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Materials and methods
Fresh leaves of bermuda grass (Cynodon dactylon L.) were
collected, washed with water and dried in an oven at 75C for 48 hr.
Samples were ground in a Wiley mill to pass through a 20 mesh screen.
Ground samples were stored in plastic bottles at room temperature.
Five levels (0, 0.5, 1.0, 1.5 and 2.0%) of dried ground residue were
kept in distilled water for 24 hr at room temperature in 250
ml conical flask. The extracts were then filtered into 100 ml
beakers using Whatman filter No. 42. Wheat (cv. Pavon) seeds were
surface sterilized with 1% sodium hypochlorite solution for 2 minutes
and then washed thoroughly with distilled water. Ten healthy seeds of
wheat cultivar were placed on the surface of 0.8% agar containing 5
ml aqueous extract of each treatment in glass bowls. The bowls with
only 0.8% agar were treated as control. The bowls were covered with
petri-dishes and incubated at 28C for 5 days. A 16 hour-light and 8
hour-dark period was also provided. Treatment at each extract level
was replicated four times in a complete randomized block design. The
number of seeds germinated were counted. Shoot and root length were
measured. The experiment was repeated and the results were expressed
in terms of the averages of duplicate trials. The data has been
analyzed and presented in Table 1 and
Table 2.
Results and discussion
Germination of wheat seeds significantly reduced as the
concentration of the extract increased, and the magnitude of effect
was more pronounced at concentration above 1%, where it was markedly
reduced (Table 1). The reduction percent in
germination at 2% level of extract was 39% compared to control. This
shows that aqueous extract from Cynodon dactylon residue
contains some germination inhibiting chemicals resulting in the
reduced germination of wheat seeds. Many investigators have suggested
phenolics as the cause of inhibition of metabolic process during
germination (Williams and Hoagland 1982; Blum et al. 1984; Kuiters
1989). Phenolic compounds generally caused inhibition in germination
due to the interference with IAA metabolism, synthesis of protein and
ion uptake by the plants (Castro et al. 1984; Rice 1984; Einhellig
1986; Hussain and Khan 1988). The seedling growth of both shoot and
root of wheat significantly decreased with the increasing
concentration of leaf extract used. The percent decreased in shoot
and root length at the highest extract concentration were 68 and 83,
respectively (Table 1). The reduced growth
and development of the receiver plants in these studies demonstrated
that allelochemicals released from the residues or produced by
microorganisms during decomposition, affected wheat growth. This
result confirmed the findings of others (Castro et al. 1984; Diaz and
Kogan 1985; Weller et al. 1985; Hussain and Khan 1988; Montermurro
1988; Seetha et al. 1990).
In nature, the crop and weed residues or their parts might release
water-soluble phytotoxins into the environment, which accumulate to
the extent of toxicity to affect the wheat species tested. Hussain
and Khan (1988) reported that aqueous leaf extract of bermuda grass
contains ferulic, p-coumaric, vanillic, p-hydroxybenzoic, caffeic and
syringic acids. Similarly. Habib and Rehman (1988) found
caffeicchlorogenic, isochlorogenic, ferulic, o-coumaric, p-coumaric
acids and scopoletin as the component of bermuda grass. All of these
phenolic compounds have also been shown to possess allelopathic
effect. This supports the assumption that extract phototoxicity of
bermuda grass in this study might be due to the presence of Phenolic
compounds, all of them are water-extractable, strong and recognized
as allelopathic agents (Rice 1988). Incorporating leaf extract as 2%
inhibited the root length more than the shoot. Roots of the species
are in direct contact with residue of Cynodon dactylon when
incorporated and subsequently are exposed to allelochemicals which in
turn may have direct or indirect effect on the root system. Other
researchers reported that plant residues caused injury, if the
residue as in contact with or in the immediate vicinity of plant
roots (Bhowmik and Doll 1982; Rice 1984). The present findings,
therefore, reveal that Cynodon dactylon L. water
extract releases toxic compounds which were inhibitory to the growth
of wheat crop.
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