ARTICLE IN PRESS

0261-2194/$ - se

doi:10.1016/j.cr

�CorrespondCollege of Life

JinKu HwaY

Tel.:+822 450 3

E-mail addr

Crop Protection 24 (2005) 421–431

www.elsevier.com/locate/cropro

Paddy weed control by medicinal and leguminous plants fromSoutheast Asia

Tran Dang Khanha, Nguyen Huu Hongb, Tran Dang Xuanc, Ill Min Chunga,�

aReasearch Team of Friendly Environmental Low Input Natural Herbicide New Material Study, Konkuk University, Seoul 143-701, Republic of KoreabFaculty of Crop Science, Thai Nguyen University of Agriculture, Thai Nguyen City, Viet Nam

cDepartment of Bioscience and Biotechnology, Faculty of Agriculture, University of the Ryukyus, Okinawa 903-0213, Japan

Received 17 September 2004; accepted 20 September 2004

Abstract

Four common medicinal and two leguminous plant species from Southeast Asia were selected from a preliminary survey and

examined for their potential use in paddy weed control. The screening indicated that all species exhibited strong allelopathic

activities, the strongest of which was Nerium oleander. Allelopathic potentials of plant parts were ranked in the order:

leaf4root4stem. The only exception was for Alocasia cucullata, where the stem displayed the highest potential. In a bioassay, N.

oleander and Helianthus tuberosus showed the highest suppression of germination and growth of Echinochloa crus-galli

(barnyardgrass) and Monochoria vaginalis (monochoria). Other than for Passiflora incarnata, the spontaneous growth of paddy

weeds was significantly suppressed in a greenhouse at 1 t ha�1. At 1.5 t ha�1, all examined species markedly reduced weed plant

growth and the dry weight of weeds by 60–100% and 70–100%, respectively. In paddy fields, weed biomass was reduced by 70–80%,

and rice yield was increased by an average of 20% compared with the control. Stylosanthes guianensis, a leguminous plant widely

used in Southeast Asia and Africa as a cattle feed, cover crop, mulch, and for soil improvement, gave the greatest increase in rice

yield (25.8%). These plants might be useful as natural herbicides and might also contain numerous growth inhibitors that could be

used for the development of biological pesticides.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Allelopathy; Inhibition; Medicinal plant; Leguminous plant; Paddy field; Weed

1. Introduction

At present, there is a trend towards searching fornovel natural plant products to develop bio-herbicidesand pesticides. Numerous plants are reported to possessallelopathic potential and efforts have been made toapply them for weed control. Although most commonallelopathic plants have potential for weed suppression,their effects are usually short-lived and weeds re-emerge(Xuan et al., 2004). Furthermore, to successfully

e front matter r 2004 Elsevier Ltd. All rights reserved.

opro.2004.09.020

ing author. Department of Applied Life Science,

& Environment Science, Konkuk University, Kwang-

angDong, Seoul 143-701, Republic of Korea.

730; fax:+822 446 7856.

ess: imcim@konkuk.ac.kr (I. Min Chung).

suppress the initial growth of weeds, a large amount ofplant material (at least 1–2 t ha�1) needs to be added tothe soil. This would require a large labour force. Theisolation and identification of allelochemicals in higherplants has been attempted, but the identified allelo-chemicals were mostly in low concentrations andinteractive. Currently, only a few natural products suchas cineole (cinmethylene, Shell, USA), leptospermones(triketones, Zeneca) and benzoxazinones and quinolinicacid (BASF, Germany) have been marketed (Kohli etal., 1998; Dayan et al., 1999). These products are used inagricultural practices and show promising results. It isestimated that there are about 1,400,000 compounds inplants with allelopathic activities, of which only 3%have been examined (Einhelling and Leather, 1988). Theremainder are unknown and might contain very

ARTICLE IN PRESST. Dang Khanh et al. / Crop Protection 24 (2005) 421–431422

promising growth inhibitors that could be successfullyexploited for the development of bio-pesticides andherbicides.Xuan (2004) suggested a method for exploiting

properties from plants for agricultural practices thatinvolved: (i) searching the ecosystem for plant specieswith strong allelopathic activities; (ii) evaluating theireffects on pests and weeds under laboratory andgreenhouse conditions and determining the most effec-tive species for field application; (iii) assessing theefficacy of these plants under field conditions, and (iv)isolating and identifying compounds involved in allelo-pathic activities and simultaneously searching for novelallelochemicals.Several studies on the screening for the allelopathic

potential of plants in ecosystems have been reported.Fujii (2001) evaluated the allelopathic potentials of 53cover crop plants (26 leguminous, 19 graminaceous andeight others) and 239 medicinal species (Fujii et al.,2003) using the Plant Box Method. They found thatsome species strongly suppressed tested plants. Theseincluded: Vicia villosa (hairy vetch), Mucuna pruriens

(velvet bean), Avena sativa (oat), Secale cereale (rye),Triticum aestivum (wheat) and Hordeum vulgare (barley)(Fujii, 2001). Artabotrys odoratissimus, Annona cheri-

mola, Dialium guianense, Emblica pectinata, Hevea

brasiliensis, Eucalyptus citridora, Garcinia oblongifolia,Paeonia lactiflora and Sandoricum koetjape showed thestrongest allelopathic activities.Besides their pharmaceutical properties, medicinal

and leguminous plants are also useful for pest controland soil improvement. Southeast Asia is rich in variousmedicinal and leguminous plants in the plant ecosystemand indigenous farmers traditionally incorporate theminto paddy soils after harvest. This is because they hadnoted that, in the cropping season after incorporation,rice yield is increased and pest damage reduced.However, no data on the use of medicinal andleguminous plants for weed suppression have beenreported.In this study, four common medicinal plants and two

leguminous species selected from a preliminary surveywere tested for their biological effects on the sponta-neous growth of paddy weeds. The allelopathic potentialof their plant parts (leaves, stems and roots) wasexamined. Greenhouse and field trials were designed toevaluate the possibility of their use as natural herbicides.Their effects on rice yield were also examined.

2. Materials and methods

2.1. Plant description

The six species examined in this study were Alocasia

cucullata (Chinese taro), Helianthus tuberosus (Jerusalem

artichoke), Nerium oleander (oleander), Passiflora incar-

nata (passionflower), Sophora japonica (Japanese pagodatree) and Stylosanthes guianensis (stylo grass). S. japonica

and S. guianensis are perennial legumes, of which S.

guianensis is widely used for cattle grazing and as a covercrop and mulch in Africa and Southeast Asia. Theremainder are medicinal plants that sometimes causetoxicity if touched or ingested in large quantities.Descriptions, identified constituents and botanical char-acteristics of these plants are described in Table 1.

2.2. Plant materials

The six species were grown in the University of ThaiNguyen Botanic Garden and sampled plants wereselected at random and taken to the Laboratory ofFood Crops, Faculty of Crop Science, Thai NguyenAgricultural University, Thai Nguyen City, Vietnam.The leaves, stems (with bark) and roots of the plantswere separated, well cleaned with tap water, choppedinto 1 cm lengths and halved for fresh and dried extracts.The fresh materials were dried at 80 1C for 2 h and 60 1Cfor 24 h, and then ground into powder. All extracts werefiltered several times and kept at 5 1C in the dark untiluse. The plant powders were used in the bioassays andgreenhouse experiments. Dried plant leaves were used inthe field trials.

