Effects of soil types on phytochemical constituents and antioxidant properties of Solanum nigrum

Solanum nigrum L., a nutraceutical vegetable found in most tropical African countries is known for its medicinal properties imparted by antioxidant rich phytochemicals. This study evaluates the influence of soil textural types on the phytochemical constituents and antioxidant properties of cultivated S. nigrum. Bioactive compounds such as phenols, flavonoids, pro-anthocyanidins, tannins, saponins, and alkaloids were evaluated in S. nigrum cultivated on sandy loam (control soil), sandy clay loam, silty clay loam, clay loam and loam soils of University of Fort Hare, Alice, Eastern Cape, South Africa. Antiradical scavenging activity was evaluated with 1, 1-diphenyl-2-picryl-hydrazyl (DPPH), 2, 2′-Azino-bis (3-ethylbenzthiazoline-6-sulfonic acid (ABTS) and Iron Reducing Power (FRAP). Data were analysed statistically using MINITAB Release 17. The highest concentrations of phenolics (73.87 mg/g), flavonoids (58.41 mg/g), pro-anthocyanidins (875.74 mg/g), tannins (49.32 mg/g), saponins (875.74 mg/g) and alkaloids (47.50 mg/g) were recorded in silty clay loam soil, which proved to be the best soil type. Moreover, this same soil type had the highest IC50 values of DPPH (0.0102 mg/g) and ABTS (0.1062 mg/g) scavenging power, and absorbance (0.8395 mg/g) in FRAP reducing power. From the foregoing, we conclude that different soil textures influence the phytochemical contents and antioxidant properties of S. nigrum cultivated on them. Specifically, silty clay loam soil increased the bioactivity as well as oxygen radical scavenging properties of S. nigrum when cultivated on it. Therefore, silty clay loam can be recommended for cultivation of S. nigrum for pharmaceutical purposes.