The allelopathic effect of root exudates of five weeds on seed germination of Radish and Cucumber 

This
study was conducted to investigate the allelopathic effects of aqueous root
exudate of 5 common weeds (Ageratum
conyzoides, Leucas aspera, Scoparia dulcis, Spilanthes acmella and Vernonia
patula) on seed germination and seedling growth (shoot and root length) of
Radish (Raphanus sativus) and
Cucumber (Cucumis sativus) under
laboratory conditions. The aqueous root exudate of these weeds were applied to
determine their effect on seed germination and seedling growth of two
agricultural test crops (Radish and Cucumber). The result observed that the
aqueous root exudate of all the weeds had significant inhibitory effects on
seed germination and seedling growth as compared to control. The aqueous root
exudates of Vernonia patula showed
highly reduced effect on seed germination as well as seedling growth as
compared to the other weeds and control. It was followed by Spilanthes acmella. The medium effect
was observed in case of Leucas aspera and
Scoparia dulcis. Minimum effect was
found in case of Ageratum conyzoides.
The result suggested that these weesd may affect Radish
and Cucumber seeds due to inhibitory effect of allelochemicals which are
present in the root exudates of these weeds.

Key words: Allelopathy,
Allelochemicals, Root exudate, Weed and Seed germination. INTRODUCTION

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      Weed may define as the plants which are
grown in undesirable place. Weeds which are grown in crop fields compete with
crop plants for nutrients, moisture, light and space. Thus weeds affect the
seedling growth and crop production. According to the
International Society of Allelopathy defined the term allelopathy as any
process involving secondary metabolites produced by plants, microorganisms,
viruses and fungi that influence the growth and the development of biological
and agricultural system (Allelopathy Journal 2009). Allelopathy is the
inhibitory or stimulatory effect of one plant to another plant by producing
chemical compounds which are released to the environment. These chemicals are
called allelochemicals which are present in different plant organs including
plant tissues, leaves, flowers, fruits, stems, roots, rhizomes and seeds
(Putnum, 1987). The quantities of organic compounds exuding from roots are not
large but they have significant effect on seed germination seedling growth.
Plant root exudates contain secondary metabolites including different organic
compounds that are released into the surrounding environment by healthy plant
roots (Rovira, 1969). There is evidence from several
research on root exudates that they can reduce seed germination and plant
growth (Pope et al. 1985; Kalburtzi et al. 1989). Some weed species
produce plant growth inhibiting substances that adversely affect growth and
development of other plants. These chemicals are released into the soil as root
exudates of the living or dead plants (Ghafoor and Sadiq, 1991). The aim of
this study was to examine the effect of some common weeds root exudates on the
two agricultural crop species. The result of this experiment shows that, root
exudates play a significant role on seed germination and seedling growth of
Radish and Cucumber.   

 MATERIALS AND METHODS

     This experiment was carried out in the
laboratory of Department of Botany, University of Chittagong at room
temperature. The selected weeds Ageratum
conyzoides, Leucas aspera, Scoparia
dulcis, Spilanthes acmella and
Vernonia patula which were common were collected from around the crop
fields of Chittagong University campus. The collected weeds were identified by
using ‘Dictionary of Plant Names of Bangladesh’by MK Pasha and SB Uddin and
Google search.The seeds of Radish and Cucumber were collected from Hathazari
bazar Hathazari, Chittagong.

    Ageratum conyzoides:  It is an herb that is 0.5-1 m high with ovate
leaves 2-6 cm long and flowers are white to mauve. Ageratum conyzoides contains pyrrolizidine alkaloids lycopsamine
and echinatine. It is an invasive weed in Africa, Australia, Southeast Asia and
the USA.

Leucas
aspera:
Leucas aspera is commonly found in
India, Philippines, Mauritius and Java. It is very common weed in India and
Philippines. It is an annual, branched, herb erecting to a height of 15-60 cm
with stout and hispid acutely quadrangular stem and branched. Leaves are
sub-sessile or shortly petiolate, linear or linearly lanceolate. The plant
contains oleic, linoleic, palmitic, stearic, oleanolic and ursolic acids.
Leaves contain glucosides, tannins, saponins and sterols. 

Scoparia
dulcis:
 It is an annual erect herb distributed
throughout tropical subtropical regions of India, America, Brazil, West Indies
and Myanmar. Scoparia dulcis is a rich source of flavones, terpenes and
steroids.

Spilanthes
acmella:
 It is an annual or short-lived herb that is
40-60 cm tall. It is grown in damp area. It is widely distributed in tropical
and subtropical regions of the world such as Africa, America, Borneo, India,
Srilanka and Asia. Plants contain a diverse group of highly valuable and
readily available resource of bioactive metabolites e.g. alkaloids, tannins,
essential oils and favonoids.

