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RESEARCH PAPER
Allelopathic potential of Acacia melanoxylon on thegermination and root growth of native specieswbm_401 18..28
MUHAMMAD I. HUSSAIN*, LUIS GONZLEZ and MANUEL J. REIGOSADepartment of Plant Biology and Soil Science, University of Vigo, Lagoas-Marcosende Campus, Vigo, Spain
Water extracts that were obtained from the flowers and phyllodes of Acacia melanoxylon wereused to determine their allelopathic potential in relation to the germination and seedlinggrowth of the native species, cocksfoot (Dactylis glomerata), perennial ryegrass (Lolium perenne),and common sorrel (Rumex acetosa), as well as a general biotest specie, lettuce (Lactuca sativa),in laboratory bioassays.The flowers and phyllodes ofA. melanoxylon were soaked separately indistilled water in a ratio of 1:1 (w/v) for 24 h in order to prepare the aqueous extracts. Distilledwater was used as the control.The seeds of the target species were germinated in Petri dishesand counted daily for up to 7 days.The A. melanoxylon flower extract (100%, 75%, and 50%)
decreased the seed germination of D. glomerata, R. acetosa, L. perenne, and L. sativa.The flowerextract caused the most reduction in the germination index and germination speed in D.glomerata, L. perenne, and L. sativa. The mean LC50 value of the A. melanoxylon flower andphyllode extracts in relation to the germination inhibition of L. perenne was 43% and 41%,respectively, 40% and 38%,respectively, in R. acetosa, and 53% and 41%, respectively, in L. sativa.All four concentrations of the flower extract proved to be more phytotoxic than the phyllodeextract, reducing the root length of all four species, while the phyllode extract decreased theroot length of L. perenne and R. acetosa at the 100% concentration. The L. perenne and D.glomerata grass seeds were more sensitive regarding germination,as compared to L.sativa and R.acetosa.The flower aqueous extract ofA. melanoxylon was more phytotoxic, as compared to thephyllode aqueous extract, even at the lowest concentration (25%).
Keywords: Acacia melanoxylon, allelopathy, Dactylis glomerata, germination, Lolium perenne,Rumex acetosa.
In the 19th century, some Acacia species were introducedas an ornamental plant in southern Europe and havenaturalized and become invasive in the Mediterraneanand Atlantic regions, from Portugal to Italy (Sheppardet al.2006).Acacia species affect crop growth by compet-ing for various environmental resources as their litterinterferes with the establishment and growth of theadjoining crop plants (Kohli et al. 2006), as well as releas-ing numerous chemical substances, including phenolic
compounds, in the litter (Seigler 2003). Carballeira andReigosa (1999) have demonstrated that the leachate from
Acacia dealbata showed strong inhibitory effects on thegermination and growth of Lactuca sativa during theflowering of A. dealbata. Allelopathy was implicated byDuhan and Lakshminarayana (1995), Rafiqul-Hoqueet al. (2003), El-Khawas and Shehata (2005), Al-Wakeelet al. (2007), and Lorenzo et al. (2008) when theyobserved that the water extracts of different Acaciaspecies inhibited the germination,root and shoot length,and dry weight of different crops and weeds. However,allelopathic plants might have inhibitory, stimulatory, orno effect on the germination and growth of other plants(Reigosa et al. 1999).