2.3. Calcareous soil

Commercial heated soil (Yamamune CommercialAssociation, Miyazaki City, Japan) with no micro-organisms or weed seeds was used in a laboratory trial(pH: 6.3; EC (ms cm�1): 0.45; NO3-N (mg 100 g drysoil�1): 19.1; exchangeable CaO, MgO, K2O andavailable P2O3 (mg 100 g dry soil�1): 251.7, 76.0, 111.4and 5.21, respectively).

2.4. Herbicide

The herbicide used in the paddy field trial, Beron100WP (Bensulfuron methyl 70%W/W), was applied atthe rate of 5 l ha�1.

2.5. Laboratory experiment

2.5.1. Screening for allelopathic potential of plant parts

The screening was conducted at the Laboratory ofFood Crops, Faculty of Crop Science, Thai NguyenUniversity of Agriculture and Forestry in 2003. Radish(Raphanus sativus L.) was used as an indicator plant inthis experiment because it is very sensitive to allelo-chemicals at low concentrations (Tsuzuki et al., 1995).Commercial radish seeds were provided by the SeedCentre of Thai Nguyen province, Vietnam, and the

ARTICLE IN PRESS

Table 1

Characteristics of screened species

Plant species and

common name

Description Identified constituents Botanical characteristics References

Alocasia cucullata

(Chinese taro)

Thick, shiny, green heart-

shaped leaves with curlicue

twisted tips and unturned leaf

margins. Forms sturdy

clumps. The flower spathe is

hooded with the whole flower

structure hiding beneath the

leaf.

Oxalic acid and

asparagine

Causes itchiness if contact

made with sap.

Goonasekera and

Vasanthathilake

(1993)The leaves and stems are most

dangerous if eaten in quantity.

Weed density is lower in

canopy than outside it.

Helianthus tuberosus

(Jerusalem

artichoke)

Perennial herb, often with

knobby white, red or purple

skinned tubers that range in

size from 7.5–10.0 cm long and

3–5 cm thick. Stems erect,

1.5–3.0m tall and hirsute.

Leaves opposite or the upper

ones are alternate, serrate-

dentate, with rough upper

surfaces and winged petioles.

Heads are 5.0–7.5 cm across,

range from few to many, and

terminal on branches. Seeds

are pubescent. Native to

North America.

Ilunin, levulose, purine

bases, arginine, histidine,

betaine, choline and

haemagglutinin. Lectin

Reported to be aperient,

aphrodisiac, cholagogue,

diuretic, spermatogenic,

stomachic and tonic.

Duke (1978, 1979),

Bourne et al. (1999)

Also good for diabetes and

rheumatism.

Good candidate as a defence

protein against plant

pathogens or predators.

Nerium oleander

(Oleander)

Evergreen shrub with fragrant

white, rose or purple flowers

and whorled leaves. Possesses

a long follicle that contains

numerous comose seeds.

Native to Mediterranean

regions and Asia.

Glycosides: oleandrin,

uzangenin,

oleandrigenin,

oleandroside

gentiobioside, diginoside,

digitoxigenin

oleagenine—oleasides

(A,E) Adyregenin,

flavonoids.

The whole plant, including the

sap, is toxic. Ingestion of plant

parts causes serious toxicity.

Mahin et al. (1984),

Osterloh et al.

(1982)

Preparations containing active

principles were used as

rodenticides, insecticides, and

as remedies for indigestion,

fever, ringworm, malaria,

leprosy, venereal diseases and

as abortifacients.

Passiflora incarnata

(Passionflower)

Native to southeastern regions

of North America, it is a

perennial climbing vine with

herbaceous shoots and a

sturdy woody stem. The

flowers are white to pale red.

Apigenin, luteolin,

kaempferol and

quercetin.

The passionflower is used as a

sedative for anxiety and

insomnia. It also has

antispasmodic, antimicrobial

and antifungal properties.

Akhondzadeh et al.

(2001), Brinker

(1998), Gruenwald

et al. (2000)

Sophora japonica

(Japanese pagoda

tree)

Native to East Asia, it is a

perennial leguminous plant

with alternate leaves that are

pinnately divided into 3–8

pairs of leaflets. The flowers

are pea-like, in terminal

clusters, creamy white and

fragrant. The fruit is contained

in yellow or yellow-brown

pods. It has a rapid growth

rate and tolerates different

weather conditions, heat and

drought.

Agglutinin, numerous

types of alkaloids,

flavones, fatty acids,

triterpenes,

phospholipids, amino

acids and

polysaccharides.

Poisonous if ingested, and

causes vomitting, diarrhoea,

delirium and coma.

Gilman and Watson

(1994), Grishkovets

and Gorbacheva

(1995), Tang et al.

(2001), Watanabe et

al. (1993)

It is a traditional herb

medicine with a number of

good attributes such as anti-

virus, anti-tumour, anti-heart

dysfunction, anti-

inflammatory, anti-

abnormality and anti-ulcer

properties. It also reduces the

symptoms of asthma.

Stylosanthes

guianensis (Stylo)

A leguminous perennial herb

with an erect form, many

branches and trifoliate leaves.

It is native to South America

and adapted to a wide range of

soil and weather conditions.

Saponin, tannin and

proanthocyanidin.

It is both a pasture legume and

popular cover crop in Africa

and Southeast Asia.

Baloyi et al. (2001)

T. Dang Khanh et al. / Crop Protection 24 (2005) 421–431 423

ARTICLE IN PRESST. Dang Khanh et al. / Crop Protection 24 (2005) 421–431424

germination rate in distilled water was examined atrandom before the experiment and found to be 490%.Thirty grams of fresh materials (leaves, stems and

roots) was ground and extracted with 100ml distilledwater. One gram of dried powder was extracted with20ml of distilled water and the extracts were kept atroom temperature (25–27 1C) for 24 h and filtered withdistiller papers. Thirty radish seeds were sown in 9 cmPetri dishes lined with filter paper, and 10ml of eachextract was added. Treatment with distilled water wasused as a control. The dishes were transferred to agrowth chamber (set at 25 1C, 4000 lux, lit time:09:00–17:00 h). Germination, shoot and root length,and radish dry weights after 7 days were calculated forall treatments. The inhibitory magnitude of each plantpart was averaged from their inhibitory levels ongermination, root length, plant height and dry weightof the indicator plant.

