Fertilization-induced changes in growth parameters and antimycobacterial activity of Euclea natalensis (Ebenaceae)

The effect of Nitrogen: Phosphorus: Potassium (NPK) (2:1:2 (44)) fertilizers on growth, accumulation of bioactive naphthoquinones (shinanolone, 7-methyljuglone, diospyrin, isodiospyrin and neodiospyrin) and antimycobacterial activity was investigated in seedlings of Euclea natalensis (Ebenaceae) grown in shadehouse and in field conditions. Seedlings were subjected to three differential fertilization regimes (40 g/l, 20 g/l and 10 g/l). Treatment 2 enhanced the vegetative performance of seedlings grown under shadehouse conditions. A significantly positive correlation was established between the concentration of shinanolone (P < 0.01), isodiospyrin (P < 0.05) and neodiospyrin (P < 0.05) with fertilization from field-grown seedlings. Application of NPK fertilizers significantly (P < 0.05) increased the accumulation of neodiospyrin in seedlings subjected to shadehouse conditions. The most potent naphthoquinone, 7-methyljuglone was found to be abundant in all the extracts and was quantified at a high mean concentration of 10.2 g/kg from shade-grown seedlings. Extracts acquired from field-grown seedlings were more active against Mycobacterium smegmatis with a lowest MIC value of 0.78 mg/ml. Extracts from the control group and Treatment 1 (10 g/l), which had less applied fertilizers, were more active against strains of M. tuberculosis with MIC value of 10 µg/ml. This shows the selectivity of E. natalensis against M. tuberculosis. Our study indicated that subjecting seedlings to homogenous environments coupled with higher levels of fertilizers could have a negative impact on the antimycobacterial activity of E. natalensis.