Introduction and review of literature - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/3426/10/10_chapter 1.pdf · Introduction and review of literature interactions. In any

Introduction and review of literature

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Chapter I

Introduction and Review of literature

A. Introduction

The agricultural country like India is facing the problem of acute shortage of

food, fodder, fuel, fiber etc. The pivotal issues behind this are population explosion,

shrinkage of cultivable land, degradation of soil fertility, pollution of land, water and

air. The changing environmental scenario including global warming, elevated levels

of CO2 and depletion of ozone have aggravated the problems. As a result of this the

productivity of agricultural as well as forest ecosystems has registered severe

reduction. In spite of all the above mentioned factors the mainstay of degradation in

soil fertility and reduction of crop productivity, is burgoing population of native

weeds and successful invasion of several exotic weeds.

To control and manage effectively this problem of invasion and encroachment

of weeds in cropland and forest ecosystem, allelopathic investigation is the dire need

to protect the green cover of mother earth. According to Narwal (2003b) and Inderjit

(2005), allelopathy will provide the best alternative solution, to get rid of above

mentioned threat in Indian agriculture and forestry.

Allelopathy

Allelopathy generally refers to any direct or indirect, harmful or beneficial

effect of one plant on another through the production of chemical compounds that are

released into the environment (Molisch 1937, Rice 1984). These donor plants may

affect germination, growth and development of the recipient plant species (Einhellig

1987). “The invasive weeds are the plant species that are new to a specific area and

have become dominant, replacing the native plant species”. These are also known as

the alien, exotic or introduced ones.

The research and development in allelopathy is of extreme urgency for the

improvement of agriculture, forestry and the global environment (Rizvi and Rizvi


1992, Chou et al. 1999, Inderjit et al. 1999, Kohli et al. 2001, and Reigosa and Pedrol

2002), because allelopathy majorly deals with invasive/ exotic and native weeds,

allelopathic crops that keep hampering agricultural practices and bring about

environmental degradation (Inderjit 2005).

It is rather difficult to use and define the term ‘weed’, but the Weed Science

Society of America defined it as “a plant growing where it is not desired” (Buchholtz

1967). Weeds have many ill / negative characters, because of which they cannot be

neglected at all. Many of the weeds cause damages to agroecosystems and also

disturb/ reduce natural phytodiversity. Weeds cause great harm to the crops in various

ways as they cause 30 – 40% yield losses, increase the expenditure of various cultural

practices, reduce the efficiency of agricultural implements. Perennial weeds reduce

quality of fertile lands, cause obstacles for water flowing in canals. Weeds reduce

crop yield and its quality as they compete with crops for resources like soil, water,

nutrients and light. Weeds are alternative hosts for many pests and pathogens. Many

weeds like Prosopis, Calotropis etc. reduce the germination capacity of crops’ seeds

due to the phytotoxins/ allelochemicals/ ecochemicals, many a times which are the

secondary metabolites, secreted by them in the soil.

Aquatic weeds like Eichhornia and different types of algae produce toxins,

which are harmful to aquatic flora and fauna. Weeds harbour organisms like

mosquitoes, which cause or transmit diseases. Some weeds are poisonous to humans

and produce pollens, which cause allergies. These studies will be more helpful, if

emphasis on interactions among the plants is highly focused by the researchers.

Studies on allelopathic potential and the biochemical characterization of native and

invasive weeds has become the top priority to get rid of the ill effects of native and

invasive weeds.

Allelopathic interactions between plants

Allelopathic interactions are primarily based on the synthesis and release of

secondary metabolites by higher plants that initiates a wide array of biochemical

reactions, which induce several biological changes, however, many of these are yet to

be understood. In nature, many plant species grow together and interact with each

other by inhibiting or stimulating the growth and development through allelopathic

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interactions. In any ecosystem, the dominant plants growing within it are exhibited in

the form of pure stands or monothickets. Such ecosystems always show the zones of

inhibition around them (Nilsen 2002). The ecosystems infested by dominant weeds

show drastic alterations in their structure and function.

All the weed species, which are the part of dynamic ecosystems, originate in

natural environment and become hurdle to the crops (Aldrich 1984, Baker 1965,

1991). These weeds have some diagnostic features, such as short seed dormancy

period, high rate of seed germination, rapid seedling growth, high reproductive

ability, life cycle of a short span, very high environmental plasticity, self-

compatibility, effective and efficient methods of dispersal of propagules, production

of different types of novel ecochemicals or allelochemicals and tolerance to biotic and

abiotic stresses (Baker 1965), which enable them to grow and survive in varied

habitats and inhospitable ecological conditions.

As a result of this these weeds are becoming dominant throughout the

world (Holm 1967, Holm et al. 1977), and threaten the native phytodiversity (Dukes

and Mooney 1999, Tilman 2000, Mc Neely 2001, Heutte and Bella 2003, Colautti and

MacIsaac 2004, Lee and Klasing 2004, Jeschke and Strayer, 2005). As stated by

Reichard and Campbell (1996) and Li et al. (2004), the invasion of exotic weeds is

mainly due to their easier establishment and faster growth under diverse

environmental conditions. Lonsdale (1999) claimed that the propagules’ pressure,

adaptive characters and susceptible environment favour the invasibility to which

Carlton (1996, 2001) called biological invasion.

Plant invasions and encroachments

The whole biosphere is facing the problem of invasion of different weed

species, hence studies on plant invasions and allelopathy will help in understanding

the mechanism of invasions, and consequences of them on global biodiversity and

ecosystem functioning. These invasions pose many ecological, economic and social

problems. A team approach to solve these complicated problems is necessary.

According to MacDougall and Turkington (2005), the alien species highly out

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compete the native species or escape from adverse environmental conditions and

dominate the community. According to vacant niche hypothesis (Elton 1958) the

empty places such as barren lands, roadsides, open grounds etc. are generally invaded

by such weeds. The different hypotheses explaining the invasion mechanisms (Inderjit

et al. 2005a, b) are summarized in Table 1.1. The diversity of these weeds is governed

by population, ecosystem dynamics, disturbances, nutrient supply and climatic

factors. The biotic restrictions on them, force to skip from their previous habitat and

start surviving in new habitats, helping in the process of invasion. The enemy release

hypothesis advocated by Mack et al. (2000) also supports the above view. If the

invader is resistant enough and tolerant to herbivory, then its competitive ability

increases and it becomes very aggressive due to production of some defensive

chemicals (Carpenter and Cappuccino 2005).

