranean) and Maize (Zea mays) at different concentrations, as well as the possible ameliorating effect of the chelant; ethylene diamine acetate (EDTA) and farmyard manure, on the enzymatic activities, the chlorophyll, total protein, and carbohydrate contents. Findings revealed that Pb and Zn increased the superoxide dismutase (SOD), peroxidase, glutathione synthetase (GSH), malondialdehyde (MDA), and catalase levels with increased concentrations from 100 mg/kg to 200 mg/kg significantly (p < 0.05) compared to the control Maize plants. While in Bambara nut, the superoxide dismutase (SOD), peroxidase, glutathione synthetase (GSH), and catalase levels decreased with increased concentrations from 100 mg/kg to 200 mg/kg compared to control except for the malondialdehyde (MDA) which was increased. For Bambara groundnut, with increased Pb concentrations, the chlorophyll content reduced from 2.94 to 2.00 mg/g. However, there was an increase (up to 4.918 mg/g) in the chlorophyll content with increased zinc nitrate concentrations augmented with EDTA at the highest concentration. Maize plants treated with Pb augmented with farmyard manure showed an increase in chlorophyll content with increased concentrations while those assisted with EDTA still experienced a decrease as metal concentrations increased. Bambara groundnut plant had a mean carbohydrate (%) of 14.79 (control), 17.60 (100 mg/kg of Pb concentration) and 11.20 (200 mg/kg of Pb concentration), indicating a decrease in carbohydrate content with increased Pb concentrations. The same trend was observed for the different Zn and Pb concentrations on the mean total proteins and carbohydrates of both test plants. Generally, Pb and Zn induced oxidative stress in treated plants. Elevated activity of anti-oxidative enzymes can assist as important components of antioxidative defense mechanism against oxidative damage. The results of this study could be beneficial in the understanding of the role of the defense system as well as the detoxification mechanism of Vigna subterranean and Zea mays in efficient tolerance and response to Pb and Zn. This signifies that these plants can act as bioindicators in environmental quality assessment.