Temperature and water stresses on antioxidant activity of selected medicinal plants have implications for sustainable use and global warming

Abstract

Some traditional healers believe that herbal medicines derived from wild populations have higher activity than cultivated medicinal plants. This has important implications for the conservation of medicinal plants if traditional healers do not want to use cultivated plants. We have previously shown that water and temperature stresses have little effect on antimicrobial activity acetone leaf extracts of Leonotis dysophylla Benth., Bulbine frutescens (L.) Willd. and Tulbaghia violacea Harv. Antioxidant activity of plant extracts may be important in treating several different diseases. We have therefore investigated the effect of water and temperature stresses on the quantitative and qualitative antioxidant activity of these species in plants with limited genetic variation using 6‑hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox) and l-ascorbic acid as positive controls. Activity was measured against two free radicals, 2,2′-azinobis (3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS) and 2, 2-diphenyl-1-picryl hydrazyl (DPPH). All plant extracts had some antioxidant activity following 182 to 240 days of exposure to temperature (15 °C and 30 °C) and water (50–500 ml/pot every second day) treatments. The IC50 values across water treatments did not show any clear patterns, which suggests that water stress had little influence on the antioxidant activity. On the other hand, the IC50 values under high temperature of 30 °C were statistically significantly higher than those under the temperature of 15 °C, which indicates that high temperature led to a reduced antioxidant activity. The effect of fluctuating temperatures higher than 30 °C because of prevailing global warming is more likely to lower the antioxidant activity of many important medicinal plants in nature. The results also indicate that plants in nature do not necessarily have higher activity to address concerns of traditional healers. Using cultivated plants may affect the quality and/or safety of medicinal properties to be used to treat different illnesses. As such, the establishment of optimal growth regimes for ex situ cultivation for important and especially threatened medicinal plants is recommended. It may help yield consistent supply of quality and quantity of bioactive material for self-medication and commercialisation. Cultivation may also contribute to conservation of species biodiversity.