In vitro and In vivo Antioxidant Capacity of Cowpea (Vigna unguiculata) Grown in Copper Polluted Soil

Abstract

Vigna unguiculata is a drought tolerant legume which serves as a staple in sub-Sahara West Africa region especially in Nigeria. Studies have shown that cowpea possesses adaptive features which enables it to strive in dry regions. The present study investigates the antioxiodant capacity of the leaf and stem of cowpea (Vigna unguiculata) grown in copper polluted soil. Exactly 2kg of soil was treated with copper sulphate stock solution (1g/L) and kocide (a copper based pesticide) to obtain eight groups: 10mg/mL, 50mg/mL, 100mg/mL and 200mg/mL of copper sulphate, and 5.20g/L of kocide (lower dose of kocide), 6.67g/L of kocide (positive control) and 10.70g/L of kocide (higher dose of kocide), and no treatment in one group (negative control). The experimental plant, cowpea, was cultivated for five weeks. Data were collected after two weeks of cultivation and subsequent weeks until the fifth week, for the assays both on the homogenate and methanolic extract of the plant leaf and stem. The data obtained from the various assays conducted were subjected to analysis of variance (ANOVA) using Statistical Product and Service Solutions (SPSS) software (version 16). The results obtained showed that the malondialdehyde (MDA) level for both the leaf and stem showed an inconsistent increase (in lower dose of kocide, higher dose of kocide, 100mg/mL of copper sulphate and 200mg/mL of copper sulphate with 63.00 ± 8.82, 50.00 ± 5.77, 163.60 ± 3.33 and 157.00 ± 3.33 unit in mol/g wet weight × 10-4 respectively as against the negative control 73.00 ± 3.33MDA unit in mol/g wet weight × 10-4) within the weeks and decrease (in lower dose of kocide, higher dose of kocide, 100mg/mL and 200mg/mL with 17.00 ± 3.33, 23.00 ± 3.33, 17.00 ± 3.33 and 10.00 ± 0.00 unit in mol/g wet weight × 10-4 respectively as against the negative control 20.00 ± 5.77MDA unit in mol/g wet weight × 10-4) between weeks: depicting a stress condition in the plant. In the case of catalase, superoxide dismutase activities (SOD), total antioxidant capacity (TAC), ferric reducing antioxidant potential (FRAP), reducing power and nitric oxide (NO) free radical scavenging potentials; a consistent and significant increases within and between the weeks for both the leaf and stem were observed. The results obtained from the present study depict that Vigna unguiculata plant could tolerate a specific level of copper pollution (200 mg/mL) by enhancing the activities of enzymic antioxidants and other features.