Use of Local Plant Aqueous Extracts as Potential Bio-herbicides against Striga hermonthica (Del.) Benth. in Burkina Faso
This study was conducted to evaluate the allelopathic properties of endogenous plant species against Striga hermonthica (Del.) Benth., a root parasitic weed. In this respect, twenty five water extracts and eight freeze dried water extracts from sixteen plant species were screened in bio-assays to test their ability to induce or inhibit the germination of S. hermonthica seeds. Striga seeds were conditioned either in 10% water extracts or in 10% diluted lyophilisats to check their inhibition effect on Striga seed germination. Three doses, 1, 5 and 10% of water extracts were applied on conditioned Striga seeds to test their ability to induce Striga germination. Aqueous extracts from four plant species reduced Striga seed germination by 95.8 to 99.8% compared to the untreated control. Aqueous extracts from two others also significantly reduced Striga germination by 93.1 and 86.3%, respectively. Lyophilisats from four species inhibited Striga seeds germination, whereas that of two others reduced Striga seeds germination by 93.5 and 99.6%, respectively. Only 1% aqueous extracts of Ceiba pentandra and Eucalyptus camaldulensis significantly stimulated Striga seed germination by 39.2 and 38.9%. These results pointed out that the metabolites produced by some of the local plant species may have the potential to be used as bio-herbicides to control S. hermonthica and enhance cereals yield.
Received: March 20, 2010;
Accepted: May 26, 2010;
Published: July 27, 2010
Root parasitic weeds of the genus Striga (Scrophulariaceae) constitute
a major biotic constraint to cereals production in sub-Saharan Africa, particularly
for the very important food crops, maize, sorghum and pearl millet. The most
devastating to cereal production in West Africa is Striga hermonthica
(Del.) Benth which causes huge losses ranging from 40-90% (Gressel
et al., 2004) and up to 75% of its overall damage to the hosts occurred
during its subterranean stage of development (Parker and Riches,
1993). A single S. hermonthica plant can produce up to 500 000 seeds
which can remain viable for more than 14 years (Bebawi et
al., 1984). This has led to the buildup of a large reserve of Striga
seeds in contaminated soils.
Several control methods against Striga species have been recommended
such as crop rotation, trap and catch cropping, injection of ethylene gas, use
of fertilizers and herbicides (Radi, 2007). Unfortunately,
techniques targeted at controlling Striga are frequently ineffective
and rarely adopted by resource-poor African farmers (Hess
and Grard, 1999). Indeed, the use of herbicides such as Dicamba and 2,4
D against S. hermonthica is effective (Radi, 2007)
but the required chemical compounds are expensive for subsistance farmers and
ill-fated for human. In addition, their application led sometimes to chemical
damage to host crop and environmental pollution.
An alternative control approach against Striga would be the use of natural
products that would inhibit or induce the germination of Striga seeds
in order to deplete the Striga seedbank in the soil. The effectiveness
of plant products to control grasses has been reported. Indeed, the roots of
Callistemon citrinus produce a toxic substance (Callisto) of which the
active compound (mesotrione) inhibits the growth of foliage, roots and seeds
germination of numerous weeds (Syngeta, 2004). In bio-assays,
about twenty and ten of Chinese medicinal herbs reduced and inhibited, respectively
S. hermonthica seeds germination (Ma et al.,
2004). Screenhouse evaluation of Azadirachta indica and Parkia
biglobosa to control S. hermonthica revealed that products based
on two species were effective to reduce Striga emergence (Marley
et al., 2004). A survey carried out in Eastern Burkina Faso reported
traditional uses of endogenous plants to control Striga infestation (Traore
and Yonli, 2001). Thus, the promotion of plant substances from local species
to control Striga controlling will contribute to develop new perspectives
for crops protection by saving annual yield losses of cereals worth US $ 7 billion
(MBoob, 1988) and decrease the importation of
chemicals in Africa. This approach should be based on economically and technically
viable technologies without damage to environment.