2.6. Effects on barnyardgrass and monochoria

This experiment was undertaken at the Laboratory ofCrop Science, Faculty of Agriculture, Miyazaki Uni-versity, Japan, in 2003. Seeds of two paddy weeds,Echinochloa crus-galli (barnyardgrass) and Monochoria

vaginalis (monochoria), were collected and preparedaccording to Xuan et al. (2004). The same aqueousextracts in the screening experiment were applied and setat the original concentration. Dilutions of 1/2 and 1/4from the original dose were made. Twenty seeds of E.

crus-galli and M. vaginalis were placed in Petri dishes(6 cm in diameter) lined with filter paper and 5ml of theaqueous extracts were added. The distilled watertreatment was used as a control.In another trial, a plastic beaker (500ml, 9 cm in

diameter) with 300 g of calcinated soil was saturatedwith distilled water. Twenty seeds of E. crus-galli weresown at a depth of 0.5 cm from the soil surface and plantpowders of 0.32, 0.64 and 0.95 g (equal to 0.5, 1.0 and1.5 t ha�1) were applied by scattering them onto thewater surface of the beaker. The distilled watertreatment was used as a control.All treatments in the laboratory trial were placed in a

growth chamber (set at 25 1C, 4000 lux, lit time:09:00–17:00 h). Plant number and elongation of weedroots and shoots were examined after 10 days.

2.7. Greenhouse experiment

This experiment was conducted in a greenhouse of theAgriculture Faculty, Miyazaki University, Japan, in2003. The paddy soil (pH: 6.2, total C: 2.21%, total N:0.17%, CEC: 8.8 meq per 100 g soil; CaO: 91, MgO: 13,K2O: 17, K2O5: 18, SiO2: 25mg per 100 g soil) camefrom the Experimental Farm of Miyazaki University,Japan, where early matured rice (variety Koshihikari)

had been cultivated. The soil was collected to a depth of10 cm and air dried and mixed until treatments werecarried out.Four hundred grams of the paddy soil was added to a

plastic beaker (500ml, 9 cm in diameter) and saturatedwith tap water. Leaf powders at 1.0 and 1.5 t ha�1 wereapplied by scattering them onto the water surface of thebeakers. Tap water treatment was used as a control.Greenhouse temperatures were maintained at 25–28 1Cand tap water was added every 2 days to the beakers.Plant numbers, weed species and dry weights ofemergent paddy weeds were recorded after 1 month.

2.8. Field experiment

This experiment was conducted at the ExperimentalFarm of the Thai Nguyen University of Agriculture andForestry, Vietnam, in summer 2003. The rice field wasdivided into plots (2m� 3m) that had received noprevious herbicide or soil treatment. Thirty-day-oldseedlings of Bao Thai, a common rice (Oryza sativa L.)variety grown in the Thai Nguyen province of NorthVietnam, were transplanted into the experimental plotsat a density of 20 cm� 10 cm. The experiment consistedof eight treatments, of which six had dried material of A.

cucullata, H. tuberosus, N. oleander, P. incarnata, S.

japonica and S. guianensis applied at 1.5 t ha�1 2 daysafter transplanting. Simultaneously, a plot received5 l ha�1. The control received neither plant nor herbicidetreatment. Other than for weed management, all plotswere treated by conventional methods. Treatments werein triplicate and samples were taken with three replica-tions in all treatments. Thirty days after application,weed species, plant number and dry weight of weeds persquare metre were determined. Rice yields were alsocalculated.All treatments in the greenhouse and field experiments

received a fertilizer treatment (N, P2O5 and K2O at therates of 8.0, 6.0 and 6.0 gm�2, respectively) 1 day beforethe soil was saturated with water.The inhibition percentage was calculated as follows:

Inhibition percentage ð%Þ

¼ ½1� ðsample extracts=controlÞ� � 100:

2.9. Statistical analyses

The laboratory experiment was conducted with fivereplications and repeated twice, and the greenhouse andfield experiments were in a completely randomizeddesign with three replications. Statistical analyses wereperformed using analysis of variance (SAS Institute,1997) to analyse treatment differences. The means wereseparated on the basis of least significant differences atthe 0.05 probability level.

ARTICLE IN PRESST. Dang Khanh et al. / Crop Protection 24 (2005) 421–431 425

3. Results and discussion

3.1. Laboratory experiment

3.1.1. Screening trial

(a) Fresh extracts: The inhibition of each plant partwas determined from the average inhibition of theirleaves, stems and roots on the germination and growthof the indicator plant. For the leaves, both N. oleander

and P. incarnata completely suppressed the germinationand growth of radish (100% inhibition), and H.

tuberosus and S. guianensis showed equal inhibition(about 30%). A. cucullata slightly stimulated radishgrowth (Table 2). Other than for A. cucullata and S.

japonica, the inhibition of all leaf treatments on thegermination and growth of radish significantly differedfrom the control (Table 2).Stem and root extracts of N. oleander also completely

suppressed radish germination and growth (Table 2),which indicated that the fresh extracts of this plant wasthe strongest of the screened plants in terms ofallelopathic potential. The next most suppressive species

Table 2

Effects of fresh extracts of screened plants on the germination and growth o

Plant species Germination (%) Root length (mm)

Leaves

Alocasia cucullata 84.4a(5.0) 5.1ab(�2.7)

Helianthus tuberosus 57.8b(35.0) 4.0c(20.0)

Nerium oleander 0.0c(100.0) 0.0e(100.0)

Passiflora incarnata 0.0c(100.0) 0.0e(100.0)

Sophora japonica 47.8b(46.2) 5.7a(�14.7)

Stylosanthes guianensis 51.1b(42.5) 2.2d(56.0)

Control 88.7a(0.0) 5.0b(0.0)

LSD (0.05) 13.0 0.7

Stem

Alocasia cucullata 71.1b(20.0) 3.4b(32.7)

Helianthus tuberosus 82.2ab(7.5) 4.5ab(9.3)

Nerium oleander 0.0d(100.0) 0.0c(100.0)

Passiflora incarnata 31.1c(65.0) 4.6ab(�8.7)

Sophora japonica 36.6c(58.8) 5.1a(�2.7)

Stylosanthes guianensis 75.0b(15.6) 4.6ab(�8.7)

Control 88.9a(0.0) 5.0a(0.0)

LSD (0.05) 9.7 1.4

Root

Alocasia cucullata 87.8a(1.2) 8.7b(�74.7)

Helianthus tuberosus 76.7b(13.7) 12.5a(�149.3)

Nerium oleander 0.0e(100.0) 0.0f(100.0)

Passiflora incarnata 32.2d(63.8) 4.6d(8.0)

Sophora japonica 65.5c(26.3) 2.2e(56.0)

Stylosanthes guianensis 81.9ab(7.9) 6.2c(�23.3)

Control 88.9a(0.0) 5.0d(0.0)

LSD (0.05) 9.8 1.0

Means with the same letters in a column are not significantly different at Po*, **, *** are species with inhibition percentages greater than 80, 50 and 20

were P. incarnata and S. japonica (about 30–40%inhibition) (Table 2), and the least effective species wasA. cucullata. Both H. tuberosus and S. guianensis had aweak impact on radish emergence. In the fresh extracts,the leaves of the screened plants showed the greatestsuppression, followed by the root and the stem.However, the stem of A. cucullata exhibited strongersuppression than did its leaves and roots. All plant partsof N. oleander (100%) and P. incarnata (about 32%) hadequal inhibition potential (Table 2).(b) Dry extracts: Three plant species, N. oleander, S.