Vernonia
patula:
This is a species of perennial herb in the Asteraceae family. The species is
native to tropical Africa and to tropical Asia and has become naturalized in
Australia, Mesoamerica, tropical South America, the West Indies and the US
state of Florida. The plant contains flavones, terpenes,
sterols (?-sitosterol, stigmasterol, ?-spinasterol), phenolic resin, caffeic
acid and dotriacontanoic acid. The root of V.
patula yield six terpenes (?-Amyrin,
?-Amyrin, ?-Amyrin acetate, ?-Amyrin acetate,

– Amyrin
acetate and 3?-acetoxyurs-13(18)-ene).

Preparation of Root Exudates    

    
The collected weeds were uprooted from the soil of
the research area (Chittagong University campus). Then the roots of collected
weeds were washed with distilled water and immediately transferred to the
conical flask containing distilled water of 250 ml. The roots of the weeds were
soaked in distilled water for 5 hours and then the plants were removed from the
conical flask. Then the root exudates were collected by filtering it through
Whatman 1 filter paper. After the collection the root exudates were preserved
in Refrigerator.

Treatment details:

    
This experiment was carried out by using two
agricultural crop species and 5 weeds. The details of which are mentioned
below:     Test species:            1.Radish(Raphanus sativus).            2.Cucumber(Cucumis sativus).     Aqueous root exudates:           C = Control (Distilled water)          T1 = Root exudate of Ageratum conyzoides.          T2 = Root exudate of Leucas aspera.          T3 =Root exudate of Scoparia dulcis.          T4 =Root exudate of Spilanthes acmella.          T5 =Root exudate of Vernonia patula.Procedure of data collection:   
The treated seeds were observed daily. The seeds
were considered germinated, when radical length was over 2 mm.The
germination percentage was calculated by using following

Seed germination and seedling
growth:        The seeds were surface sterilized with
0.1% Mercuric chloride for 5 minutes then washed with distiied water several
times to remove chemicals. The seeds of Raphanus and Cucumber were spread on
sterile filter paper in each petridish and soaked in aqueous root exudates of
each weeds. Each weeds had three replicates with equal quantity of seeds in
petridishes. The seeds are allowed to grow for 10 days at room temperature. The
seeds treated with sterile distilled water was termed as control. The seeds
treated with weeds aqueous root exudates of Ageratum
conyzoides, Leucas aspera, Scoparia dulcis, Spilanthes acmella and Vernonia  patula were termed respectively as T1,
T2, T3, T4 and T5. During the experiment period, care was taken to add equal
volume of root exudates in each petridish periodically. After 10 days the
seedlings were harvested. Then the effect of root exudates on the seed
germination and seedling growth were studied. The shoot and root length of the
seedlings were measured in cm and recorded. Then the average shoot and root
length were calculated. The result observed are presented in Tables and photo
plates. RESULTS AND DISCUSSION 
A.Germination test: The aqueous root exudates of five
weeds showed inhibitory effect on seed germination. The effects of germination
inhibition were different for different weeds root exudates.  The inhibition of germination was found acute
in Cucumis sativus than Raphanus sativus. The seeds treated with
root exudates of Ageratum conyzoides,
in case of Cucumis sativus it showed
87% germination, while for Raphanus
sativus it was 90%. The germination inhibition of C. sativus was 70%, while it was 78% for R. sativus, when the seeds were treated with root exudates of Vernonia patula. The seeds treated with
root exudates of Vernonia patula were
showed highest reduced incidence of seed germination than other treatments for
both the tes crops. The seeds treated with Ageratum
conyzoides were showed least inhibitory effect than the other treatments.
The average germination percentage of Raphanus
sativus was 86.17% and 81.67% of Cucumis
sativus.Table-1: The effect of aqueous root
exudates of five common weeds on
seed germination of Radish and   
Cucumber.              