The disturbances by some plant species, grazing pressure (Bright, 1998),

fluctuation in resource availability (Davis et al. 2000), soil moisture, available light

(Meekins and Mc Carthy 2001), phenotypic plasticity and hybridization (Daehler

2003) results in to successful invasion. The novel weapon hypothesis (Callaway and

Ridenour 2004), biotic resistance hypothesis (Maron and Vilà 2001), and the genetic

shift hypothesis (DeWalt et al. 2004a) also explain the mechanism of invasion. To

understand the distribution of invasive weeds and their associates in a natural

community, the eco-distribution mapping is of paramount importance.

Eco – distribution (GPS) mapping of invasive weeds and co- occurring species

Eco – distribution mapping and frequent monitoring of invasive plants’

distribution are important for natural resource management and habitat preservation

(Anderson et al. 1993, NISC 2001). Eco-distribution mapping by using Geographical

Information System (GIS) is greatly contributing in understanding the weed

distribution and their growth modeling. It also helps in getting the knowledge of

magnitude and complexity of exotic weeds and their increasing ecological impact on

reduction of phytodiversity. The unique capability of GIS software is helpful for the

sustainable management and utilization of natural resources throughout the world

(Crossman and Kochergen 2002).

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Morphological and reproductive features of invasive weeds

According to Baker (1965), the weeds have rapid seedling growth and ability

to reproduce at young stage. They undergo maturation very quickly and often have a

short vegetative stage. The root system penetrates deep in the soil. Their vegetative

parts may store large amount of food which enables them to withstand environmental

stress. The environmental plasticity of such weeds is helpful to escape from the harsh

environment. They are usually self-compatible, but some are cross pollinated. They

produce large number of seeds having long or short range dispersal mechanisms.

They have great competitive ability for nutrients, light and water. Their ability to

repel grazing, offensive nature and allelopathic potential helps to become dominant

and occupy diversified habitats including fallow lands, degraded land masses etc.

Physiological and biochemical characters of invasive weeds

The competitive success of many invasive plant species over co-occurring

native species has proved clearly in many ecosystems (McDowell 2002), however, the

physiological attributes that influence this competitive success have often remained

unexplored. However, understanding of these aspects may provide valuable insight

into this problem.

Pattison et al. (1998) claimed that successful invasive species should have

some elite physiological traits which can increase their ability to capture photons and

utilize them efficiently. The invasiveness of alien species depends on their

photosynthetic efficiency and ability to capture energy at the minimum cost, as a

result of which they get more resources for their growth, reproduction and metabolic

processes (Durand and Goldstein 2001). Apart from this they have very high abiotic

stress tolerance capacity. This enables them to survive and reproduce successfully

under extreme environmental conditions due to presence of various antioxidants and

antioxidant enzymes. Hence biochemical, physiological and enzymological

characterization of weeds is necessary for better understanding of the mechanism of

invasion.

Allelopathic potential of weeds

The invasive and native weeds may affect germination, growth and

development of the recipient plant species, as they have very high allelopathic 5


potential. According to Rizvi and Rizvi (1992), Rizvi et al. (1999) and El-Khatib et al.

(2004a) allelopathic potential has predominant role in deciding the patterns of

vegetation and succession in any ecosystem. The dominance, substitution or

extinction of species may be primarily due to the impact of human activities, and

secondly due to allelopathy. Allelopathic interactions exhibited through the

allelopathic potential of invasive as well as native weeds play a crucial role in natural

as well as manipulated ecosystems. The allelopathic nature of any plant is due to the

presence of different types of allelochemicals in it. All the details regarding

allelochemicals, the master molecules in allelopathy are discussed further.

Phytochemicals and allelochemicals in weeds

The weeds attain dominance in any ecosystem because of prodigality

(overproduction), and different ecochemicals / allelochemicals present in their

different parts. Such allelochemicals detected in different weeds are shown in Table

1.2. The phytochemicals as well as allelochemicals are of primary (sugars, amino

acids, proteins) and secondary (phenols, alkaloids, steroids, flavonoids etc.) nature.

The correct understanding of the mechanism of invasion is not possible without the

isolation, identification and characterization of these phytochemicals and

allelochemicals.

The success of allelopathy and invasion depends on types of allelochemicals

and their functional groups. Every aspect of allelopathy is interlinked with

allelochemicals, which are secondary metabolites, produced by the plants as the

byproducts of their metabolic processes (Levin 1976). The higher concentrations of

allelochemicals usually inhibit the growth of recipient plants and soil microorganisms

or both. However, they have stimulatory effects at lower concentrations on growth,

development, flowering, fruiting and yield (Mallik and Williams 2005). These

positive or negative effects of leachates, extracts and residues on recipient plants are

due to the stimulatory or inhibitory action of allelochemicals. The action of

allelochemicals is governed by their presence as single compound or mixtures

(Macias et al. 1998).

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Classification, nature, occurrence, biosynthesis and release of allelochemicals

The allelochemicals naturally occurring in various forms are divided into

eleven different categories by Putnam (1985) e.g. toxic gases, organic and aromatic

acids, aldehydes, simple unsaturated lactones, coumarines, quinones, flavonoids,

tannins, alkaloids, terpenoids, steroids and miscellaneous unknown compounds. These

allelochemicals occur in various plant organs such as roots, rhizomes, stems, barks,

leaves, flowers, fruits etc. (Rice 1979) in stored form, which are released into the

environment through different processes (Putnam and Duke 1978, Fisher 1979) like

volatilization, root exudation, leaching and decomposition of plant residues. Leaves

may be the most consistent source (Putnam 1985), while roots are considered to

contain fewer and less potent allelochemicals (Kobayashi 2004).