japonica and P. incarnata, showed the strongest inhibi-tion percentages, with extracts from their plant partscompletely inhibiting the germination and growth ofradish (100% inhibition) (Table 3). In general, the leavesshowed greater inhibition than did the stem and roots.For instance, the leaves of H. tuberosus had 80% of thetotal inhibition, whereas the stems had 9% and the rootssignificantly promoted radish growth (Table 3).Although the leaves of S. guianensis also significantlyreduced radish germination and growth, its stems androots showed negligible suppression of radish emer-

f radish

Plant height (mm) Dry weight (g) Inhibition rank average

7.7a(�22.8) 0.3a(0.0) �5.1

6.2b(1.1) 0.1c(55.6) 28.0*

0.0c(100.0) 0.0d(100.0) 100.0***

0.0c(100.0) 0.0d(100.0) 100.0***

6.3b(0.0) 0.2b(33.3) 16.2

6.4b(�1.6) 0.2b(33.3) 32.6*

6.3b(0.0) 0.3a(0.0) 0.0

0.8 0.07

4.0c(36.0) 0.2b(33.3) 30.5*

10.3a(�63.5) 0.2b(33.3) �3.4

0.0d(100.0) 0.0d(100.0) 100.0***

6.3b(0.0) 0.1c(66.7) 30.8*

7.1b(�12.7) 0.1c(66.7) 27.5*

7.3b(�15.3) 0.2b(33.3) 6.2

6.3b(0.0) 0.3a(0.0) 0.0

1.2 0.05

6.7bc(�6.3) 0.2b(22.2) �14.4

9.5a(�50.3) 0.2b(22.2) �40.9

0.0e(100.0) 0.0d(100.0) 100.0***

5.3d(15.9) 0.1c(66.7) 38.6*

6.4c(-2.1) 0.1c(66.7) 36.7*

7.3b(�15.3) 0.2b(22.2) �1.9

6.3c(0.0) 0.3a(0.0) 0.0

0.8 0.05

0:05: Values in parentheses are inhibition percentages over the control.

of the control, respectively.

ARTICLE IN PRESS

Table 3

Effects of dry extracts of screened plants on the germination and growth of radish

Plant species Germination (%) Root length (mm) Plant height (mm) Dry weight (g) Inhibition rank average

Leaves

Alocasia cucullata 75.5a(9.3) 14.1a(�241.1) 10.8a(�62.5) 0.2b(33.3) �65.3

Helianthus tuberosus 11.1c(86.7) 0.1d(97.6) 0.5c(92.0) 0.1c(55.6) 83.0***

Nerium oleander 0.0d(100.0) 0.0d(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Passiflora incarnata 0.0d(100.0) 0.0d(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Sophora japonica 0.0d(100.0) 0.0d(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Stylosanthes guianensis 38.9b(53.3) 2.4c(41.9) 7.1b(�7.0) 1.7b(44.4) 33.2*

Control 83.3a(0.0) 4.1b(0.0) 6.7b(0.0) 0.3a(0.0) 0.0

LSD (0.05) 9.1 1.7 0.7 0.05

Stem

Alocasia cucullata 77.8ab(6.6) 11.5a(�178.2) 8.5a(�27.5) 0.2b(33.3) �43.0

Helianthus tuberosus 33.3c(60.0) 5.4b(�30.6) 8.7a(�31.0) 0.2b(33.3) 7.9

Nerium oleander 0.0d(100.0) 0.0c(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Passiflora incarnata 0.0d(100.0) 0.0c(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Sophora japonica 0.0d(100.0) 0.0c(100.0) 0.0c(100.0) 0.0d(100.0) 100.0***

Stylosanthes guianensis 71.3b(14.3) 4.5b(�8.9) 6.6b(0.5) 0.1c(55.6) 15.4

Control 83.3a(0.0) 4.1b(0.0) 6.7b(0.0) 0.3a(0.0) 0.0

LSD (0.05) 11.6 2.1 1.4 0.04

Root

Alocasia cucullata 83.3a(0.0) 10.5b(�54.8) 8.9b(�34.0) 0.2b(33.3) �13.9

Helianthus tuberosus 78.6a(5.6) 12.1a(�193.5) 11.3a(�70.0) 0.3a(0.0) �64.5

Nerium oleander 0.0c(100.0) 0.0d(100.0) 0.0e(100.0) 0.0c(100.0) 100.0***

Passiflora incarnata 0.0c(100.0) 0.0d(100.0) 0.0e(100.0) 0.0c(100.0) 100.0***

Sophora japonica 38.9b(53.3) 4.7c(�12.9) 7.6c(�13.4) 0.2b(33.3) 100.0***

Stylosanthes guianensis 78.1a(6.2) 4.4c(�6.5) 6.5d(2.5) 0.2b(33.3) 8.9

Control 83.3a(0.0) 4.1c(0.0) 6.7d(0.0) 0.3a(0.0) 0.0

LSD (0.05) 6.6 1.4 0.7 0.05

Means with the same letters in a column are not significantly different at Po0:05: values in parentheses are inhibition percentages over the control.

*, **, *** are species with inhibition percentages greater than 80, 50 and 20 of the control, respectively.

T. Dang Khanh et al. / Crop Protection 24 (2005) 421–431426

gence. Of the six species studied, A. cucullata exhibitedthe least inhibition on the germination and growth ofradish (Table 3). Observations in this experimentdemonstrate that all the studied plants showed allelo-pathic potential, and the magnitudes of their inhibitoryeffects were species dependent and varied among plantparts. Leaves possessed the strongest inhibition, fol-lowed by roots and stems. N. oleander showed thestrongest allelopathic potential, followed by P. incarnata

and S. japonica. A. cucullata exhibited the weakestpotential.

3.1.2. Effects on the germination and growth of E. crus-

galli and M. vaginalis

The abilities of aqueous extracts to suppress thegrowth of E. crus-galli (barnyardgrass) were evaluated(Table 4), and extracts from N. oleander and H.

tuberosus were the most effective. At the original dose,germination and growth of E. crus-galli was completelyinhibited. At 1/2 dilution, the inhibition was slightlylowered (Table 4). At the 1/4 dose, although suppressionon weed germination was weakened, weed growth wasstill retarded by 70–90%. S. japonica and P. incarnata

showed the next best suppression. Their original dosesgave only 20–40% inhibition of barnyardgrass germina-tion, but severely reduced weed growth (75–100%).Inhibition from 1.0 and 1/2 dilution of S. japonica and P.

incarnata on barnyardgrass emergence was slightlydifferent. At the 1/4 dose, the impact on germinationof barnyardgrass was significantly reduced, although theinhibition on root length was still strong (70–80%).However, the suppression on shoot length was lowered(25–35% inhibition). Weed suppression by A. cucullata

and S. guianensis was weaker than that by the otherplant species (Table 4).Table 5 shows that H. tuberosus and N. oleander

completely inhibited the germination and growth of M.

vaginalis (monochoria) at all applied doses. At theoriginal dose, all plants completely controlled theemergence of M. vaginalis (Table 5). A. cucullata wasthe next most effective plant at the 1/2 dose, followed byP. incarnata and S. japonica (Table 5). At the 1/4dilution, other than for N. oleander and H. tuberosus,germination and growth of M. vaginalis was notinfluenced. However, root elongation was stunted byP. incarnata and S. japonica (Table 5).