Blackwood (Acacia melanoxylon R. Br.) is an intro-duced species that has its origin in the temperate forestsof the south-eastern Australian mainland andTasmania. It
Communicated by B.S. Ismail.*Correspondence to: M.I. Hussain, Department of Plant Biology andSoil Science, University of Vigo, Lagoas-Marcosende Campus, 36310 Vigo, Espaa.Email: [email protected]
Received 21 January 2010; accepted 29 October 2010
Weed Biology and Management 11, 1828 (2011)
2011 The AuthorsJournal compilation 2011 Weed Science Society of Japan
doi:10.1111/j.1445-6664.2011.00401.x
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is a versatile and highly adaptive tree specie that hasspread all over the world (Knapic et al. 2006). It covers aconsiderable area in the coastal zone of the north-western Iberian Peninsula, both in monocultures and inmixed stands, with Eucalyptus globulus, characterized byvigorous tree or root sprouts and seed germination that
is stimulated by fire. On invasion,it establishes quickly inthe alien environment, thereby resulting in changes inthe structure and dynamics of the native ecosystem.Although the species is found mostly in wastelands, italso grows well in cultivated fields, pastures, and alongroadsides.This dominance might be related to chemicalinterference or allelopathy, which gives it an additionaladvantage over native plants.The forest plantations ofA.melanoxylon started in the north-western Iberian Penin-sula at the beginning of the 20th century (Areses 1953)and the plant is presently considered as invasive (Xuntade Galicia 2007). Its leaves have allelopathic effects onthe germination and seedling growth of L. sativa (Soutoet al.2001).Allelochemicals are found in different parts ofa plant body (roots, stems, flowers, and leaves) and prob-ably enter the soil from foliar leaching in rain water,exudation from roots into the soil water, or after leaf falland the subsequent incorporation into the soil (Inderjit& Duke 2003).Different parts of the same plant also varyin their allelopathic effect on the germination andgrowth of crops. However, there is no information aboutthe allelopathic effect of the water extract of the flowersand phyllodes ofA. melanoxylon on the native species thatare present near the Acacia stands.
Seed germination and seedling growth bioassays are
widely used for the study of the depletion of germina-tion and the stunting of growth of a susceptible plantdue to the release of allelochemicals from a donor plant(Lottina-Hennsen et al. 2006). The aim of the presentinvestigation was to assess the allelopathic potential ofthe flower and phyllode extracts ofA. melanoxylon on thegermination, emergence, root elongation, and seedlinggrowth of three native species. This can provide basicinformation about the management of A. melanoxylon.
MATERIALS AND METHODS
Target species
Two grasses, Dactylis glomerata L. cv.Amba (Poaceae) andLolium perenne L. cv. Belida (Poaceae), and Rumex acetosaL. cv. Belleville (Polygonaceae) were used as the repre-sentative species because they naturally grow in the Gali-cian forest ecosystem (north-western Spain) and underthe canopy stands of A. melanoxylon. Lactuca sativa L. cv.Great Lakes California (Asteraceae) was selected as ageneral biotest specie because it is frequently used as a
model specie in allelopathic bioassays (Macas et al.2000).The use ofL. sativa could demonstrate the possiblemechanisms by which A. melanoxylon competes withother species, while the results concerning the threenative species have more ecological significance. Theseeds of the test species were purchased commercially
from Semillas Fito (Barcelona, Spain).
Water extraction of Acacia melanoxylon
The fresh shoots of A. melanoxylon were collectedfrom a natural population in the surrounding area ofLagoas Marcosende Campus, University of Vigo, Spain,during the flowering period.The flowers and phyllodes(expanded petioles that form simple lamina; Atkin et al.1998) were separated from the branches and each wassoaked separately in distilled water in the ratio of 1:1(w/v). Similar extraction techniques were used byMolina et al. (1991) in their studies ofEucalyptus spp.and
Lorenzo et al. (2008) in their studies of the allelopathiceffects ofAcacia dealbata L.The extracts were prepared atroom temperature and left in the laboratory for 24 h.Theextracts were collected, filtered through filter paper, anddescribed as 100%. Distilled water was added to thesolutions to make different dilutions (75, 50, and 25%).
Germination bioassays
Glass Petri dishes (9 cm diameter) were used and con-tained blotting paper (3MM; Whatman, Maidstone,England).Twenty-five seeds of each species were placed
in the Petri dishes to which 3 mL of solution were addedat the start, while the control received 3 mL of distilledwater. An additional 1 mL of each solution was addedevery 48 h thereafter.Three replicates of each treatmentwere incubated in a germination chamber with the fol-lowing germination conditions, each with a relativehumidity of 80% (Hussain et al. 2008):
1 L. perenne: 25/15C day/night temperatures and12/12 h light/darkness.
2 L. sativa: 18/8C day/night temperatures and 12/12 hlight/darkness.
3 R. acetosa: 28/20C day/night temperatures and
9/15 h light/darkness.4 D. glomerata: 25/20C day/night temperatures and
14/10 h light/darkness.