According to Whittaker (1970), biosynthesis of allelochemicals takes place

through the metabolism of carbohydrates, fats and amino acids via various

biosynthetic pathways like acetate, shikimic and mevalonic acid (Fig.1.1). According

to Aldrich (1984), Rice (1984) and Einhellig (1996) the diseases, insect damage and

abiotic factors such as temperature, nutrients and moisture deficit enhance the

induction of biosynthesis of allelochemicals (Fig. 1.2).

Once released in the environment, these chemicals may be degraded or

transformed into other compounds (Inderjit and Duke 2003). The water-soluble

compounds such as phenolics and alkaloids leach out due to the rains, mist and fog

and / or after the decomposition of plant residues in the soil (Gallet and Pellissier

1997). While scopoletin and hydroquinones are released through the root exudates.

The biosynthesis, concentration and flux rate of allelochemicals depend on

prevailing climatic as well as environmental conditions (Einhellig 1996,

Weidenhamer 1996, Gallet and Pellissier 1997, Nilsson et al. 1998, Devi et al. 1997,

Burgos et al. 1999, Cambier et al. 2000). The several factors causing induction of

allelochemicals, their transport and allelopathic responses are shown in Fig. 1.2. The

allelochemicals act through specific mechanism, which differs from species to

species.

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Role of leachates, root exudates, extracts and decomposition of plant residues in

allelopathy

The various organic and inorganic substances, secondary metabolites,

allelochemicals or ecochemicals existing in any plant species are released in to the

environment through processes like leaching, volatilization, root exudation, extraction

and decomposition. The allelopathic potential of leachates, extracts, decomposed

residues depends on the type and quantity of allelochemicals existing in them. The

allelopathic stimulation or inhibition in recipient plants and their response also

depends on in what way they are exposed to the allelochemicals. Hence a brief

highlight is made on all such aspects.

Leachates

It is the removal of the substances from plants by the action of rain, snow, fog,

dew or mist. This process releases large number and amount of allelochemicals in to

the environment (Kumari and Kohli 1987). The degree of leachability depends upon

the type of plant tissue, age of the plant and the amount and nature of precipitation

(Molina et al. 1991). The major allelochemicals released via leaching include many

different organic and inorganic compounds such as phenolic compounds, terpenoids,

alkaloids etc. (Rice 1984), that are characterized for their toxicity to the plants and the

microorganisms in the vicinity (Macias et al. 1994). The amount of rainfall

determines the leachability of a particular compound and its toxicity either to the

leached plant (autotoxic) and/ or to an adjacent plant (allotoxic).

Root exudates

The natural oozing out through the roots into the soil are generally described

as root exudates. Root exudates contribute greatly towards allelopathy, as they include

many of the allelochemicals. Many of the compounds released via roots are known to

reduce seed germination, root and shoot growth, nutrient uptake, and nodulation

(Pandya et al. 1984, Weston 1986, Yu and Matsui 1994). The factors such as plant

age, nutrition, light and moisture influence the process of root exudation both

qualitatively and quantitatively (Einhellig 1987).

Extracts

The different plant parts such as roots, leaves, stems, branches, fruits and

seeds are harvested and extracted in suitable solvents like water, alcohol, chloroform

etc. The tissue homogenization/ crushing of such plant parts in the solvent is able to

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extract the various organic and inorganic substances existing in respective plant part.

The extractive values depend upon the age and type of the plant part, chemical nature

of the solvent, temperature etc. As extraction is the mechanical process as opposite to

the natural processes like leaching and exudation, most of the allelochemicals and

other organic and inorganic substances are extracted with a great efficiency. The

concentration of such chemical compounds in the crude extracts is usually very high,

as compared to leachates, root exudates and decomposing plant residues. The

inhibitory or stimulatory effect of such plant extracts is comparatively higher than the

above mentioned types. The crude extracts are usually filtered/ centrifuged to get

purified forms.

Decomposition of residues

The plants’ waste materials such as dried roots, leaves, stems, branches, fruits

and seeds left behind after harvest and/ or completion of life cycle is generally termed

as residue. Litter is one of the important forms of decomposing residue. The

decomposition of plant residues adds a large quantity of allelochemicals to the

rhizosphere (Goel 1987), which is influenced by nature of the residue, soil type and

the conditions of decomposition (Mason-Sedum et al. 1986). The decomposing plant

materials may not get evenly distributed throughout the soil and hence as the roots

will grow through the soil and wherever and whenever they come in contact with

decomposing residue, they may get affected by allelochemicals. The compounds

released into the soil are subjected to transformation by soil microflora and produce

biologically more active products than original compound (Blum and Shafer 1988).

These residues influence not only the crop emergence, growth and productivity but

also influence similar aspects of weed growth.

Mechanism of action of allelochemicals

It is difficult to understand the exact mechanism of action of allelochemicals.

But Rice (1984) and Mandava (1985) indicated that the allelochemicals act through

positive or negative impact on i) cell division and cell elongation, ii) phytohormone

induced growth, iii) membrane permeability, iv) mineral uptake, v) stomatal opening

and photosynthesis vi) respiration, vii) protein synthesis and changes in lipid and

organic acid metabolism viii) inhibition and stimulation of specific enzymatic

activities (Fig.1.3).

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Impact of allelochemicals

In nature the impact of allelochemicals is centralized on a fine-tuned

regulatory process in which these chemicals act together on one or more of the above

mentioned processes (Rizvi et al. 1999). They further divided the effect of

allelochemicals on target plant into a direct and an indirect type. Effects through the

alternation of soil properties, nutritional status and an altered population or activity of

micro-organisms and nematodes represent the indirect action. While direct action

involves the biochemical/ physiological effects of allelochemicals on various

important processes of plant growth and metabolism.

After release, allelochemicals cause both inhibitory and stimulatory effects (Waller et al. 1995, Mitzutani 1999) and various factors like concentration, flux rate, age, metabolic state and environmental conditions determine their toxicity (Wyman-Simpson et al. 1991, Kohli et al. 1993, Wardle et al. 1993, Weidenhamer 1996, Gallet and Pellissier 1997, Nilsson et al. 1998). Their amount and production varies in quality and quantity with age, cultivars, plant organ, and time of the year (Devi et al. 1997, Burgos et al. 1999 and Cambier et al. 2000). The receiver plants show varied types of responses to the allelochemicals released from donor plants.

