ABSTRACT This study was conducted to investigate theallelopathic effects of aqueous root exudate of 5 common weeds (Ageratum conyzoides, Leucas aspera, Scopariadulcis, Spilanthes acmella and Vernonia patula) on seed germination andseedling growth (shoot and root length) of Radish (Raphanus sativus) and Cucumber (Cucumissativus) under laboratory conditions. The aqueous root exudate of these weedswere applied to determine their effect on seed germination and seedling growthof two agricultural test crops (Radish and Cucumber).
The result observed thatthe aqueous root exudate of all the weeds had significant inhibitory effects onseed germination and seedling growth as compared to control. The aqueous rootexudates of Vernonia patula showedhighly reduced effect on seed germination as well as seedling growth ascompared to the other weeds and control. It was followed by Spilanthes acmella. The medium effectwas observed in case of Leucas aspera andScoparia dulcis. Minimum effect wasfound in case of Ageratum conyzoides.The result suggested that these weesd may affect Radishand Cucumber seeds due to inhibitory effect of allelochemicals which arepresent in the root exudates of these weeds.Key words: Allelopathy,Allelochemicals, Root exudate, Weed and Seed germination. INTRODUCTION Weed may define as the plants which aregrown in undesirable place.
Weeds which are grown in crop fields compete withcrop plants for nutrients, moisture, light and space. Thus weeds affect theseedling growth and crop production. According to theInternational Society of Allelopathy defined the term allelopathy as anyprocess involving secondary metabolites produced by plants, microorganisms,viruses and fungi that influence the growth and the development of biologicaland agricultural system (Allelopathy Journal 2009). Allelopathy is theinhibitory or stimulatory effect of one plant to another plant by producingchemical compounds which are released to the environment. These chemicals arecalled allelochemicals which are present in different plant organs includingplant tissues, leaves, flowers, fruits, stems, roots, rhizomes and seeds(Putnum, 1987). The quantities of organic compounds exuding from roots are notlarge but they have significant effect on seed germination seedling growth.
Plant root exudates contain secondary metabolites including different organiccompounds that are released into the surrounding environment by healthy plantroots (Rovira, 1969). There is evidence from severalresearch on root exudates that they can reduce seed germination and plantgrowth (Pope et al. 1985; Kalburtzi et al. 1989). Some weed speciesproduce plant growth inhibiting substances that adversely affect growth anddevelopment of other plants.
These chemicals are released into the soil as rootexudates of the living or dead plants (Ghafoor and Sadiq, 1991). The aim ofthis study was to examine the effect of some common weeds root exudates on thetwo agricultural crop species. The result of this experiment shows that, rootexudates play a significant role on seed germination and seedling growth ofRadish and Cucumber. MATERIALS AND METHODS This experiment was carried out in thelaboratory of Department of Botany, University of Chittagong at roomtemperature. The selected weeds Ageratumconyzoides, Leucas aspera, Scopariadulcis, Spilanthes acmella andVernonia patula which were common were collected from around the cropfields of Chittagong University campus.
The collected weeds were identified byusing ‘Dictionary of Plant Names of Bangladesh’by MK Pasha and SB Uddin andGoogle search.The seeds of Radish and Cucumber were collected from Hathazaribazar Hathazari, Chittagong. Ageratum conyzoides: It is an herb that is 0.5-1 m high with ovateleaves 2-6 cm long and flowers are white to mauve.
Ageratum conyzoides contains pyrrolizidine alkaloids lycopsamineand echinatine. It is an invasive weed in Africa, Australia, Southeast Asia andthe USA.Leucasaspera:Leucas aspera is commonly found inIndia, Philippines, Mauritius and Java. It is very common weed in India andPhilippines. It is an annual, branched, herb erecting to a height of 15-60 cm withstout and hispid acutely quadrangular stem and branched. Leaves are sub-sessileor shortly petiolate, linear or linearly lanceolate.
The plant contains oleic,linoleic, palmitic, stearic, oleanolic and ursolic acids. Leaves containglucosides, tannins, saponins and sterols. Scopariadulcis: It is an annual erect herb distributedthroughout tropical subtropical regions of India, America, Brazil, West Indiesand Myanmar. Scoparia dulcis is a rich source of flavones, terpenes andsteroids.Spilanthesacmella: It is an annual or short-lived herb that is40-60 cm tall.
It is grown in damp area. It is widely distributed in tropicaland subtropical regions of the world such as Africa, America, Borneo, India,Srilanka and Asia. Plants contain a diverse group of highly valuable andreadily available resource of bioactive metabolites e.g. alkaloids, tannins,essential oils and favonoids.Vernoniapatula:This is a species of perennial herb in the Asteraceae family. The species isnative to tropical Africa and to tropical Asia and has become naturalized inAustralia, Mesoamerica, tropical South America, the West Indies and the USstate of Florida.
The plant contains flavones, terpenes,sterols (?-sitosterol, stigmasterol, ?-spinasterol), phenolic resin, caffeicacid and dotriacontanoic acid. The root of V.patula yield six terpenes (?-Amyrin,?-Amyrin, ?-Amyrin acetate, ?-Amyrin acetate, – Amyrinacetate and 3?-acetoxyurs-13(18)-ene).Preparation of Root Exudates The collected weeds were uprooted from the soil ofthe research area (Chittagong University campus). Then the roots of collectedweeds were washed with distilled water and immediately transferred to theconical flask containing distilled water of 250 ml. The roots of the weeds weresoaked in distilled water for 5 hours and then the plants were removed from theconical flask. Then the root exudates were collected by filtering it throughWhatman 1 filter paper. After the collection the root exudates were preservedin Refrigerator.
