Allelopathic Effects of Aqueous Extract from Siam Weed on the Growth of Cowpea

L.S. Gill, G.O Anoliefo and U.V. Iduoze
Department of Botany, University of Benin, Benin City, Nigeria





This study reports on the effect of aqueous extract taken from the stem, leaf and root of Chromolaena odorata, on the growth of Vigna unguiculata. Generally, leaf extract for 48 hours had significant inhibitory effect on growth of the root, stem and leaf. However, results indicate that leaf growth was significantly inhibited by aqueous extract of root, stem and leaf.


DeCandolle (1832) was probably the first person to suggest the possibility that many plants may excrete something from their roots which is injurious to other plants. Molisch (1937) proposed the term "allelopathy" for expressing the harmful effect that one plant species has on another through the formation of chemical retardants escaping into the environment. The concept of allelopathy was further supported and further developed by Bonner (1950), Grummer and Beyer (1960) Evenari (1961), Whittaker (1970), Pitman and Duke (1978) and Fischer et al. (1978). According to Lavabre (1991), allelopathic effects are controversial and still poorly understood. Allelo-chemicals (inhibitors) are produced by plants as end products, by-products, and metabolites, and are contained in the stem, leaves, roots, flowers, inflorescence, fruits and seeds of the plant. Of these plant parts, leaves seem to be the most consistent producers of these allelochemicals.
The four ways in which allelochemicals escape from a plant are: (i) volatilization, during which the terpenes are released from the leaves of some plant species; (ii) leaching (which has shown that living or dead leaves of many plants contain growth inhibitors); (iii) exudation by which case roots of several crop and non-crop species release large quantities of organic compounds that inhibit the growth of other plants; and (iv) decomposition, through which allelochemicals are released from plant residue.
Considerable research work has been done on the allelopathic effect of weeds on natural plant communities viz, abandoned fields or old fields (Abdul-Wahab & Rice 1967, Neil & Rice 1971). Although information on the allelopathic effects of weeds on many tropical crop plants is lacking, such information is available for some temperate crop plants.
Chromolaena odorata is a troublesome weed of arable fields, roadsides and plantation crops such as oil palm, coffee and cashew (Eze & Gill 1992). The leaves of C. odorata contain a large amount of allelochemicals (Ambika & Jayachandra 1980), which may retard the growth of crop plants. Tijani and Fawusi (1989) have reported on the allelopathic activities of crude methanol extract of C. odorata, on seed germination and seedling growth of the tomato. A review of the literature indicates that work has not yet been done on the effect of aqueous extract of Siam weed on the growth of cowpea (Vigna unguiculata). Thus, the present study was undertaken in an attempt to record the effect of aqueous extracts on the growth of cowpea.


C. odorata was collected from University of Benin, Benin City (Lat. 6.5 N, Long. 6.0 W.), Edo State, Nigeria, and immediately brought to the laboratory where the leaves were removed from their stems. The roots and stems were then cut into 8 cm pieces. One kilogram each of leaves, stems and roots were soaked in 1 litre of distilled water, for periods of 36, 48 and 60 hours. The extracts were then filtered and stored in a refrigerator until they were to be used.
Seeds of two varieties of V. unguiculata, white and brown, purchased from the local market were used. These seeds were placed in petri dishes for germination. The seeds used in the study were steeped in water to determine their viability; those that floated were not used.
Sets of 10 seeds each with three replicates per treatment, were allowed to imbibe water on "Whatman No. 1" filter paper saturated with 4 ml. of respective extracts. This was done for a period of 36, 48 and 60 hours, for the leaf stem and root extracts. The seeds were allowed to germinate and grow in petri dishes. One set of petri dishes from each treatment was then put in the dark while the other received continuous light. The seeds which were maintained in a continuously dark area were kept in a cupboard, while those receiving continuous light were placed under a fluorescent lamp (1100 m/m at bench level). The filter papers were constantly moistened with the appropriate extracts. The seedlings were left to grow for four days at room temperature (28+2°C), and measurements were taken of the leaf, stem and root. The dishes in the dark were observed under red light. Emergence of 1 mm of the radicle was used as the criterion for germination.