Response of recipient plants to allelochemicals

Biological activities of receiver plants in response to allelochemicals are

known to be concentration dependent. These responses are characteristically,

stimulation or attraction at low concentrations and inhibition or repellence as the

concentration increases (Lovett 1989). When receiver plants are exposed to higher

concentrations of allelochemicals, their growth and development are adversely

affected. These effects include inhibition or retardation of seed germination, reduced

root and shoot growth, swelling or necrosis of root tips, curling of the root axis,

discolouration, lack of root hairs, increased number of seminal roots, reduced dry

weight accumulation, and lowered reproductive capacity.

These allelochemicals also inhibit the growth of recipient soil

microorganisms. The allelochemicals often regulate the distribution and vigour of

plants. The existence and distribution of allelochemicals in plants and microorganisms

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had been well documented. These are generally stored in plant cells in bound form,

and released into the environment from the special glands on the stems and leaves

(Putnam and Duke 1978, Fisher 1979).

Cytotoxic and larvicidal effects

The allelochemicals from donor plants affect cell division which results in to positive or negative impact on growth of recipient plant. Hence the screening of cytotoxic activities of such plants is necessary.

The control of mosquito larvae is dependent on regular applications of

organophosphates and different insecticides (Yang et al. 2002b). The major drawback

with the use of chemical insecticides is that they are non-selective and could be more

harmful to other non-target organisms. Moreover, after few years the mosquitoes

develop resistance against the insecticides due to frequent use of them (Severini et al.

1993, WHO 1970). Plants, being a natural source of various compounds, are known to

contain larvicidal agents, which may act in combination or independently, hence

necessiting to carry out the studies on the larvicidal activities (Gershenzon and

Dudareva 2007, Wink 1993).

In nature the weed – weed, weed – crop and crop – crop interactions are

observed which are mainly due to the allelochemicals released into the environment.

Weed-weed interactions

The weed-weed interaction greatly influences the weed biodiversity in natural

ecosystems. Rahman (2002) reported the inhibition of Cassia tora due to Parthenium

hysterophorus. Similarly, Nie et al. (2003a, b) also reported the inhibitory effect of

aqueous extract of Wedelia trilobata on Brassica parachinensis . The weed extract

caused reduction in seed germination percentage, fresh weight of roots and aerial

parts, plant height and chlorophyll contents. They claimed that these negative effects

were due to inhibited activities of peroxidase, superoxide dismutase, nitrate reductase

and disruption of nitrogen metabolism. Similar was the finding of Penna et al. (2003).

They had reported the inhibitory effects of aqueous extracts of Chenopodium

ambrosioides on seed germination of Bidens pilosa. The research on allelopathic

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properties of weeds, weed-weed interactions, characterization of allelochemicals,

mechanism of release and weed management, is well documented.

Weed – crop interactions

Since the origin of agriculture, weeds are associated with crops, reducing their

yield considerably. Many researchers have reported the allelopathic potential of

numerous weeds on the crops. Inderjit (2004 a, b) had given very comprehensive

account of biology and ecology of invasive weeds in agricultural systems. These

weeds have evolved with indigenous or traditional agriculture e.g. Echinochloa

evolved with rice cultivation in Japan. The allelopathy researchers like Ambika and

Smitha (2005), Batish et al. (2005) and Tajuddin et al. (2002) reported that the

residues, root exudates as well as leaf and seed leachates/ extracts of weeds had

inhibitory allelopathic effects on different crops.

About Pune University campus

Geographical location

The big campus of University of Pune established in 1949, at Ganeshkhind

occupies an area of 164.8 hectares, which is situated about seven km north-west of

Pune city proper (Plate I) and lies between 18034’ North latitude and 73053’ East

longitude at an elevation of about 1880 m. At present 1/4 th area is occupied by roads,

buildings and gardens. At the South – East corner range hill is situated while a small

nallah is flowing at North – Eastern boundary.

Geology, soil and climate

Ganeshkhind stands on pediment surface of cretaceous rock with – eocene or

amygdaloidal basalt and sometimes even it shows porphyritic and compact basalts.

These rocks are traversed by many veins and veinlets of silica and chalcedony. The

poor soil of study area is reddish brown on higher grounds and deeper dark brown

(black cotton soil) on flat areas. The soils are alkaline and are of pedocal type

(Varadpande 1972). The average rainfall, climate and other environmental conditions

of the campus are more or less similar to that of Pune city. The railfall is restricted to

couple of months in monsoon and the maximum annual rainfall is 31.78cm. The

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temperature during hot season goes up to 40 – 420C but normally it is cool as

compared to Pune city.

Floristic composition of the campus

The Pune University campus is occupied by the typical sub-deciduous,

xerophytic vegetation, in addition to the herbaceous monsoon flora. Dicotyledons are

forming the major vegetation followed by grasses. The entire area is covered by about

780 species and 540 genera belonging to 118 different families. The largest number of

species are from Leguminosae, Asteraceae, Euphorbiaceae and Poaceae. While

Acacia chundra Willd., Dalbergia melanoxylon Guill. and Acacia arabica Willd. are

representatives of under trees. The invasive weeds like Synedrella nodiflora (L)

Gaertn. Cassia uniflora Mill. non Spreng and Alternanthera tenella Colla. As well as

native weed species like Achyranthes aspera, Oplismenus compositus are mixed with

shrubby vegetation of Dalbergia melanoxylon (Plate II).

The campus is dominated by majority of seasonal and annual herbaceous

plants and few perennials like Dalbergia melanoxylon Guill., Gliricidia maculata H

.B. & K., Rouwolfia canescens L. etc. (Varadpande 1972). The invasive herbaceous

plant species like Cassia uniflora Mill. non Spreng, Alternanthera tenella Colla.,

Synedrella nodiflora (L) Gaertn, Parthenium hysterophorus L., Bidens biternata L.,

Acalypha ciliata Forsk are significantly suppressing the population of native weeds

like Boerhaavia diffusa, Achyranthes aspera. These native and invasive weeds

influence each others’ growth throughout their life. (Almeida 1998).