The effect of extracts from endogenous plants on S. hermonthica has not almost been explored in Burkina Faso. This study aimed to evaluate through bio-assays the allelopathic properties of local plants to control Striga seed germination. Since, water is presumed to be the most readily available solvent, aqueous extracts were made and assessed for their ability to induce or to inhibit the germination of Striga seeds. In this way, we consider to develop a plant-based protection method that would be available for resource-poor farmers.
MATERIALS AND METHODS
S. hermonthica Seeds
Striga hermonthica seeds were harvested in 2006 from a sorghum field
located at the Kouaré agricultural research station (11°9503
N and 0°3058E) in Eastern Burkina Faso, air dried and stored
at ambient temperature (30°C). Seeds were surface sterilized with alcohol
(70°) and sodium hypochlorite (NaOCl. 1%) amended with Tween 80 respectively
for 3 and 5 min before using them in germination tests.
The utilizable parts of sixteen local species were collected from the Sudanian
zone of Burkina Faso (Table 1). The plant materials were washed
and dried in the dark at room temperature and were separately ground into fine
powder (<1 mm) and stored until use.
Plant Aqueous Extracts
Aqueous extracts at 10% concentration were obtained by pickling at room
temperature. Ten gram of powdered part of plant were placed in a 250 mL glass
beaker with 100 mL of sterile distilled water.
|| Names and materials of plants tested against S. hermonthica
The glass beaker containing suspension was stirred (7/24 h) on an agitator
(Edmund Bühler, 7 400 Tübingen, SM 25) at 100 rpm for 24 h and each
suspension was then filtered through two tools, the first (nylon cloth) served
to move big debris and the second (Whatman® filter paper N°1)
to set an homogeneous solution. Eight water extracts were freeze-dried for lyophilisat
tests and twenty three were stored at 4°C until use; they were screened
for inhibition effect on Striga seed germination. Dilutions of 5 and
1% from 10% concentration of sixteen plants were prepared and the three doses
were assessed for stimulation effect on germination of water-conditioned Striga
seeds. For each assay a Completely Randomized Block (CRB) design was used with
Thirty to 40 surface sterilized Striga seeds were placed on glass
microfibre filter paper (GF/A) discs (6 mm ø) in Petri dishes (9 cm ø)
lined with double moistened Wathman No. 1 filter papers (Botanga
et al., 2003). Eight milliliters of each 10% water extracts or 10%
diluted lyophilisats was used to condition Striga seeds and treatments
were as follows: (1) Seeds conditioned with sterile distilled water (Control);
(2) seeds conditioned with water extract or diluted lyophilisat. Sealed Petri
dishes were wrapped in aluminum foil and black polyethylene and incubated in
darkness at 28°C for 10-12 days. The treatments were replicated three times
and arranged as a Completely Randomized Block (CRB) design. After the conditioning,
Striga seeds were placed on discs and transferred into new Petri dishes
(9 cm) lined with double moistened Whatman No. 1 filter papers. Then, a 20 μL
of GR24 (0.0001%) was used per dick to check the ability of Striga seeds
to germinate. Petri dishes were again sealed, wrapped in aluminum foil and black
polyethylene and incubated in the darkness at 28°C for 48 h. After 48 h,
the effectiveness of each treatment to inhibit Striga seeds germination
was determined. The experiment was repeated three times.
In this assay, Striga seeds were previously conditioned with sterile
distilled water as described above. Four discs with Striga seeds (30-40)
were introduced into new Petri dish (9 cm) lined with double moistened Whatman
No. 1 filter papers. For each water extract, a 20 μL was applied on water-conditioned
Striga seeds per dick in order to stimulate their germination and treatments
in comparison were: (1) seeds germination induced with sterile distilled water
(Control (-)), (2) seeds germination induced with GR24 (0.0001%) (Control (+)),
(3) seeds germination induced with 1% water extract, (4) seeds germination induced
with 5% water extract and (5) seeds germination induced with 10% water extract.
Three replications were used per treatment. Sealed Petri dishes were wrapped
in aluminum foil and black polyethylene and incubated in the darkness at 28°C
for 48 h. After 48 h, the ability of each treatment to stimulate Striga
seed germination was evaluated by counting the number of germinated seeds. The
bio-assay was repeated three times using a Completely Randomized Block (CRB)
Germination data of Striga seeds were arcsine-transformed (Gomez
and Gomez, 1984) before performing ANOVA (SAS Institute. Cary. NC) and then
back-transformed. Means were separated using Newman Keuls Multiple Range test
and differences between treatments were considered significant at p<0.01.
