Allelopathic Effects of Aqueous Extract from Siam Weed on the Growth
L.S. Gill, G.O Anoliefo and U.V. Iduoze
Department of Botany, University of Benin, Benin City, Nigeria
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.
(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
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.
MATERIALS AND METHODS
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.
RESULTS AND DISCUSSION
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.
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