The central barren area shows accumulation of soil for proper growth of

monsonaceous flora (ephemerals) like Mollugo pentaphylla L., Cyanotis fasciculata

Schult., Oldenlandia corymbosa L. etc. The early sprouting ephemerals and tuberous

plants include members of Liliaceae, Cyperaceae and Commelinaceae. These plants

sprout in succession with the onset of monsoon. When monsoon sets in by about

middle of June, the campus gets totally different appearance and the ground becomes

carpeted temporarily, with variety of herbs and grasses. They belong to the families

like Leguminosae, Asteraceae, Convolvulaceae, Amaranthaceae, Tiliaceae,

Capparidaceae, Rubiaceae, Acanthaceae, Lamiaceae, Commelinaceae, Cyperaceae,

Euphorbiaceae and Poaceae. The annual and perennial climbers and twinners mostly

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belong to Leguminosae, Convolvulaceae, Asclepiadaceae, Passifloraceae and

Menispermaceae. The vegetation of this area represents two seasonal aspects as

winter and summer.

The flora of the Pune University campus is an artificial complex, as the

original vegetation is being partly replaced or altered due to human interference which

caused the introduction of new trees, seed mixtures of exotic and invasive plants.

The natural invasion of many exotic weed species have occupied the major

area of the campus (Varadpande 1972), which has also affected the phytodiversity of

the entire campus. According to Almeida (1998) and Almeida et al. (2003), Cassia

uniflora Mill. non Spreng. and Synedrella nodiflora (L.) Gaertn. are annual,

herbaceous weeds, now distributed all over India which have also become most

dominant in this campus.

Selected sites of study area

For the present study the vast campus of Pune University was conveniently

and randomly divided into four different sites, with an area of approximately 40

hectares. All the four selected sites are representing the variety of soil types of the

campus and distributed in all the directions. These sites have rich phytodiversity.

Site I

This site includs the area around Chhatrapati Shivaji Maharaj statue, the

departments of Botany, Zoology, Jaykar library, Environmental science, Geology and

Geography. The weed species such as Cassia uniflora, Blainvillea, Acalypha,

Synedrella, Achyranthes and Euphorbia are highly dominant at this site. Monospecific

thickets of Cassia uniflora occupied the open area in front of Botany department.

Communities of grass species (Oplismenus composites) are also mixed with it. This

site is also occupied by tree species like Gliricidea and D. melanoxylon.

Site II

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It is covering the area around international students’ hostel, Gents’hostel

no.VI, main playground and IUCAA as well as SET guest house campus. It is

showing dominance of Cassia uniflora with its pure stand along with Bidens,

Achyranthes, C. absus, Alternanthera and Synedrella. At marshy places Cynotis was


growing. Parthenium, Xanthium and other grasses like Paspalum, Sporobolus were

fairly dense in number. The shrubs and trees like D. melanoxylon, Acacia chundra,

Leucena leucocephala and Tamarindus are also intermixed with the vegetation. But

there was no Gliricidea.

Site III

This site included the area near Late General B. C. Joshi gate of the University

and clusters of residential quarters of nonteaching staff. It is having the dominance of

Synedrella as pure stand and very big communities of Rauwolfia, Acalypha,

Achyranthes, C. uniflora and Tithonia. Trees in this area were Pongamia, Albizzia,

Acacia chundra, few Gliricidea and rare occurance of D. melanoxylon and Caryota

urens.

Site IV

This site includes the area at the footsteps of Rangehills, Gents’ hostel no. 8 ,

residential quarters and university press. The area is dominated by C. uniflora,

Achyranthes, Synedrella and Alternanthera. This is highly disturbed site due to

human activities and new constructions like NCCS, PUMBA, Health science,

Microbiology departments, hostels and school. It is showing distribution of

Gliricidea, D. melanoxylon and Lantana camara.

About the selected weeds

Frequent visits and ecological surveys conducted during the study period

indicated that the plant species like Cassia uniflora Mill.non Spreng. and Synedrella

nodiflora Gaertn. were highly dominant as compared to other plants in the campus of

Pune University

Cassia uniflora Mill.non Spreng. (Syn. Cassia sericea Sw.) is a member of

family Caesalpinaceae having origin in tropical South America and worldwide

distribution. It is an annual, erect herb with rufous hairs on whole plant body and

paripinnate leaves with 4-5 pairs of leaflets. Leaves stipulate, 6 – 9 cm long, rachis

grooved, with a stalked gland between each pair of leaflets. Flowers are yellow,

polypetalous with five petals and with seven fertile stamens. The pods are small,

clustered (5-8 pods in each cluster), 4-10 seeded, sub-compressed, thick on both the

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sutures, septate. Flowering period is between August to November (Plate III). But if

plants get water, they keep on flowering throughout the year. This weed grows

luxuriantly if it gets full sunlight and high soil moisture. It poorly grows under low

soil moisture conditions (Wakte and Nadaf, 2007). It has been reported for the first

time from Pune in 1998, (M.S.) by Almeida. It has also been recorded along the

highways, roads, railway tracks, barren lands and sometimes even in agricultural

lands.

The another selected invasive weed, Synedrella nodiflora (L.) Gaertn. is a

member of Asteraceae having origin in tropical America. It is an erect, dichotomously

branched annual herb with stems and branches terete, glabrous, serrate, 3- nerved

leaves. Inflorescence is made up of heterogamous heads with 1-2 involucral bracts.

The heads are with 1 – 2 seriate, fertile ray florets. Achenes of the ray and disc florets

differ in their morphology (dimorphic). The ray florets have dorsally compressed

achenes, with lacerate and spinous wings, while that of disc florets are erect, stout

and with spines longer than achenes (Plate IV). The flowering period is between

August to January. Wakte and Nadaf (2007) reported that, Synedrella nodiflora needs

deep shade, high soil moisture levels and soil depth for its survival. Asteraceae plants

with their leachates, extracts and residues of different plant parts are well known for

their allelopathic activity (Chon et al. 2003, Chon and Kim 2003, Fujii et al. 2003,

Barney et al. 2005). They are reported to contain allelochemicals like phenolic acids

and terpenoids (Chon et al. 2003).

