An assessment of the uptake of selected heavy metals, antioxidant response and lipid peroxidation in Spinacia oleracea vegetables, cultivated on soil from a coal mining area in Matabeleland North Region of Zimbabwe

Abstract

Coal mining in Matabeleland North region plays an important role in the production of energy in Zimbabwe. Coal mining, however, results in the release of pollutants such as heavy metals into the surrounding soil environments, putting communities involved in vegetable gardening at risk. This study aimed to quantify the levels of the heavy metals, cadmium (Cd), copper (Cu) and zinc (Zn) in soil from a coal mining area in Matabeleland North region, and in spinach (Spinacea oleracea) grown in these soils, as well as assess superoxide dismutase (SOD) and catalase (CAT) antioxidant enzyme activities in the cultivated spinach. Malondialdehyde (MDA) levels were also measured as an index of lipid peroxidation. Soil was collected from 3 sites and the pH and electrical conductivity were measured. Spinach seeds were planted in the soils in polythene bags and left to germinate and grow outdoors for 30 days, with daily watering. Spinach leaves were harvested. Heavy metals were quantified, SOD and CAT activities assessed and MDA levels measured. Soil pH from Sites 1, 2 and 3 ranged between 6.2 and 6.9, while conductivity was in the 1.50-1.59 μS range. Compared to the heavy metal levels in the reference soil, Cd levels from Sites 2 and 3 were significantly (p< 0.05) higher. Copper and Zn levels from each of the sites were significantly higher than in the reference soil. In spinach, Cd levels ranged between 1.20-2.23 mg/kg. Both Cu and Zn levels were significantly (p< 0.05) higher than in spinach grown on reference soil, ranging between 38-47 mg/kg and 64-89 mg/kg respectively. There was a significant increase in SOD and CAT enzyme activities in spinach grown on Sites 1-3 soils, compared to enzyme activities of plants grown on reference soil. Malondialdehyde levels were significantly higher in plants grown on Site 1 and 2 soils compared to plants grown on reference soil. The findings suggest that spinach bioaccumulates pollutants from soil in the coal mining area.