ARTIC

LEIN

PRES

STable 4

Effects of plant aqueous extracts on the germination and growth of barnyardgrass

Plant species Concentration

1.0 1/2 1/4 Row LSD

Germination

(%)

Root length

(mm)

Shoot length

(mm)

Germination

(%)

Root length

(mm)

Shoot length

(mm)

Germination

(%)

Root length

(mm)

Shoot length

(mm)

Germination Root length Shoot length

A. cucullata 60.8b 0.6b 2.0c 33.3d 1.7b 3.3c 46.7c 1.6e 3.7c 12.5 0.2 0.7

(�20.7) (�96.5) (�83.7) (�56.6) (�90.1) (�73.2) (�39.1) (�90.6) (�69.9)

N. oleander 0.0d 0.0b 0.0d 26.7d 0.0c 0.5d 66.7b 0.7e 3.5cd 13.2 0.1 0.3

(�100.0) (�100.0) (�100.0) (�65.2) (�100.0) (�95.9) (�13.0) (�95.9) (�71.5)

P. incarnata 60.0b 0.0b 3.1bc 60.0b 0.7bc 6.7b 66.7b 3.8d 9.2b 7.1 0.4 0.9

(�21.8) (�100.0) (�74.8) (�21.8) (�95.9) (�45.5) (�13.0) (�77.8) (�25.2)

H. tuberosus 0.0d 0.0b 0.0d 1.4e 0.0c 0.1d 53.3c 0.6e 2.4d 8.1 0.1 0.03

(�100.0) (�100.0) (�100.0) (�98.2) (�100.0) (�99.2) (�30.5) (�96.5) (�80.5)

S. japonica 40.0c 0.2b 1.0cd 46.7c 1.6b 3.9c 73.3ab 5.3b 8.0b 11.6 0.7 1.3

(�47.8) (�98.8) (�91.9) (�39.1) (�90.6) (�68.3) (�4.4) (�69.0) (35.0)

S. guianensis 73.3a 0.8b 4.4b 60.0b 0.8bc 4.6c 66.7b 6.7b 8.6b 12.9 0.4 1.9

(�4.4) (�95.3) (�64.2) (�21.8) (�95.3) (�62.6) (�13.0) (�60.8) (�30.1)

Control 76.7a 17.1a 12.3a 76.7a 17.1a 12.3a 76.7a 17.1a 12.3a

(0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0)

LSD (0.05) 10.2 1.3 1.9 11.5 1.2 1.4 9.6 1.5 1.3

Means with the same letters in a column are not significantly different at Po0:05: Values in parentheses are inhibition percentages over the control.

Table 5

Effects of plant aqueous extracts on the germination and growth of monochoria

Plant species Concentration

1.0 1/2 1/4 Row LSD

Germination

(%)

Root length

(mm)

Shoot length

(mm)

Germination

(%)

Root length

(mm)

Shoot length

(mm)

Germination

(%)

Root length

(mm)

Shoot (mm) Germination Root length Shoot length

A. cucullata 0.0b 0.0b 0.0b 34.0c 0.0c 1.2d 100.0a 2.8c 6.1a 12.1 0.2 0.9

(�100.0) (�100.0) (�100.0) (�66.0) (�100.0) (�75.5) (0.0) (�70.5) (24.5)

H. tuberosus 0.0b 0.0b 0.0b 0.0d 0.0c 0.0e 0.0b 0.0 0.0c — — —

(�100.0) (�100.0) (�100.0) (�100.0) (�100.0) (�100.0) (�100.0) (�100.0) (�100.0)

N. oleander 0.0b 0.0b 0.0b 0.0d 0.0c 0.0e 0.0b 0.0 0.0c — — —

(�100.0) (�100.0) (�100.0) (�100.0) (�100.0) (100.0) (�100.0) (�100.0) (�100.0)

P. incarnata 0.0b 0.0b 0.0b 74.0b 0.5c 2.6c 100.0a 5.5b 4.9b 16.2 0.8 0.4

(�100.0) (�100.0) (�100.0) (�26.0) (�94.7) (�46.9) (0.0) (�42.1) (0.0)

S. japonica 0.0b 0.0b 0.0b 80.0b 0.0c 1.7d 94.0a 2.2c 4.7b 14.3 0.1 0.7

(�100.0) (�100.0) (�100.0) (�20.0) (�100.0) (�65.3) (�6.0) (�76.8) (�4.1)

S. guianensis 0.0b 0.0b 0.0b 94.0ab 4.7b 4.0b 100.0a 10.3a 5.0b 17.1 1.7 0.5

(�100.0) (�100.0) (�100.0) (�6.0) (�50.5) (�18.4) (0.0) (8.4) (2.0)

Control 100.0a 9.5a 4.9a 100.0a 9.5a 4.9a 100.0a 9.5a 4.9b

(0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0) (0.0)

LSD (0.05) 15.3 1.8 0.8 14.7 1.3 0.8 12.7 1.7 0.6

Means with same letters in a column are not significantly different at Po0:05: Values in parentheses are inhibition percentages over the control.

T.

Da

ng

Kh

an

het

al.

/C

rop

Pro

tection

24

(2

00

5)

42

1–

43

1427

ARTICLE IN PRESS

nandgrowth

ofbarnyardgrass

1.0

0.5

RowLSD

Shootlength

(mm)

Germination

(%)

Rootlength

(mm)

Shootlength

(mm)

Germination

(%)

Rootlength

(mm)

Shootlength

(mm)

Germination

Rootlength

(mm)

Shootlength

(mm)

2.0c

6.7d

1.3e

1.1cd

50.0b

6.6d

5.8d

6.1

0.4

0.5

(�90.8)

(�92.6)

(�95.6)

(�94.9)

(�44.4)

(�77.9)

(�73.3)

0.8c

56.7b

9.6d

4.3c

100.0a

46.6a

32.1a

13.6

7.7

3.9

(�96.3)

(�37.0)

(�67.8)

(�80.2)

(+11.1)

(+56.4)

(+47.9)

0.0c

10.0d

0.5e

0.5d

3.3d

0.3e

0.2e

1.1

0.2

0.1

(�100.0)

(�88.9)

(�98.3)

(�97.7)

(�96.3)

(�99.0)

(�99.1)

12.2b

66.7b

24.3b

20.6a

60.0b

18.7c

15.9c

9.8

3.4

3.7

(�43.8)

(�25.9)

(�18.5)

(�5.1)

(�33.3)

(�37.2)

(�26.7)

5.9c

66.7b

14.1c

11.9b

53.3b

17.1c

14.8c

14.3

4.2

3.3

(�72.8)

(�25.9)

(�52.7)

(�45.2)

(�40.8)

(�94.3)

(�31.8)

2.0c

26.7c

3.0e

3.3c

20.0c

6.7d

9.4d

6.6

0.7

1.2

(�90.8)

(�70.3)

(�89.9)

(�84.8)

(�77.8)

(�77.5)

(�55.7)

21.7a

90.0a

29.8a

21.7a

90.0a

29.8b

21.7b

(0.0)

(0.0)

(0.0)

(0.0)

(0.0)

(0.0)

(0.0)

6.1

14.7

3.1

2.7

11.4

5.7

3.6

notsignificantlydifferentat

Po0:05:Values

inparentheses

are

inhibitionpercentages

over

thecontrol.