The distribution of this species has been observed all over the world. In

Maharashtra it has been reported from Mumbai, Konkan, Mahabaleshwar and

Vidarbha regions. The weed is not very common in and around Pune city, but it has

wide occurrence in the campus of Pune University.

About the selected test crops

Both the selected invasive weeds are dominant in the campus of Pune

university, hence to check their allelopathic potential, mungbean and mustard were

selected as test crops. The brief information about them is given below.

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Mungbean (Vigna radiata L.)

This member of Fabaceae, reaches 50 to 60 cm in height, with trifoliate leaves,

flowers pale yellow, borne in clusters. Mature pods yellowish-brown, contain 10 to 15

green seeds. The seeds of mungbean var. ‘Vaibhav’ sown in February, flowers within

45 to 50 days. It is fairly adapted to dry conditions. This short duration, nitrogen

fixing, sensitive crop, is usually used for screening allelopathic potential.

Mustard (Brassica juncea L.)

This oil seed crop belongs to family Brassicaceae, and grows about 40 – 50

cm tall with lyrately lobed leaves, yellow flowers in racemes, the siliqua fruits

appearing like legumes. The crop is of winter season and flowers within 40 – 50 days

after sowing.

These entire phytosociological, morphological, reproductive, physiological

and biochemical studies were focused and pertaining to herbaceous, dominant,

invasive and few native weeds for investigating their allelopathic potential and

invasion in the campus of Pune University with the following objectives:

1) To understand the luxuriant growth and dominance of Cassia uniflora and

Synedrella nodiflora through ecophysiological investigations.

2) Analysis of allelochemicals to know their allelopathic influence.

3) To understand the allelopathic potential of Cassia and Synedrella, through

bioassays on test crops like mungbean and mustard.

4) To evaluate the allelopathic potential of leachates, extracts and residues of

Cassia and Synedrella through foliar and soil applications.

5) To screen the cytotoxicity, antimicrobial and larvicidal activities for exploring

the mechanism of action and possible sustainable use in agriculture.

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B. Review of literature

To have the information about the previous work done on allelopathy in

general, its role in different fields of agriculture and botany, different types of

interactions such as weed – weed, weed – crop, the impact of leachates, extracts and

residues on recipient plants, allelochemicals existing in different donor plants, their

chemical structures, mode of release of these ecochemicals in the environment, their

accumulation, mechanism of action, their effect on seed germination, seedling growth,

mineral nutrition, microbial activity in the soil etc. a review of literature is given in

nutshell.

Studies on allelopathy were made thousands of years before the term was

coined by Molisch (1937). The term allelopathy is derived from two Latin words

Allelon means each other and pathos means to suffer. He, for the first time studied the

effect of numerous plant species and their plant parts viz.- roots, shoots, leaves,

flowers, fruits, leachates, extracts and residues on seed germination, seedling growth

and maturity of crops. Later on many scientists at different corners of the world,

contributed to this field by carrying out the research on various aspects of allelopathy

(Proskurnin et al. 2003). At present the research on allelopathy is being carried out in

more than 85 countries. In India, the research in this field took a great speed after

1950. Presently the allelopathy research work is mainly focused on identification of

allelochemicals, their mode of action and ecological significance.

According to Fujii et al. (2002) allelopathy now refers to any process

involving secondary metabolites produced by plants, microorganisms, viruses and

fungi, that influence the growth and development of agricultural and biological

systems. The allelopathy workers like Bhatt and Chauhan (2000), Singh and

NarsingRao (2003) and Leather and Einhellig (2005) also claimed that secondary

metabolites produced by donor plants, when released into environment, play a key

role in ecology and physiology of recipient plants. They further advocated that the

released allelochemicals as well as the phytochemicals present in the leachates /

extracts have stimulatory or inhibitory influence on seed germination, seedling growth

and yield of recipient plants.

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Today this subject has come into lime-light because of its multidisciplinary

nature, which covers agriculture, biological sciences, biochemistry, physiology,

biotechnology and even genetic engineering. Vilai-Santisopasri (2003) studied the

allelopathic effects of Eupatorium adenophorum Spreng. on growth of some crops

and weeds. Hierro and Callaway (2003) had investigated in detail the invasion of

exotic plants and their role in allelopathy. Many workers like Rice (1979), Gill and

Sandhu (1996), Pawar and Chavan (1999), Chou (1999), Wang et al. (2001), Cheema

et al. (2002) had great contribution in allelopathy through their basic research.

Recently, many researchers like Narwal et al. (2003a, b), Podolska et al. (2003),

Navaz et al. (2003), Batish et al. (2003), Singh and Singh (2003) and Azania et al.

(2003) have introduced multidisciplinary approach in allelopathy.

The allelopathic impact of invasive weeds on seed germination, seedling

growth, growth parameters like plant height, number of leaves per plant, leaf area,

yield contributing parameters like number of flowers and fruits per plant, weight of

fruit and grains etc in different crops had been studied in detail by Rice (1979), Patil

and Hegde (1988), Devi et al. (1997), Kulvinder et al. (1999), Bhalerao et al. (2000a,

b), Wang et al. (2001), Kong and Hu (2001), Lin et al. (2002), Bhalerao (2003)

Jadhav (2006), Hase (2008) and Vaidya (2009). The brief review of literature on

major aspects of allelopathy is summarized below:

Invasion success of weeds

Number of researchers like Carlton (1996), Kong et al. (2000), Mack et al.

(2000), Richardson et al. (2000), Rejmanek (2000), Sharma et al. (2005b) have

thrown light on the success of invasive alien weeds outside their native boundary and

probable causes of this. Studies have been carried out with respect to changing flora

in Indian continent due to the threat of invaded species by Nayar (1977). Bryson and

Carter (2004) indicated that biological processes and specific characteristics of

invasive weeds are important factors in their introduction, spread, and establishment,

that threatens the ecosystems, habitats, or species with economic/ environmental

harm (GISP, 2001). Kruse et al. (2000) and Foy and Inderjit (2000) advocated that

detailed investigations on the ecological, physiological and molecular aspects of

invasive weeds’ allelopathy should be conducted in order to understand community

structure and declining phytodiversity.

