Effect of water stress on antimicrobial activity of selected medicinal plant species

Some traditional medicinal practitioners believe that cultivated medicinal plants will lose healing powers, compared to plants growing in nature. Several scientists have shown that a plant under stress will produce secondary metabolites that may influence its medicinal properties. This perception has negative effects on the cultivation of medicinal plants to control local extinctions by uncontrolled collection of medicinal plants. To investigate this perception we examined the effect of water treatments on the antimicrobial activity of selected plant species grown under controlled conditions. Firstly, Tulbaghia violacea and Hypoxis hemerocallidea plants were subjected to watering regimes of 1000 ml of water in intervals of 3,14 and 21 days in a preliminary experiment. Subsequently, T. violacea, Leonotis dysophylla and Bulbine frutescens were subjected to different water regimes of 50, 100, 200 and 500 ml every second day for 240 days. Air-dried leaves of matured plants were finely ground and extracted with acetone. Minimum inhibitory concentration (MIC) and total activity were determined in order to evaluate the antimicrobial activity and potential use of plant extracts against four bacteria and two fungi. In general there were no statistically significant differences in MIC and total activity values and there were no clear trends across treatments. The applied water treatments did not cause significant effects on the antimicrobial activity of extracts. The fact that water stress did not materially influence antimicrobial activity does not mean that water stress does not affect other medicinal properties. Nevertheless our results question the general perception of traditional medicinal practitioners that plants growing under natural stress conditions have higher biological activities at least as far as water stress related to antimicrobial activities is concerned. (C) 2015 SAAB. Published by Elsevier B.V. All rights reserved.