T. Dang Khanh et al. / Crop Protection 24 (2005) 421–431428

In calcinated soil, N. oleander exhibited the greatestinhibition on the germination and growth of E. crus-galli.At 0.5 t ha�1, more than 90% emergence of the weed wascontrolled. However, H. tuberosus strongly stimulatedweed emergence at this dose (Table 6). Other plants thatseverely suppressed weed emergence were A. cucullata,S. guianensis and S. japonica. P. incarnata was the leasteffective.The 1.0 t ha�1 dose gave greater inhibition than the

0.5 t ha�1 dose. A. cucullata and N. oleander controlledE. crus-galli by about 90%. The inhibition of H. tuberosus

on E. crus-galli germination and growth was 37% and70%. The other plant species did not give a greatersuppression of E. crus-galli emergence at the 1.0 t ha�1

dose compared with the 0.5 t ha�1 dose (Table 6).The 1.5 t ha�1 dose showed the greatest weed suppres-

sion. N. oleander completely inhibited germinationand growth of E. crus-galli, whereas A. cucullata,H. tuberosus, S. guianensis and S. japonica reducedweeds by about 70–90%. P. incarnata was the leasteffective, although it suppressed E. crus-galli by about50% (Table 6).The findings in Tables 4–6 demonstrate that the six

species strongly suppressed the germination andgrowth of two noxious paddy weeds, E. crus-galli andM. vaginalis. In bioassays, their aqueous extracts weremore effective on M. vaginalis than on E. crus-galli.Their influence on the two weeds varied between plantspecies; N. oleander showed the greatest inhibition,followed by H. tuberosus. S. japonica and P. incarnata

inhibited E. crus-galli to a greater extent than didA. cucullata. However, A. cucullata suppressed M.

vaginalis more than did S. japonica and P. incarnata

(Tables 4 and 5).The results in Table 6 indicate that the influence of

these screened plants on weed emergence differedbetween bioassays and field conditions. Weed reductionwas greatest for N. oleander in both the bioassay and soiltrials. A. cucullata and S. guianensis were more effectivein the soil, whereas H. tuberosus and S. japonica wereless effective in the soil than in the bioassays. Theeffectiveness of P. incarnata on barnyardgrass did notdiffer between the bioassay and soil trials (Tables 4–6).

Table6

Effectsofplantmaterialsonthegerminatio

Plantspecies

Concentration(tha�1)

1.5

Germination

(%)

Rootlength

(mm)

A.

cucu

llata

13.3c

2.8c

(�85.2)

(�90.6)

H.

tub

ero

sus

10.0cd

1.4c

(�88.9)

(�95.3)

N.

ole

an

der

0.0d

0.0

(�100.0)

(�100.0)

P.

inca

rnata

43.3b

15.7b

(�51.9)

(�47.3)

S.

japonic

a33.3b

7.0c

(�63.3)

(�76.5)

S.

gu

ian

ensi

s13.3c

2.8c

(�85.2)

(�90.6)

Control

90.0a

29.8a

(0.0)

(0.0)

LSD

(0.05)

12.6

7.9

Meanswiththesamelettersin

acolumnare

3.2. Greenhouse experiment

3.2.1. Effects of plant materials on the germination and

emergence of weeds in paddy soil

(a) Weed biomass: At 1.0 t ha�1, three plant species, A.

cucullata, N. oleander and S. japonica, strongly sup-pressed weed plant growth and dry weight by about70% and 90%, respectively, compared with thecontrol (Table 7). H. tuberosus achieved about 50%weed control and was more effective than S. guianensis.

P. incarnata was the least effective, slightly inhibiting

ARTICLE IN PRESS

Table 7

Effects of plant materials on the spontaneous emergence of weeds in the paddy soil

Plant species Weed species Total weeds (plant) Total dry weight (g)

A B C D E F G H

1.5 t ha�1

A. cucullata — — — — — — — — 0.0g (�100.0) 0.0e (�100.0)

H. tuberosus 0.6 3.7 — — 5.3 — — — 9.3c (�60.9) 1.1cd (�75.6)

N. oleander 0.3 5.3 0.3 — 0.6 — — — 6.4d (�73.1) 0.3de (�93.3)

P. incarnata 0.3 1.0 0.3 — 3.0 0.3 — — 4.9de (�79.4) 0.2de (�95.6)

S. japonica — — — 1.3 0.6 — — — 1.9fg (92.0) 0.1e (�97.8)

S. guianensis 0.6 4.0 — — 2.3 — — — 6.9d (�71.0) 1.5c (�66.7)

1.0 t ha�1

A. cucullata — 0.7 1.7 — — — — — 2.4f (�89.9) 0.2d (�95.6)

H. tuberosus 2.3 1.0 2.7 2.3 2.7 — — — 11.0c (�53.8) 1.6c (�64.4)

N. oleander 0.3 5.3 0.3 � 0.6 — — — 6.5d (�72.7) 0.4d (�91.1)

P. incarnata 1.0 6.0 1.0 10.0 7.0 — — 25.0a (+5.0) 4.4a (�2.2)

S. japonica — 0.7 — 6.3 — — — — 7.0d (�70.6) 0.3de (�93.3)

S. guianensis 0.7 4.3 0.7 4.0 5.7 — — — 15.4b (�35.3) 2.1b (�53.3)

Control 0.7 1.7 4.7 8.0 4.7 0.7 2.3 1.0 23.8a (0.0) 4.5a (0.0)

LSD (0.05) 2.1 0.3

Means with the same letters in a column are not significantly different at Po0:05: Values in parentheses are inhibition percentages over the control.

A: E. crus-galli, B: M. vaginalis, C: Rotala indica, D: Ludwigia prostrata, E: Scirpus juncoides, F: Eleocharis kuroguwai, G: Ceratophylum demersum,

H: Myriophyllum verticillatum.