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Phytosociological studies and GIS mapping

The phytosociological associations, interactions and their implications in

different ecosystems were investigated in detail by Chou and Hou (1981), Chou and

Kuo (1986), Thelen et al. (2005). They have studied the allelopathic impact of

invasive weeds on native phytodiversity, frequency, abundance and vegetational

changes. According to Yu et al. (2004), Ghayal et al. (2007a) the dominance of

Eupatorium, C. uniflora, and Synedrella in different ecosystems was due to their

allelopathic influence. For understanding the distribution of weeds and their

containment, interference with the growth of associated plant species and their

ecological impact, GIS mapping was done for an aquatic, invasive weed species

Limnocharis flava (L.) Bu. by Abhilash et al. (2008).

Physiological and biochemical characterization of dominant invasive and co-

occurring weed species

To understand the plant-plant interactions and allelopathic influence of

invasive weeds on their natives, the physiological, biochemical and enzymological

studies are essential. Similarly the investigations on leachates, extracts and residues

and their impact on recipient plants are of paramount importance. Several workers

like Tyagi et al. (1995), Tripathi et al. (2000), Bhalerao (2003), Guha et al. (2003),

Pawar (2004), Dim et al. (2004), Castells et al. (2005), Jadhav (2006) and Ghayal et

al. (2009), Vaidya (2009) have investigated photosynthetic pigments, photosynthetic

rate, carbohydrates, proteins, phenols, proline, glycine betaine, mineral constituents

and activities of various types of antioxidant enzymes in different alien and native

weeds. Such types of investigations have given explanations for their luxuriant

growth, high reproductive abilities and tolerance to biotic and abiotic stresses. The

success of invasion lies in their morpho-physiological superiority.

Photosynthetic rate and DMAR studies

According to Pattison et al. (1998) successful invasive species should have

elite morphological and physiological traits, which could increase photon capture and

light utilization efficiency. Invasiveness of alien species could be affected by

photosynthetic efficiency, (Durand and Goldstein 2001). They further claimed that

invasive species have higher ability to capture energy at the minimum cost, which

20


allows more resources for their growth, physiology and reproduction. But

unfortunately this has remained obscure due to paucity of experimental support. The

light capturing mechanism, photon-saturated maxima (Smith and Knapp 2001, Ewe

and Sternberg 2003), and specific leaf area (Durand and Goldstein 2001, Smith and

Knapp 2001, McDowell 2002) were studied in different invasive weed species. The

high efficiency in solar energy harvesting, noted in different invasive weeds enabled

them to become dominant over co-occurring species in different ecosystems.

Photosynthetic rate and DMAR are the key processes in success of invasion by exotic

weeds.

Allelochemicals in invasive and native weed species

Isolation, identification and characterization of allelochemicals present in

roots, stems, leaves, flowers, fruits, seeds, bark, residues, litter, dried leaves (trash)

and their leachates, extracts and residues have a pivotal role in allelopathy research,

without which any predictions, possibilities, hypothesis and explanations are not

possible. Asteraceae plants with their leachates, extracts and residues of different

plant parts are well known for their allelopathic activity (Chon et al. 2003, Chon and

Kim 2003, Fujii et al. 2003, Barney et al. 2005), because of their allelochemicals like

phenolic acids and terpenoids (Chon et al. 2003).

Many researchers like Saharia and Seshari (1980), Ansari et al. (1981), Joshi et al. (1981), Ghosh et al. (1981), Ansari et al. (1983), Sotheeswaran (1989), Ding et al. (1999), Kong et al. (1999), Bras et al. (2001), Sukul and Chaudhuri (2001), Okhunde (2002), Batish et al. (2002), Patil et al. (2003), Kong et al. (2004), Hoque et al. (2003), Devaranavadgi et al. (2004), Mara et al. (2005), El-Khawas and Shehata (2005), Mundhada and Takte (2005), Jadhav (2006), Al-Wakeel et al. (2007), Ghayal et al. (2007a, b, c) and Li et al. (2009) have given prime importance for identification of allelochemicals, ecochemicals, novel bioactive compounds which are the secondary metabolites existing in their leachates, extracts and residues. They have characterized diverse groups of allelochemicals like terpenoids, flavonoids, phenolic compounds and essential oils existing in the invasive and native weeds.

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Allelopathic potential of weeds

Seed germination bioassay

The most common and widely accepted technique in allelopathy to determine the inhibitory or stimulatory nature of different allelochemicals existing in different parts of invasive and native weeds, is seed germination bioassay studies, which in fact are the first stages in allelopathy research (Inderjit and Callaway 2003). Since the inception of allelopathy, several workers have studied this aspect in detail and concluded that the positive or negative influence on seed germination, seedling growth, fresh and dry biomass, vigour index was mainly due to allelochemicals present in the donor plants. The seed germination bioassay helps for the rapid and easy screening of allelopathic potential of the invasive and native weeds.

Many allelopathy researchers like Sukul and Chaudhuri (2001), Patil et al. (2003), Hoque et al. (2003), Devaranavadgi et al. (2004), Jadhav (2006), Al-Wakeel et al. (2007), Ghayal et al. (2007a, b), Travlos and Paspatis (2008) and Li et al. (2009) have studied the allelopathic influence of leachates, extracts and residues on seed germination, seedling growth as well as physiology of seedlings in different crops.

Physiology, biochemistry and enzymology of test plants treated with leachates, extracts and residues

The impact of foliar applications of leachates, extracts and soil applications of residues on the test plants are expressed in terms of phenotypic expressions, which are the manifestations of their metabolic status. The analyses of such changes induced in physiological parameters such as organic and inorganic constituents, macromolecular contents and enzymatic activities, provide the meaningful information on the positive or negative influence of the leachates, extracts and residues of invasive and native weeds.

The research workers like El-Khatib and Hegazy (1999), Venkataraju (1999), Padhy et al. (2000), Vidyasagar and Rajasab (2001), Singh and Singh (2003), Pawar (2004), Bajaj et al. (2005), Jadhav (2006), Al-Wakeel et al. (2007) and Garg et al. (2007), Turker et al. (2008) carried out similar physiological, biochemical and enzymological investigations in different crops treated with leachates, extracts and residues of different allelopathic plants.

