Comparative Study of Phytochemical Composition and Antioxidant Properties of <I>Vigna unguiculata</I> (Cowpea) Cultivated on Mining and Non-Mining Soils in Edo North

Abstract

From a nutritional standpoint, Cowpea (Vigna unguiculata) is gaining conspicuous recognition as an important source of proteins, calories, minerals, vitamins and contain significant amounts of polyphenols such as flavonoids, phenolic acids, and lignin, which are considered natural antioxidants. This study evaluated the phytochemical constituents, nutritional composition, and antioxidant properties of cowpea cultivatedon mining (Okpella and Ikpeshi) and non-mining (Afuze) soils. Phytochemical screening, proximate composition and antioxidant activity were investigated using standard analytical methods. Phytochemical screening revealed the presence of alkaloids, saponins, phlobatannins, steroids, terpenoids, flavonoids, and phenols in all cowpea extracts. However, Cardiac glycosides with cardenolides (CG3) was absent in all samples. Additionally, Cardiac glycosides with steroidal nucleus, Cardiac glycosides with deoxy sugar (CG1 and CG2) and anthraquinone were exclusively detected in cowpea from the Afuze soil. The highest total phenolic and flavonoid content was recorded for cowpea from the non-mining Afuze site A 34.18 ± 0.07μg/mg and 27.10 ± 0.02 μg/mg. Ikpeshi site A demonstrated the highest ABTS, hydroxyl radical scavenging activity and ferric reducing power with 0.89 ± 0.01 μg/mg, 0.89 ± 0.04 μg/mg, and 0.80 ± 0.12 μg/mg respectively while Ikpeshi site B has the highest DPPH activity 0.90±0.09μg/mg. Conversely, Afuze cowpea had the lowest DPPH, ABTS, OH and ferric reducing power. Non-mining soils consistently yielded cowpea with higher percentages of fat, carbohydrate, nitrogen, dry matter, and protein compared to mining soils. These findings suggest that mining activities negatively impact soil quality leading to decreased nutrient content.