T. Dang Khanh et al. / Crop Protection 24 (2005) 421–431 429

dry weight by 2.2% and negligibly promoting weedplants by 5% (Table 7).At 1.5 t ha�1, inhibition of the applied plant species on

the spontaneous growth of paddy weeds increased. A.

cucullata completely controlled the emergence of paddyweeds (100% inhibition). The second most effectivespecies was S. japonica (about 95% weed reduction)(Table 7). Suppression by H. tuberosus, P. incarnata andS. guianensis was greater at 1.5 t ha�1 than at 1.0 t ha�1.No significant difference in weed suppression betweenthe two applied doses of N. oleander was observed(Table 7).(b) Weed species: Eight weed species, E. crus-galli, M.

vaginalis, Rotala indica, Ludwigia prostrata, Scirpus

juncoides, Eleocharis kuroguwai, Ceratophylum demer-

sum and Myriophyllum verticillatum, were found in thecontrol (Table 7). At 1.0 t ha�1, H. tuberosus, P.

incarnata and S. guianensis completely controlled theemergence of three weed species: E. kuroguwai, C.

demersum and M. verticillatum. At 1.5 t ha�1, P.

incarnata completely suppressed the emergence of L.

prostrata, and H. tuberosus and S. guianensis completelycontrolled two further weed species, R. indica and L.

prostrata (Table 7). Although N. oleander completelycontrolled four weed species, L. prostrata, E. kuroguwai,C. demersum and M. verticillatum, at 1.0 t ha�1, noadditional weed species was completely controlled at1.5 t ha�1 (Table 7).Applications of A. cucullata and S. japonica at

1.0 t ha�1 completely controlled the emergence of mostpaddy weeds found. Exceptions were for M. vaginalis,

which was found in both treatments, and R. indica andL. prostrata, which were observed in the plots ofcucullata and S. japonica, respectively. At 1.5 t ha�1, A.

cucullata completely controlled weed emergence (100%inhibition). In the treatment with S. japonica, M.

vaginalis was not found and, although L. prostrata wasnot completely controlled, its density was stronglyreduced (Table 7).Results from the greenhouse trial showed that the

magnitude of weed suppression was proportional to theapplied dose. The concentration of 1.5 t ha�1 gave atotal reduction in weed biomass of greater than 70%and completely controlled most weed species. Theapplied plant species showed selective effects on paddyweed emergence (Table 7).

3.3. Field experiment

3.3.1. Weed biomass

All six species achieved about a 70–80% reduction inpaddy weed biomass (weed density and dry weight). A.

cucullata and S. japonica exhibited greater weedsuppression (about 80%) than did the other plantspecies (about 70%) (Table 8). The herbicide was mosteffective (about 90% weed control).

3.3.2. Weed species

In the field trials conducted in paddy fields in ThaiNguyen City in North Vietnam, 11 weed species werefound in the control: M. quadrifolia, F. millicacea,L. chinensis, C. difformis, A. indica, D. aegyptium,

ARTICLE IN PRESS

Table 8

Effects of plant material on the spontaneous growth of weeds and rice yield in fields

Plant species Weed species (plant m�2) Total weed (plant) Total dry weight (gm�2) Rice yield (t ha�1)

A B C D E F G H I J K

A. cucullata 2.7 1.7 3.3 5.3 0.3 3.7 2.7 1.3 3.3 0.3 0.3 24.9b(�78.4) 2.9c (�81.5) 4.13ab(+17.0)

H. tuberosus 2.0 2.0 3.0 5.3 0.3 3.3 3.3 0.0 2.7 1.0 2.7 25.6(�77.8) 4.7b (�70.1) 4.13ab (+17.0)

N. oleander 0.0 1.3 4.3 4.3 4.0 2.0 4.3 1.7 2.7 4.0 0.7 29.3b(�74.5) 4.5b (�71.3) 4.22ab (+19.5)

P. incarnata 2.7 0.7 2.7 3.0 0.3 6.3 3.0 2.7 4.7 1.3 1.3 28.7b(�75.1) 4.0bc (�74.5) 4.29ab (+21.5)

S. japonica 1.3 0.3 1.3 3.0 1.0 0.0 4.0 0.0 1.3 0.0 0.0 12.2c(�89.4) 2.5c (�84.1) 3.88bc(+9.9)

S. guianensis 1.3 0.0 5.3 3.3 1.7 2.7 3.3 0.7 2.3 2.3 0.3 23.2b(�79.8) 4.4b (�72.0) 4.44a (+25.8)

Herbicide 1.0 0.3 1.0 2.0 0.0 0.3 1.3 1.0 0.3 1.3 0.0 8.5c(�92.6) 1.3d (�91.7) 3.81bc (+7.9)

Control 15.0 4.7 12.7 14.0 12.3 11.7 15.0 1.7 12.7 12.3 3.0 115.1a (0.0) 15.7a (0.0) 3.53c (0.0)

LSD (0.05) 7.6 1.2 0.51

Means with the same letters in a column are not significantly different at Po0:05: Values in parentheses are inhibition percentages over the control.

A: Marsilea quadrifolia, B: Fimbristylis millicacea, C: Leptochloa chinensis, D: Cyperus difformis, E: Ammania indica, F: Dactyloctenium aegyptium, G:

Sogittaria sogitfolia, H: Polygonum odoratum, I: Jussiacea repens , J: Phyllanthus amarusihum, K: Axomopus offionis.

T. Dang Khanh et al. / Crop Protection 24 (2005) 421–431430

S. sogitfolia, P. odoratum, J. repens, P. amarusihum andA. offionis. S. japonica completely suppressed four weedspecies: D. aegyptium, P. odoratum, P. amarusihum andA. offionis (Table 8). H. tuberosus, N. oleander and S.

guianensis completely inhibited the emergence of P.

odoratum, M. quadrifolia, and F. millicacea, respectively(Table 8). Under field conditions, although the sixspecies did not completely control the emergence of agreat number of weed species, weed density was stronglyreduced compared with the control. This resulted in atotal reduction of weed density of about 80% (Table 8).

3.3.3. Rice yield

All species improved rice yields more than theherbicide application. S. guianensis recorded the greatestimprovement in rice yield (25.8%). Both N. oleander andP. incarnata improved rice yield by about 20%, andcucullata and tuberosus improved rice yield by 17%(Table 8). S. japonica recorded the greatest weedreduction, but had the lowest improvement in rice yield(9.9%) of the applied species. The herbicide improvedrice yield by only 7.9% compared with the control(Table 8). Observations from this experiment indicatethat applications of these medicinal and leguminousplants at 1.5 t ha�1 reduced weed biomass by 70–80%and increased rice yield by up to 25.8% (S. guianensis).

4. Conclusions

The medicinal and leguminous plants in this studyexhibited a strong allelopathic potential on the germina-tion and growth of R. sativus and two nuisance paddyweeds, E. crus-galli and M. vaginalis, in the bioassays. Inthe greenhouse and paddy fields, they caused a morethan 70% reduction in paddy weed biomass and

increased rice yield by up to 25.8% at the applied dosesof 1.0 and 1.5 t ha�1. Furthermore, their impacts onpaddy weeds were selective and proportional to theapplied dose. This study indicates that a preliminarysurvey should be carried out to select plant species fromthe ecosystem with strong allelopathic potential throughbioassays. Although the allelopathic potential mightvary among plant parts (leaves, stems and roots), leavesare commonly chosen under field conditions becausethey can provide a larger plant biomass than the stemand roots. Before determining the plant species thatshould be used for weed control in paddy fields, bothbioassay and greenhouse trials should be conducted toexamine their impacts on the spontaneous emergence ofmajor weed species. Different applied doses should alsobe tested; however, the dose should not exceed 2 t ha�1,because it might cause considerable fieldwork, and thetime of application should be 1–5 days after transplant-ing (Xuan et al., 2003). The bioassays can accuratelyevaluate the allelopathic potential of a plant because itcan minimize competitive factors. However, in the field,the effects on weed growth might differ from bioassayresults. Although a plant might show strong allelopathicpotential on indicator weeds in bioassays, under fieldconditions, the level of weed suppression might decreasebecause soil pH, organic carbon, organic matter andavailable nitrogen affect the allelopathic expression of aplant (Xuan, 2004). Therefore, the impacts of plantspecies on the spontaneous growth of weeds in the soilshould be carefully examined before applying them topaddy fields. We suggest that, for an allelopathic plantto be used as a natural herbicide, it should meet thefollowing criteria. The plant should: (i) be available in alarge biomass in the plant ecosystem and be tradition-ally used as a cover crop or green manure, or for soilimprovement or pest reduction; (ii) exhibit strong