RESULTS AND DISCUSSION
Inhibition of Striga hermonthica Seed Germination
No water extract from the 16 plant species completely inhibited Striga
seed germination. However, they significantly reduced percentage of Striga
seed germination in comparison to the untreated control. Striga seeds
germination was strongly inhibited when conditioned with water extracts from
Thevetia neriifolia (leaves), Azadirachta indica (roots), Parkia
biglobosa (peels), Balanites aegyptiaca (roots), Jatropha gossypifolia
(leaves) and Eucalyptus camaldulensis (leaves and roots). Indeed, water
extracts from seven species reduced the potential germination rate of Striga
seeds by more then 76% (Table 2).
|| Inhibition effect of 10% water extracts from local plant
species on Striga hermonthica seed germination
|*Means with the same letter(s) are not significantly different.
!Arcsine transformations of percentage of Striga hermonthica seeds
germination. §Means in brackets are back-transformations
of percentage of Striga hermonthica seeds germination
||Inhibition effect of lyophilisats of water extracts from local
plant species on Striga hermonthica seed germination. Dark gray color
indicate standard errors of the means
ANOVA revealed that the lyophilisats of water extracts from nine plant species significantly affected S. hermonthica seed germination in comparison to the control (GR 24). Indeed, the rate of Striga seed germination from the control was the greatest (77.3%) (Fig. 1). Striga seeds germination was 0% when seeds were conditioned with the lyophilisats from A. indica (roots), B. aegyptiaca (roots), P. biglobosa (peels) and Sclerocarya birrea (leaves) whereas that recorded with the lyophilisats from T. neriifolia (leaves) and J. gossypifolia (leaves) were 5.03 and 0.28%, respectively. Among the rates of Striga seed germination obtained with the lyophilisats, that of seeds treated with the lyophilisats from Accacia gourmaensis (bark), Calotropis procera (leaves) and Azadirachta indica (leaves) were the highest namely 14.3, 17.5 and 20.2%, respectively (Fig. 1).
Stimulation of Striga hermonthica Germination
Striga seeds stimulated with GR24 alone (control +) showed the highest
germination rate (about 72%) whereas that treated with sterile distilled water
did not germinate. Aqueous extracts from Ceiba pentandra (bark) and Eucalyptus
camaldulensis (leaves) significantly induced Striga seed germination.
Water extracts (1%) from both species respectively stimulated Striga
seeds germination by 39.2 and 38.9%, which are the greatest after that of control
+ (Table 3). Weak germination rates ranging from 1 to 4.3%
were recorded when Striga seeds were stimulated with 1% water extracts
from five other species: E. camaldulensis (roots), Sclerocarya birrea
(roots), Calotropis procera (leaves), Lantana camara (leaves+stalks)
and Jatropha curcas (leaves). Considering 5 and 10% water extracts of
the different species, only that of E. camaldulensis (leaves) induced
a Striga germination rate more than 10%, while that of C. pentandra
(bark) showed no effect. Water extracts from Accacia gourmaensis (bark)
at the dose 5% and from Chrysanthellum americanum (leaves+stalks) at
the dose 10% weakly stimulated Striga germination by 3.2 and 8.3%, respectively.
Water extracts from six local plant species showed significant inhibitory effects
on the germination of Striga hermonthica seeds. The current study pointed
out that plant water extracts may have potential inhibition on Striga
infestation and widened the list of allelopathic plants to Striga germination
(Ma et al., 2004). Indeed, similar evaluation
of water extracts from 383 Chinese traditional herbs showed that 27 herbs inhibited
S. hermonthica seed germination and among them, undiluted extracts from
sixteen herbs reduced Striga germination by more than 50% (Ma
et al., 2004).
|| Percentage of Striga hermonthica seed germination
induced by water extracts from local plant species
|*Means followed by the same letter(s) are not significantly
different. !Arcsine transformations of percentage of Striga
hermonthica germination. §Means in brackets are back-transformations
of percentage of Striga hermonthica germination
The undiluted extract from Curcuma longa L. was found to inhibit completely
S. hermonthica germination (Ma et al., 2004).
Present results obtained with Azadirachta indica and Parkia biglobosa
confirmed the observations reported by Marley et al.
