PLANTS ARE THE POSSIBLE SOURCE OF ALLELOCHEMICALS THAT CAN BE USEFUL IN PROMOTING SUSTAINABLE AGRICULTURE

Abstract

Plants are sources of diverse classes of natural products, some of which have biological activity such as phytotoxins, nematocidal, antimicrobial, phytoalexins, provide nutrition to soil microbes and are allelopathic. In fact the chemical compounds that are released from plant organs (i.e. leaves, stem, roots, flowers and seeds), have an effect on the environment and are known as allelochemicals. These allelochemicals comprises of low molecular weight compounds (such as sugars, inorganic ions, vitamins, nucleotides, amino acids and phenolics), as well as high molecular weight substances (polysaccharides and enzymes and other proteins). Furthermore, the allelochemicals from the source plant can enter the environment through leaching, volatilisation, root exudation and seed-coat exudation in order to protect themselves from the threat around them. The aim of this study is to give a picture of different classes of allelochemicals and their properties. Besides, this review will provide an insight regarding the utilisation of the compounds of interest as a growth regulator, antimicrobial, nematicidal, and/or natural herbicide. Certainly, this paper will help in promoting green agriculture and controlling pollution caused by synthetic chemicals.

Full text

© by PSP Volume 28 – No. 2A/2019 pages 1040-1049 Fresenius Environmental Bulletin
1040
PLANTS ARE THE POSSIBLE SOURCE OF
ALLELOCHEMICALS THAT CAN BE USEFUL IN
PROMOTING SUSTAINABLE AGRICULTURE
Amjad Iqbal1,2,*, Farooq Shah2, Muhammad Hamayun3, Zafar Hayat Khan2, Badshah Islam2,
Gauhar Rehman4, Zia Ullah Khan2, Sadiq Shah2, Anwar Hussain3, Yousaf Jamal5
1Institute of Molecular Plant Sciences, The University of Edinburgh, Mayfield road, Edinburgh, EH9 3JH, United Kingdom.
2Department of Agriculture, Abdul Wali Khan University Mardan-Pakistan.
3 Department of Botany, Abdul Wali Khan University Mardan-Pakistan.
4 Department of Zoology, Abdul Wali Khan University Mardan-Pakistan.
5Department of Agriculture, University of Swabi-Pakistan.
ABSTRACT
Plants are sources of diverse classes of natural
products, some of which have biological activity
such as phytotoxins, nematocidal, antimicrobial,
phytoalexins, provide nutrition to soil microbes and
are allelopathic. In fact the chemical compounds
that are released from plant organs (i.e. leaves,
stem, roots, flowers and seeds), have an effect on
the environment and are known as allelochemicals.
These allelochemicals comprises of low molecular
weight compounds (such as sugars, inorganic ions,
vitamins, nucleotides, amino acids and phenolics),
as well as high molecular weight substances
(polysaccharides and enzymes and other proteins).
Furthermore, the allelochemicals from the source
plant can enter the environment through leaching,
volatilisation, root exudation and seed-coat
exudation in order to protect themselves from the
threat around them. The aim of this study is to give
a picture of different classes of allelochemicals and
their properties. Besides, this review will provide
an insight regarding the utilisation of the
compounds of interest as a growth regulator,
antimicrobial, nematicidal, and/or natural herbicide.
Certainly, this paper will help in promoting green
agriculture and controlling pollution caused by
synthetic chemicals.
KEYWORDS:
Allelochemicals, Bioactive compounds, Bio-herbicides,
Nematocides, Phytotoxic, Fungicides
INTRODUCTION
Plants can serve as a reserviour for diverse
classes of natural products,some of them have
biological activity, including nematocidal,
phytotoxic, antimicrobial, phytoalexins, signalling
[1-8], and provide nutrition to soil microbes [9-14].
The exudates from various plant organs contain low
molecular weight compounds (such as sugars,
inorganic ions, vitamins, nucleotides, amino acids
and phenolics) [6, 15-20], higher molecular weight
substances (polysaccharides and enzymes and other
proteins) and root border cells (RBCs) [4, 21-25].
Many of the compounds that exude from
various plant organs e.g. seeds, stems, leaves,
flowers and roots have a positive or negative effect
on environment [26-31]. For example, root exudate
was observed to contribute to the complex set of
chemical, physical and biological interactions in the
rhizosphere, including root–root, root–nematode,
and root–microbe interactions [12, 32-36].
Plant root vs plant root, plant root vs nematode
and plant root vs microbial interactions can either
be positive or negative [37]. For example when root
exudate provide nutrition to the beneficial
microorganisms then the interactions are regarded
as positive while root exudate containing
nematocides or antimicrobial agent against
nematodes or microorganism the interactions are
said to be negative. In short, plant exudates consist
of complex mixtures of large and small molecules,
which might have an effect on neighbouring
organisms.
Allelopathy. Allelopathy, which means the
effect of one plant species on the germination,
growth and development of other plant species, has
been known for over 1000 years. The allelopathic
effect of chickpea (Cicer arietinum) and barley
(Hordeum vulgare) on weeds and other plant
species were known even before 300 BC (Rice,
1984).
The word allelopathy was first introduced by
Molish in 1937 and is derived from Greek words
allelon ‘of each other’ and pathos ‘to suffer’, which
means the injurious effect of one species upon the
other species. In 1996 allelopathy was defined by
the International Allelopathy Society as “Any
process involving secondary metabolites produced
by plants, micro-organisms, viruses, and fungi that
influence the growth and development of