Treatment details: This experiment was carried out by using twoagricultural crop species and 5 weeds. The details of which are mentionedbelow: 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 seedswere considered germinated, when radical length was over 2 mm.Thegermination percentage was calculated by using following Seed germination and seedlinggrowth: The seeds were surface sterilized with0.
1% Mercuric chloride for 5 minutes then washed with distiied water severaltimes to remove chemicals. The seeds of Raphanus and Cucumber were spread onsterile filter paper in each petridish and soaked in aqueous root exudates ofeach weeds. Each weeds had three replicates with equal quantity of seeds inpetridishes. The seeds are allowed to grow for 10 days at room temperature.
Theseeds treated with sterile distilled water was termed as control. The seedstreated with weeds aqueous root exudates of Ageratumconyzoides, 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 equalvolume of root exudates in each petridish periodically. After 10 days theseedlings were harvested. Then the effect of root exudates on the seedgermination and seedling growth were studied. The shoot and root length of theseedlings were measured in cm and recorded.
Then the average shoot and rootlength were calculated. The result observed are presented in Tables and photoplates. RESULTS AND DISCUSSION A.Germination test: The aqueous root exudates of fiveweeds showed inhibitory effect on seed germination.
The effects of germinationinhibition were different for different weeds root exudates. The inhibition of germination was found acutein Cucumis sativus than Raphanus sativus. The seeds treated withroot exudates of Ageratum conyzoides,in case of Cucumis sativus it showed87% germination, while for Raphanussativus 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 withroot exudates of Vernonia patula wereshowed highest reduced incidence of seed germination than other treatments forboth the tes crops.
The seeds treated with Ageratumconyzoides were showed least inhibitory effect than the other treatments.The average germination percentage of Raphanussativus was 86.17% and 81.67% of Cucumissativus.Table-1: The effect of aqueous rootexudates of five common weeds onseed 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 onthe seedling growth of the Radish and Cucumber. There was found significantinhibitory effect on seedling growth of both the tested crops as compared tocontrol. The effects of five weeds aqueous root exudates on growth of shoot androot length of R. sativus and C.
sativus are presented in Table-2 and3. There was found differences in the inhibition effects on shoot and rootlength growth for different root exudates of weeds. The average shoot and rootlength of both the test crops were highest in T1, which indicates that the inhibitory effect of root exudate of Ageratum conyzoides was minimum than theother treatments.
The medium inhibitory effects were observed while treatedwith Leucas aspera, Scoparia dulcis andSpilanthes acmella. The lowest average shoot and root length were observedin both the test species, while treated with the root exudates of Vernonia patula. That means theinhibition effect of V. patula onseedling growth was maximum among the five weeds. It was noticed that the rootinhibition was more than shoot inhibition in both the test crops. DISCUSSIONEarlierworkers reported that the root exudates of different weeds significantlyinhibited the germination and seedling growth of several crop species. Xuan etal.
(2006) concluded the root exudates of barnyardgrass suppressed the growthof 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 theallelopathic effect of the fresh shootaqueous extract (FSE) and water soluble root exudate (WRE) of Tithonia diversifolia (Hemsl) A. Gray onthe germination, growth and chlorophyll content of pepper (Capsicum annum L.) and tomato (Lycopersiconesculentum Mill.
).Thisstudy demonstrated that, the aqueous root exudates of these selected five weedshad significant inhibitory effects on seed germination and seedling growth ofboth the test crops. The previousworkers also observed similar type of inhibitory effect on seed germination andseedling growth of Raphanus sativus and Cucumis sativus, whilestudied the different allelopathic effect on them. Meissner et al. (1979) reported that, waterextracts of Cyperus rotundus decreasedthe seed germination and seedling growth of Cucumissativus. A.
T.M. Rafiqul Hoque et al. (2003) observed that Eupatoriumodoratum aqueous leaf extract caused significant inhibitory effect ongermination, shoot and root elongation and development of Raphanus sativus and Cucumis sativus. Ahmed et al. (2007) also reported similarinhibitory effect on seed germination and seedling growth of R. sativus and C.
sativus, when treated with the leaf leachates of differentconcentrations of Lantana camara. Inthis study it was also observed that, the root growth was inhibited more thanthe shoot growth of the both test crops. Zimdahl and Stachon (1980) found that,the allelopathic plant extracts had more inhibitory effect on root growth thanon hypocotyl growth, because the root absorb allelochemicals first from thesurrounding environment. The aqueous root exudates of these selected five weedsinhibited the germination and seedling growth of Raphanussativus and Cucumis sativus because of the presence of differentallelochemicals (growth inhibitors) inthe 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 chemicalsare released into the surrounding environment through the various process, suchas volatilization, leachation, microbial decay of dead or fallen plant partsand root exudation. These allelochemicals play an important role in crop-weed(plant-plant) interacton.
Table-2: The effect ofaqueous 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 ofCucumber. 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 CONCLUSIONThedetailed study revelead that these five weeds aqueous root exudates treatmentaffected the seed germination as well as seedling growth performance of Radishand Cucumber. The release of toxic compounds from these weeds showed strongallelopathic potential activity and characterized from reduction in seedgermination and seedling growth of Raphanussativus and Cucumis sativus. The plants growing in the surrounding areashave different capacity of tolerance to these chemical compounds. These fiveweeds are considerably allelopathic and can inhibit the seed germination andseedling growth of Radish and Cucumber. Consequently, these weeds should be carefully monitored and managedparticularly in susceptible crops like Raphanussativus and Cucumis sativus.