The results of the effect of aqueous extract of leaf, stem and root of C. odorata on the root growth (cm) of V. unguiculata are shown in Table 1. The soaking of seeds in extracts for 48 hours seems to have given marked inhibition of root growth (1.80 and 1.29 cm., and 3.12 and 0.25 cm for light and dark, respectively). The same trend was observed for treatment of seeds with stem extract (2.48 and 0.41 cm, and 2.57 and 0.93 cm for mixed and white seeds in continuous light and dark, respectively). However, there was no significant difference between the effects of leaf, stem and root extracts on the mixed and white seed varieties kept under continuous light. Leaf extracts saturated for 48 hours inhibited the height of the white more than the mixed variety of V. unguiculata (Tab. 2). In general, the root growth of white and mixed varieties was more affected by leaf, stem and root extracts (Tables 2 & 3). Table 3 also shows that leaf growth under continuous light was inhibited with seeds treated for 36 hours.
Eze and Gill (1992), report that C. odorata contains a large amount of allelochemicals especially in the leaves, which inhibit the growth of many plants in nurseries and plantations. The results of this study show that leaf extract of C. odorata had an inhibitory effect on the general growth of V. unguiculata. Pandya (1975) recorded similar results on the effect of Celosia argentia extract on root and shoot growth of Sorghum vulgare seedling. More recently, similar results on the effect of Cyperus rotundus leaf extract on seedling growth of both shoots and roots of wheat were found. Their results indicate that root growth may have been affected more than stem and leaves because roots were in continuous contact with the extracts. However, McCalla and Haskins (1964) suggest that allelochemicals or toxins are released from the weed by the action of micro-organisms during decomposition. The growth inhibition caused by allelochemicals released from C. odorata may be due to its interference with the plant growth processes. Or the allelochemicals may be reducing cell division or auxin induced growth of roots.
The differential degree of inhibitory effect on leaves, stem and root at various durations (36, 48 and 60 hours), indicates that these 3 fractions have either different quantities of inhibitory allelochemicals, or that the nature of allelochemicals may differ. Based on this study, it is apparent that the leaf contains the highest inhibitory allelochemicals. The results demonstrate that Siam weed leaf extract has inhibitory effects on the growth of roots, stems, and the leaves of V. unguiculata. Thus it is recommended that the Siam weed should be physically removed from cowpea fields before the allelochemicals wash down with the rains.


Abdul-Wahab, A.S., & E.L. Rice. 1967. Plant inhibition by Johnson grass and its possible significance in old field succession. Bull. Torrey Bot. Club 94: 486-487.

Ambika, S.R. & Jayachandra. 1980. Influence of light on seed germination in Eupatorium odoratum L. Indian Forester 106: 637-640.

Bonner, J. 1950. The role of toxic substances in interaction of higher plants. Bot. Rev. 16: 51-65.

DeCandolle, M.A.P. 1832. Physiologic vegetable. Vol. 111 Bechet Jeune Library Faculty Medicine.

Evenari, M. 1961. Chemical influence of other plants (allelopathy) Handbuch der pflanzen-physiol. 16: 691-736.

Eze, J.M.O. & L.S. Gill. 1992. Chromolaena odorata - a problematic weed. Compositae Newsletter 20: 14-18.

Fischer, R.F., R.A. Woods & M.R. Glavicic. 1978. Allelopathic effects of goldrod and ashes on young sugar maple. Canadian J. Res. 8: 1-9.

Grummer, G. & H. Beyer. 1960. The influence exerted by species of flax by means of toxic substances. In J.L. Harper (ed.), The Biology of Weeds pp. 153-157. Blackwell, Oxford.

Lavabre, E.M. 1991. Weed control. McMillan Education Ltd., London.

McCalla, T.M. & F.A. Haskins. 1964. Phytotoxic substances from oil micro-organism and crop residues. Bacterial Rev. 28: 181-207.

Molisch, H. 1937. Der Einfluss einer pflanze auf die andere. Allelopathic Fischer, Jena.

Neil, R.L. & E.L. Rice. 1971. Possible role of Ambrosia psitostachyva patterning and succession in old fields. Amer. Midland Naturalist 86: 344-357.

Pandya, S.M. 1975. Effect of Celosia argentea extracts on roots and shoot growth of bajra seedlings. Geobios 2: 175-178.

Pitman, A.R. & W.B. Duke. 1978. Allelopathy in agroecosystems. Annu. Rev. Phytopathology 16: 431-451.

Tijani-Eniola, H.A. & O.A. Fawusi. 1989. Allelopathic activities of crude methanol extract of stem weed and wild poinsetta on seed germination and seedling growth in tomato. Nigerian. J. Weed Sci. 2: 15-20.