PHYTOTOXIC EFFECTS OF METABOLITES PRODUCED BY WILD AND MUTANT STRAINS OF ACTINOBACTERIA ON POST-EMERGENCE OF Chromolaena odorata (L.) R. M. KING and H. E. ROBINS2

Abstract

ABSTRACT Chromolaena odorata, known as Siam weed, is one of the most invasive weeds in Nigeria and West Africa. The allelochemicals produced by this plant may inhibit the growth of many crop plants, causing yield losses. Chemical herbicides are commonly used to control this weed but their use has contributed to health hazard and environmental pollution. This study evaluated the phytotoxic effects of metabolites produced by wild and mutant strains of actinobacteria species on post-emergent C. odorata. Soil samples were collected from the rhizosphere and rhizoplane of young seedlings and mature plants of C. odorata from four farms within and outside the university. Actinobacteria were isolated from the soil samples using standard microbiological techniques. The actinobacterial isolates were then identified by morphological and biochemical characterization methods. They were then screened for their phytotoxic effects using seed germination and detached leaf necrosis assays. Two isolates with the highest phytotoxic potentials were exposed to UV light and different concentrations of ethidium bromide solution. Secondary metabolites were extracted from the wild and mutant actinobacterial species with higher phytotoxic potentials. Screen house studies on the post-emergence of C. odorata were carried out to assess the phytotoxic effects of two most effective metabolites. In the screen house, 6 treatments were laid out in a completely randomized design in triplicates. The two isolates were then characterized by 16S rRNA gene sequencing method. The bioactive constituents of the metabolites were also determined using High Performance Liquid Chromatography (HPLC). Data were analysed by one-way analysis of variance, followed by separation of means using Duncan’s multiple range test at p ≤ 0.05. The total actinobacterial counts of the rhizospheric and rhizoplane soil samples of C. odorata ranged from 0.00 – 2.67 × 103 CFU/g and 0.00 – 5.0 × 103 CFU/g, respectively. A total of 31 isolates belonging to the genera Streptomyces, Actinomyces, Corynebacterium, Nocardia, Microbacterium, Micromonospora, Micrococcus, Bifidobacterium, Kocuria, Frankia and Mycobacterium were obtained. Only 18 (58.06 %) isolates inhibited the seed germination of C. odorata. Results of leaf necrosis assay revealed that among the isolates, 5 exhibited highest phytotoxic potentials, inducing 100% necrosis on C. odorata leaves. The results also showed that the metabolites produced by mutant strains of Streptomyces malaysiensis and Frankia asymbiotica caused 100% and 96% necrosis on the leaves of C. odorata, respectively. The screen house experiment revealed that the metabolites produced by mutant and wild strains of S. malaysiensis and F. asymbiotica exhibited moderate to high phytotoxic activities (35.8 % to 89.1 %) on the post-emergence of C. odorata causing necrosis curling, wilting and chlorosis at 21 days after application. The HPLC analyses detected 19 and 9 bioactive constituents in metabolites of S. malaysiensis and F. asymbiotica mutants respectively, with 2, 4-Di-tert-butylphenol found in both metabolites. The study therefore showed that metabolites produced by mutant and wild strains of S. malaysiensis and F. asymbiotica possessed phytotoxic properties on the post-emergent Chromolaena odorata under screen house conditions.