The effects of varying pH and shading on antioxidant content-and -capacity of Tulbaghia violacea, cultivated hydroponically under greenhouse conditions

Abstract

Environmental stress factors such as high or low soil pH, water deficit, high temperature, and shade may induce accumulation of reactive oxygen species in plants, which may lead to oxidative stress when in excess. The effects of varying the environmental variables such as pH and light intensity on secondary metabolite contents and the antioxidant capacity of extracts of T. violacea under greenhouse conditions, with the aim of improving the hydroponic cultivation of T. violacea, were assessed. In this study, six weeks old seedlings of T. violacea were separately exposed to two levels of light intensity and two levels of pH (4 and 8) in a greenhouse. The total polyphenol, alkaloid, and flavonol contents and antioxidant activities in the leaf and bulbous root extracts were determined using spectrophotometric methods. The total polyphenol contents of the leaves were significantly higher (df = 1, 4; P < 0.05) in plants under 40% shading and pH 8 compared with 0% shading and pH 4, respectively. Although generally, higher total polyphenol and flavonol contents occurred in the roots than in the leaves, the roots had a significantly (P < 0.05) reduced flavonol content compared with the leaves of plants in the 40% shade treatment. The antioxidant activity was more pronounced in the leaf extract from plants maintained under 40% shade or pH 8 than 0% shade or pH 4, respectively. In conclusion, 40% shading and high pH (pH 8) correlated with a high accumulation of antioxidants and antioxidant activities in T. violacea.