Many allelopathy researchers like Al-Khatib and Abd-Elaah (1999), Duary (2002), He and Ding (2004), Mahmood et al.(2004), Bajwa (2005a, b), Ruan et al.

22


(2005), Bhakat et al. (2006), Choudhary and Bhansali (2006), Sarika and Rao (2006), Thapar and Singh (2006a, b), Cruz-ortega et al. (2002, 2007, 2008) and Vaidya (2009) had mainly concentrated their research on the changes caused by the treatments of leachates and extracts on chlorophylls, carotenoids, activity of PSI, PSII, photosynthetic rate, photosynthetic enzymes, reducing and non reducing sugars, starch, free amino acids, proteins, phenols, proline, glycine betaine, mineral constituents, their uptake and distribution, nitrogen metabolism, nitrate reductase activity and antioxidant enzymes. However, the survey of literature has clearly indicated the scarcity of such research.

Growth parameters in treated plants

Several allelopathy researchers have investigated the favorable and adverse

allelopathic impact on growth parameters like plant height, number of branches,

number of leaves and leaf area in the plants treated with leachates, extracts and

residues of allelopathic plant species. All such findings are summarized below.

Plant height

The available literature on this aspect revealed that the workers like Sukul and

Chaudhuri (1995, 2001), Nie et al. (2003a, b and 2004b), Prasad (2004), Guo et al.

(2004), Siddiqui et al. (2004), El-Khatib et al. (2004b), Ercisli and Turkkal (2005),

Abeyeskera et al. (2005), Thapar and Singh (2006a, b) and Garg et al. (2007) have

recorded the negative impact at higher concentrations of leachates and extracts while

positive impact at lower concentrations on plant height.

Number of branches, leaves per plant and leaf area

Like plant height, the number of branches and leaves per plant as well as leaf

area are affected positively or negatively, depending on the concentrations of

leachates and extracts applied to the test plants. Leaf being the main assimilatory

organ, favorable or adverse impact on its number and area will directly affect

photosynthesis and productivity. Because of this central role has been assigned to

above mentioned parameters by Cassini et al. (1999), Yamada et al. (2002), Cheema

et al. (2002), Narwal et al. (2003), Dias et al. (2004), Ambika and Smitha (2005),

Ercisli et al. (2005), Jadhav (2006), Thapar and Singh (2006a, b), Garg et al. (2007)

and Vaidya (2009).

23


Yield contributing parameters, yield and its quality in treated test plants

Since the inception of agriculture farmers have concentrated on maximization

of yield, either sustainably or by over exploiting the natural resources. Hence, the

allelopathy researchers have duly focused on positive or negative influence of

leachates, extracts and residues of weeds on various yield parameters and quality of

yield.

The influence of leachates, extracts and residues or litter of different

allelopathic plants when applied as foliar spray or through soil to the crop plants,

there was significant increase or reduction in yield, depending on the concentrations

used. The favourable or adverse impact of lower and higher concentrations was

reported on various yield attributes such as number of flowers and fruits per plant,

number of pods/ ear heads per plant, total yield per plant, weight of 1000 grains and

dry biomass per plant (Tripathi et al. 2000, Bayala et al. 2003, Jadhav 2006, Misra

2006, Garg et al. 2007).

The quality aspects of the agricultural produce like nutrients, vitamins,

storage, post harvest life, consumers’ acceptability, market quality etc. are also

affected by leachates, extracts and residues of allelopathic plants. But in fact, very few

attempts have been made by allelopathy researchers on this aspect. Manikandan

(1998), El-Khatib and Hagazy (1999), Zhu et al. (1999), Tripathy et al. (1999), Al-

Khatib and Abd-Elaah (1999), Tripathi et al. (2000) had worked on these aspects in

different crops.

The allelochemicals are known to induce the changes even at molecular level

in recipient plants. Many workers like El-Khawas and Shehata (2005), Jadhav (2006)

and Sarika and Rao (2006) have worked on these aspects.

Sustainable utilization of weed biomass in agriculture

Many researchers have proposed the probable use of weed biomass for

herbicides, antimicrobial compounds and insecticides. Bhowmik and Inderjit (2003)

claimed that the techniques of allelopathy can be employed in weed and pest

management. Batish (2002) reported that the invasive weed Parthenium has a

significant allelopathic potential suppressing the growth of several native weeds.

24


Vyuyan (2002) and Duke et al. (2002) clearly indicated the use of allelochemicals as

new herbicides and agrochemicals. Bais et al. (2002) recorded the antimicrobial

activity of some invasive weeds. Saxena and Pandey (2001) proposed that different

types of allelochemicals (secondary metabolites) may act as the ecofriendly

agrochemicals for the coming millennium. They considered that the different types of

allelochemicals will be useful as fungicides, pesticides and herbicides.

Cytotoxic and larvicidal effects

It has also been observed that the leachates, extracts and residues of dominant

weeds (invasive and native) affect cell division and cause negative impact on the

growth of plants in their vicinity (Einhellig 1996). Phenolic acids from different

allelopathic plants cause cytotoxicity (Einhellig 2004). Pires (2001) noted the

reduction of mitotic index and enhanced peroxidase activity in maize roots treated

with Leucaena extracts. Hayet et al. (2008) also reported cytotoxic activities of

Conyza canadensis. Weaver et al. (1991) Hostettmann and Marston (1995), Babu and

Murugan (1998), Venketachalam and Jebasan (2001a), Md. Islam et al. (2003),

Siddiqui et al. (2003) Chaubal et al. (2005), Cantrell et al. (2005) have studied the

effect of different plants against mosquitoes and their larvae.

The complete review of research done on allelopathy revealed that there is

paucity of work on physiological, biochemical and enzymological changes induced in

recipient plants, as influenced by extracts, leachates or residues of different invasive

and native weeds. The studies on characterization of allelochemicals in these weeds

have not been seriously attempted. Hence the present study was undertaken to

characterize the allelopathic potential and to identify the various allelochemicals in

dominant invasive weeds from the Pune University campus. An attempt is also made

to screen the physiological, biochemical and enzymological changes induced in test

crops. The cytotoxic, antimicrobial as well as insecticidal activities of these weeds by

using their leachates and extracts were also investigated.

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