ARTICLE IN PRESST. Dang Khanh et al. / Crop Protection 24 (2005) 421–431 431

suppression of weed emergence and control the emer-gence of major weeds, and (iii) provide weed suppressiongreater than 70% and increase rice yield by more than20% at 1.0–2.0 t ha�1.By incorporating allelopathic plants, agronomic

benefits from both allelopathy and competition can betransferred to target crops. Weeds are biologicallycontrolled and the amount of inorganic fertilizer issimultaneously minimized because some nutrients andnitrogen from plant materials are added to the soilduring decomposition (Xuan et al., 2004). All of the sixplant species studied are very promising for paddy weedcontrol and for increasing rice yield, even though theirsuppression levels varied. S. guianensis (stylo) is themost effective plant because it is a popular legume thatis used as a cover crop in Africa and Southeast Asia(Baloyi et al., 2001), it reduced paddy weeds by 70% andit gave the largest improvement in rice yield (25.8%).The allelochemical composition of these plants shouldbe elucidated because the growth inhibitors responsiblefor their allelopathic activities might be useful for thedevelopment of bio-herbicides. Their impacts on plantpathogens and pests should be further examined.

Acknowledgements

The authors wish to acknowledge the financialsupport of the Rural Development Administrationmade in the BioGreen 21 project year of 2003. Thanksare also due to the fund provided by Japan Showa ShellSekiyu (2003–2004).

References

Akhondzadeh, S., Naghavi, H.R., Vazirian, M., Shayeganpour, A.,

Rashidi, H., Khani, M., 2001. Passionflower in the treatment of

generalized anxiety: a pilot double-blind randomized controlled

trial with oxazepam. J. Clin. Pharm. Ther. 26, 369–373.

Baloyi, J.J., Ngongoni, N.T., Topps, J.H., Acamovic, T., Hamudiku-

wanda, H., 2001. Condensed tannin and saponin content of Vigna

unguiculata, (L.) Walp, Desmodium uncinatum, Stylosanthes guia-

nensis and Stylosanthes scabra grown in Zimbabwe. Trop. Anim.

Health Prod. 33, 57–66.

Bourne, Y., Zamboni, V., Barre, A., Peumans, W.J., Van, D., Els, J.M.,

Rouge, P., 1999. Helianthus tuberosus lectin reveals a widespread

scaffold for mannose-binding lectins. Struct. Fold. Des. 7, 1473–1482.

Brinker, F., 1998. In: Sandy, O. (Ed.), Herb Contraindications and

Drug Interactions, second ed. Eclectic Medical Publications, pp.

109–110.

Dayan, T., Romagni, J., Tellez, M., Rimando, A., Duke, S., 1999.

Managing weeds with natural products. Pesticide Outlook.

October, pp. 185–188.

Duke, J.A., 1978. The quest for tolerant germplasm. In: ASA Special

Symposium 32, Crop Tolerance to Suboptimal Land Condition.

Am. Soc. Agron. Madison, WI, USA, pp. 1–61.

Duke, J.A., 1979. Ecosystematic data on economic plants. Q. J. Crude

Drug Res. 17, 91–110.

Einhelling, F.A., Leather, G.R., 1988. Potentials for exploiting

allelopathy to enhance crop production. J. Chem. Ecol. 14,

1829–1842.

Fujii, Y., 2001. Screening and future exploitation of allelopathic plants

as alternative herbicides with special reference to hairy vetch.

J. Crop Prod. 4, 257–275.

Fujii, Y., Parvez, S.S., Parvez, M.M., Ohmae, Y., Iida, O., 2003.

Screening of 239 medicinal plant species for allelopathic activity

using the sandwich method. Weed Biol. Manage. 3, 233–241.

Gilman, E., Watson, D.G., 1994. Sophora japonica: Scholar tree. Fact

Sheet ST-592, Environmental Horticulture Department, Florida

Cooperative Extension Service, Institute of Food and Agricultural

Services, University of Florida, 3pp.

Goonasekera, C.D.A., Vasanthathilake, V.W.J.K., 1993. Is Nai

Habarala (Alocasia cucullata) a poisonous plant? Toxicon 31,

813–816.

Grishkovets, V.I., Gorbacheva, L.A., 1995. Triterpene glycosides of

Sophora japonica seeds. Chem. Nat. Compounds 31, 596–599.

Gruenwald, J., Brandler, T., Jaenicke, C., 2000. In: Montvale, N.J.

(Ed.), PDR for herbal medicines. Medical Economics Company.

pp. 573–575.

Kohli, R.K., Batish, D., Singh, H.P., 1998. Allelopathy and its

implication in ecosystems. J. Crop Prod. 1, 169–202.

Mahin, L., Marzou, A., Huart, A., 1984. A case report of

Nerium oleander poisoning in cattle. Vet Hum. Toxicol. 26,

303–304.

Osterloh, J., Herold, S., Pond, S., 1982. Oleander interference in the

digoxin radioimmunoassay in a fatal ingestion. JAMA 247,

1596–1597.

SAS Institute, 1997. SAS/STAT User’s Guide, 6.12 Ed., SAS Institute,

Cary, NC.

Tang, Y.P., Lou, F.C., Wang, J.H., Zhuang, S.F., 2001. Four new

isoflavone triglycosides from Sophora japonica. J. Nat. Products 64,

1107–1110.

Tsuzuki, E., Shimazaki, A., Naivaluevu, L.U., Tomiyama, K., 1995.

Injury by continuous cropping to taro and its related factors. Jpn.

J. Crop Sci. 64, 195–200 (in Japanese with English abstract).

Watanabe, K., Kinjo, J., Nohara, T., 1993. The new isoflavonol

glycosides from Lupinus luteus and L. Polyphenyllus arboreus.

Chem. Pharm. Bull. 41, 394–396.

Xuan, T.D., 2004. Utilization of allelopathy in higher plants for

control of paddy weeds. Ph.D. Thesis, School of Agricultural

Sciences, Kagoshima University, Kagoshima, Japan. 167pp.

Xuan, T.D., Tawata, S., Hong, N.H., Khanh, T.D., Chung, I.M.,

2004. Assessment of phytotoxic action of Ageratum conyzoides L.

(billy goat weed) on weeds. Crop Prot. 23, 915–922.

Xuan, T.D., Tsuzuki, E., Terao, H., Matsuo, M., Khanh, T.D., 2003.

Alfalfa, rice by-products, and their incorporation for weed control

in rice. Weed Biol. Manage. 3, 137–144.