The light was provided by cool, white fluorescenttubes with an irradiance of 35 mmol m-2 s-1.The germi-nation was assessed after every 24 h by counting thenumber of germinated seeds for up to 7 days. Germina-tion was considered as the rupture of the seed coat andradicle emergence of1 mm.
Allelopathic effect ofAcacia melanoxylon 19
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Seedling growth bioassays
The Petri dishes were placed in a cold chamber at 4Cafter 1 week in order to stop seedling growth and theradicle length was measured with a measuring tape(Reigosa & Pazos-Malvido 2007).
Statistical analysis
The germination rate index (GT) was determined, asdescribed by Jderlund et al. (1996), and the speed ofgermination (S) was calculated, as proposed by Ahmedand Wardle (1994). The collected data were statisticallyanalyzed by using a one-way ANOVA (Sokal & Rohlf1995) and the Dunnett test was used to determine thedifferences between the treatment means at the 5% prob-ability level. The mean LC50 value (the dose for 50%inhibition of seedling growth) was calculated by using aprobit analysis, as described by Finney (1971). A logistic
equation was fitted to the germination data as a functionof the logarithm of the concentrations of the A. melan-oxylon flower and phyllode extracts by using SPSS forWindows (v. 15.0; SPSS, Chicago, IL, USA):
Y a bX = + ,
where Y = the probit value, a = the intercept, b = theslope of the line, and X = the log10 concentration.Thevalue of X was obtained in order to calculate the LC50values of the concentrations of the A. melanoxylon flowerand phyllode extracts.
RESULTS
Effect of the Acacia melanoxylon extracts on
germination at each exposure time
The laboratory test showed that the A. melanoxylonflower aqueous extract (100, 75, and 50%) significantlyinhibited the germination of D. glomerata, R. acetosa, L.
perenne, and L. sativa (Tables 14). The A. melanoxylonphyllode extract (100% and 75%) completely inhibitedthe germination process ofR. acetosa during the second,third, and sixth day, ofL. perenneduring the fourth, fifth,and sixth day, and ofL. sativa during the first and secondday. However, there was a tendency for a stimulation ofthe germination ofR. acetosa and L. perenne by differentconcentrations of the phyllode extract.
Effect of the Acacia melanoxylon extracts on the
germination indices
As shown in Table 5, the inhibitory effect of the waterextracts on the GT and S depended on the extract T
able1.
Effectso
ftheAcaciamelanoxylon
floweran
dp
hy
llo
dewaterextractsonthegerm
inat
ionof
Dactylisglomerataateac
hexposuretim
eduring
1wee
k
Acaciamelanoxylon
Exposuretime
(daysafte
rsow
ing
)
Day
1
Day
2
Day
3
Day
4
Day
5
Day
6
Day7
Floweraqueousextract
Contro
l
0.0
0
0.0
0
0.4
2
0.3
3
4.9
2
0.9
0
4.7
5
0.6
5
3.5
0
0.8
9
3.0
9
0.8
0
2.3
3
0.6
3
100%
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
75%
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
50%
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
25%
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
0.0
0
0.0
0*
Phy
llo
deaqueousextract
Contro
l
0.0
0
0.0
0
0.4
2
0.3
3
4.9
2
0.9
0
4.7
5
0.6
5
3.5
0
0.8
9
3.0
9
0.8
0
2.3
3
0.6
3
100%
0.0
0
0.0
0
0.0
0
0.0
0
2.0
0
1.1
5
4.0
0
2.0
0
3.0
0
0.5
7
0.0
0
0.0
0
1.6
7
0.8
8
75%
0.0
0
0.0
0
0.0
0
0.0
0
4.0
0
1.1
5
4.0
0
1.1
5
2.6
7
0.6
6
2.6
7
1.7
6
3.3
3
2.0
2
50%
0.0
0
0.0
0
0.0
0
0.0
0
3.3
3
0.4
1
2.0
0
2.0
0
2.2
2
0.8
3
3.3
3
2.0
2
2.0
0
0.5
7
25%
0.0
0
0.0
0
0.0
0
0.0
0
3.6
7
0.8
8
4.3
3
1.2
0
2.1
3
0.7
6
1.3
3
0.6
6
1.6
7
0.8
8
*Sign
ificantdifferences,compare
dtothecontro
l,atP