Chromolaena odorata, Euphorbia heterophylla and Tridax procumbens are common weeds that are prevalent in cowpea fields. The physiological influence of three dilution concentrations of the aqueous root and shoot extracts of the weeds were examined on seed germination, plumule length, radicle length, fresh and dry weights of plumule and radicle of two varieties of cowpea in the laboratory. The experiment was laid out in a completely randomized design replicated three times. Results revealed susceptibility of two varieties of cowpea (Vigna unguiculata L. Walp) (IT99K-573-1-1 and IT07K -292-10) to the allelopathic potential of all the extract concentrations of the selected weeds. Although, all the extracts reduced germination and seedling growth, shoot extracts at 75% concentration of the selected weeds significantly inhibited germination and seedling growth of the variety IT99K-573-1-1 compared with the control which produced 97% (germination %); C. odorata, E. heterophylla and T. procumbens shoot extracts produced 22, 20 and 50% germination, respectively. Consequently, C. odorata, E. heterophylla and T. procumbens shoot extracts produced 25, 18 and 28% germination respectively for variety IT07K -292-10 while the control yielded 99%. Bioassays also indicate that the inhibition was concentration dependent; the inhibition in the extract-treated seeds increased with the increase in the concentration of the extracts. Also, the degree of seed germination inhibition was higher in shoot extracts than root extracts of selected weed. It was clear from the investigation that the extracts of E. heterophylla exerted a stronger inhibitory effect on the germination process and seedling growth of the two cowpea varieties than that of C. odorata and T. procumbens.
Key words: Allelopathy, allelochemicals, Chromolaena odorata, Euphorbia heterophylla, Tridax procumbens.
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