Varieties
 
 
Treatments

Germination
percentage of Radish

Mean

Germination
percentage of  Cucumber

Mean

      Control

       100%

 

       100%

 

        T1

       90%

 

       87%

 

        T2

       86%

86.17%

        81%

81.67%

        T3

       83%

 

        78%

 

        T4

       80%

 

        74%

 

         T5

       78%

 

        70%

 

  B.Seedlings growth:
There were observed noticeable effect of aqueous root exudates of 5 weeds on
the seedling growth of the Radish and Cucumber. There was found significant
inhibitory effect on seedling growth of both the tested crops as compared to
control. The effects of five weeds aqueous root exudates on growth of shoot and
root length of R. sativus and C. sativus are presented in Table-2 and
3. There was found differences in the inhibition effects on shoot and root
length growth for different root exudates of weeds. The average shoot and root
length of both the test crops were highest in T1, which indicates that  the inhibitory effect of root exudate of Ageratum conyzoides was minimum than the
other treatments. The medium inhibitory effects were observed while treated
with Leucas aspera, Scoparia dulcis and
Spilanthes acmella. The lowest average shoot and root length were observed
in both the test species, while treated with the root exudates of Vernonia patula. That means the
inhibition effect of V. patula on
seedling growth was maximum among the five weeds.  It was noticed that the root
inhibition was more than shoot inhibition in both the test crops.  DISCUSSIONEarlier
workers reported that the root exudates of different weeds significantly
inhibited the germination and seedling growth of several crop species. Xuan et
al. (2006) concluded the root exudates of barnyardgrass suppressed the growth
of rice, lettuce (Lactuca sativa L.)
and monochoria (Monochoria vaginalis)
during the early growth stages.  O.O.
Otusanya et al (2008) carried out an experiment to evaluate the
allelopathic  effect of the fresh shoot
aqueous extract (FSE) and water soluble root exudate (WRE) of Tithonia diversifolia (Hemsl) A. Gray on
the germination, growth and chlorophyll content of pepper (Capsicum annum L.) and tomato (Lycopersicon
esculentum Mill.).This
study demonstrated that, the aqueous root exudates of these selected five weeds
had significant inhibitory effects on seed germination and seedling growth of
both the test crops. The previous
workers also observed similar type of inhibitory effect on seed germination and
seedling growth of  Raphanus sativus and Cucumis sativus, while
studied the different allelopathic effect on them. Meissner et al. (1979) reported that, water
extracts of Cyperus rotundus
decreased the seed germination and seedling growth of Cucumis sativus. A.T.M. Rafiqul Hoque et al. (2003) observed that Eupatorium odoratum aqueous leaf extract
caused significant inhibitory effect on germination, shoot and root elongation
and development of Raphanus sativus and
Cucumis sativus. Ahmed et al. (2007)
also reported similar inhibitory effect on seed germination and seedling growth
of R. sativus and C. sativus, when treated with the leaf
leachates of different concentrations of Lantana
camara. In this study it was also observed that, the root growth was
inhibited more than the shoot growth of the both test crops. Zimdahl and
Stachon (1980) found that, the allelopathic plant extracts had more inhibitory
effect on root growth than on hypocotyl growth, because the root absorb
allelochemicals first from the surrounding environment. The aqueous root
exudates of these selected five weeds inhibited the germination and seedling
growth of  Raphanus sativus and Cucumis sativus because of the presence of
different allelochemicals (growth inhibitors) in the root exudates. The allelochemicals such as phenolics, alkaloids,
flavonoids, terpenoids, organic acids, tannins, saponins and steroids (Rice, 1984
and Chou, 1995) are present in these selected five weeds. These chemicals are
released into the surrounding environment through the various process, such as
volatilization, leachation, microbial decay of dead or fallen plant parts and
root exudation. These allelochemicals play an important role in crop-weed
(plant-plant) interacton.      Table-2: The effect of
aqueous root exudates of weeds on the 
seedling growth of Radish.

Treatments

Average shoot
length (cm)

Average root
length (cm)

Control

11.88

5.30

T1

10.12

4.34

T2

8.58

4.26

T3

7.55

3.32

T4

5.48

2.91

T5

3.78

2.52

 

 Table-3:
The effects of aqueous root exudates of 5 weeds on the Seedlings growth of
Cucumber.

Treatments

Average shoot
length (cm)

Average root
length (cm)

Control

14.32

8.32

T1

12.91

7.26

T2

11.83

6.21

T3

10.86

5.51

T4

9.42

4.91

T5

7.51

3.42

  CONCLUSION

The
detailed study revelead that these five weeds aqueous root exudates treatment
affected the seed germination as well as seedling growth performance of Radish
and Cucumber. The release of toxic compounds from these weeds showed strong
allelopathic potential activity and characterized from reduction in seed
germination and seedling growth of Raphanus
sativus and Cucumis sativus. The plants growing in the surrounding areas
have different capacity of tolerance to these chemical compounds. These five
weeds are considerably allelopathic and can inhibit the seed germination and
seedling growth of Radish and Cucumber. 
Consequently, these weeds should be carefully monitored and managed
particularly in susceptible crops like Raphanus
sativus and Cucumis sativus.