(2004) in Nigeria (West Africa). Their screenhouse evaluation of products
based on A. indica (seeds, leaves) and P. biglobosa (fruits, peels)
to S. hermonthica control revealed that seeds of A. indica, fruits
and peels of P. biglobosa were effective reducing Striga emergence
(Marley et al., 2004). Other studies reported
that A. indica (bark and leaves) inhibited germination and growth of
three weeds: Echinochloa crus-galli, Monochoria vaginalis and
Aeschynomene indica in a bio-assay and in soil (Xuan
et al., 2004). Previous findings on allelopathic plants suggested
that effective compounds can be isolated and characterized to further use for
Striga control. Indeed, six phenolic compounds having potential allelopathic
activity were isolated from A. indica (Xuan et
al., 2004) while (+)-5-deoxystrigol was isolated from Lotus japonicus
root culture (Sugimoto and Ueyama, 2008).
The lyophilisats of water extracts from plant species used in this study significantly
reduced S. hermonthica seed germination. The lyophilisats from A.
indica (roots), Balanites aegyptiaca (roots), P. biglobosa
(peels) and Sclerocarya birrea (leaves) completely inhibited Striga
seed germination whereas that of Thevecia neriifolia (leaves) and Jatropha
gossypifolia (leaves) reduced Striga seed germination by 93.5 and
99.6%, respectively. Since no significant effect of water extract from Sclerocarya
birrea (roots) was observed on Striga germination, the inhibition
effect of its lyophilisat showed that the aqueous extract may contain allelochemicals
in low dose. So, the high concentration of compounds in lyophilisat resulted
in stronger inhibition activity from S. birrea (roots).
In this study, we have targeted seeds as susceptible organ to the extracts
of allelopathic plants. In this respect, our results may be additional to that
of Salam and Noguchi (2010). These authors evaluated
the inhibitory effects of methanol extracts from rice seedlings on shoots and
roots elongation of three target weed species.
With regard to the stimulation of S. hermonthica seed germination, only
1% water extracts from Ceiba pentandra (bark) and Eucalyptus camaldulensis
(leaves) significantly induced Striga seed germination. Present results
are similar to that of Ma et al. (2004), who
used Chinese plants. The evaluation of Chinese traditional herbs revealed that
distilled water and methanol extracts of 26 and 22 species, respectively, stimulated
the germination of S. hermonthica (Ma et al.,
2004). In this perspective, Tsanuo et al. (2003)
managed to isolate an isoflavanone (uncinanone B) from Desmodium uncinatum
(Jacq.) DC. which induced S. hermonthica seeds germination. Stimulants
of Striga germination cannot induce germination at high doses as oppose
to low doses (Siame et al., 1993; Yasuda
et al., 2003). The results from this study revealed that the inhibition
effect on Striga germination of water extracts from some plant species
such as E. camaldulensis (leaves) is probably due to a high concentration
of the applied compounds.
Ongoing evaluation of acetonic and methanolic extracts from these local plant species will argue the data above and the characterization of the compounds will specify their chemical nature. However, the results from this study have indicated that metabolites produced by some local plant species may have potential to be used as bioherbicides to control S. hermonthica. Indeed, they suggest that the prolonged use of non-host plants that produce S. hermonthica stimulants or inhibitors may reduce Striga seeds bank in the soil. A potential use of local plant species by farmers to control Striga infestations in the field could be through crop seed coating with plant metabolites before sowing. So, the use of local plant could be a biological component in the integrated Striga management in West Africa.
We thank the International Foundation of Science (IFS) and the International
Sorghum and Millet collaborative research support program (INTSORMIL) for funding
this study. We are grateful to the Institut de lEnvironnement et
des Recherches Agricoles (INERA) of Burkina Faso for the infrastructure that
permitted us to conduct this study.The authors greatfully acknowledge Dr. Aad
Van Ast from Wageningen University (The Netherlands) for providing GR 24.
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