ALLELOPATHIC POTENTIAL OF Brachiaria brizantha AND B. milliformis ON SEED GERMINATION OF SELECTED BIOASSAY SPECIES

Brachiaria brizantha and B. milliformis are widely grown improved pasture species of the coconut plantations in Sri Lanka. Field observations indicate that these two grass species suppress ground vegetation in coconut plantations. Therefore, the aim of this study was to test the allelopathic effect of B. brizantha and B. milliformis using their root components from where they possibly release allelochemicals to the environment. Soils where B. brizantha and B. milliformis are grown and rootexudates, aqueous extracts of fresh and dry roots were investigated to verify their allelopathic effect on seed germination of five bioassay species; Raphanus sativus, Capsicum annum, Lycopersicom esculantum, Crotalaria junica and Chromoleana odorata. Fifty seeds from each bioassay species were placed in a petri dish containing root exudates, root extracts and contaminated soil and the percentage of seed germination was examined after 3 days. The experiment was repeated four times. Seed germination percentage of Capsicum annum, Lycopersicom esculantem and Chromoleana odoratawas significantly inhibited by B. brizantha contaminated soil. However, the degree of inhibition varied among the bioassay species. The fresh aqueous root extracts of B. brizantha was highly phytotoxic and it significantly reduced seed germination of all the bioassay species than the dry root extract. Themaximum reduction in seed germination of all the bioassay species was caused by root exudates of the two grass species. B.brizantha and B. milliformis species incorporated root aqueous extracts; root exudates and its rhizosphere soil suppress seed germination of the five bioassay species and suggest that these responses are attributed to allelopathic effects which should be investigated further in the field.