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agricultural and biological systems (excluding
animals)” [38].
The secondary metabolites that are released by
plant organs such as roots, rhizomes, leaves, stems
and seeds into the environment and that affect other
plants in the rhizosphere are known as
allelochemicals (Fig. 1). These allelochemicals
might enter the environment through different
routes like leaching, volatilisation, root exudation,
seed-coat exudation after imbibition and
decomposition of different parts of the plant [26,
39-42]. When susceptible plant species are exposed
to allelochemicals, germination might be inhibited
and if they germinate they might show abnormal
growth and development. The most visible effects
observed are retarded germination, short or no
roots, lack of root hairs, abnormally long or short
shoots, swollen seeds and low reproductive ability
[43].
FIGURE 1
Various routes through which allelopathic
compounds interact with the environment.
Leaf extracts of both Eucalyptus camadulensis
and Juglans regia and seed extracts of Onobrychis
sativa contain allelopatheic compounds that can
inhibit the germination and seedling growth of
other plant species [14]. Some of the putative
allelochemicals (cis-xanthoxin and trans-xanthoxin)
from the leaf extracts of Pueraria thumbergiana
caused growth inhibition of L. sativum roots [44].
Extracts from the flowers of Chrysanthemum
cinerariefolium contain a biologically active
compound known as pyrethrin with herbicidal
effect [28]. Pisatin 1 is also a good example of an
allelopathic compound found in the extracts of pea
stem residues that effected the growth of L. sativum
[45].
Allelopathy vs competition. Differentiation
between allelopathy and competitionis very
important. In allelopathy a plant exudes into the
environment a compound or compounds [46, 47],
which affect the rhizosphere, while in competition a
plant has the ability to remove or reduce the
existing reserves (e.g. water and nutrients in the
soil, and light from sun) in the environment [48].
In past an attempt was made to differentiate
between allelopathy and competition [49]. A boreal
dwarf shrub Empetrum hermaphroditum was used
as donor species and Scots pine (Pinus sylvestris) as
receiver species. The seedlings of P. sylvestris were
grown for 27 months among E. hermaphroditum.
The different treatments used were: (a) P. sylvestris
were grown in PVC(polyvinyl chloride) transparent
plastic tubes (root-exclusion tubes), which
minimized the effect of resource competition by
roots, (b) activated carbon was used to adsorb the
allelochemicals that were released by the E.
hermaphroditum, (c) PVC tubes and activated
carbon was used to minimize the effect of both
resource competition and allelopathic effect of E.
hermaphroditum, and (d) seeds of P. sylvestris were
grown in untreated E. hermaphroditum vegetation
in order to examine the consequences of both
allelopathy and resource competition on the growth
inhibition and weight gain of the P. sylvestris
seedlings.
After 27 months of plant growth, shoot weight
and length of the P. sylvestris seedlings was studied
[49]. The seedlings from treatments a, b, and d had
shorter shoots and lower dry weight than those from
c. The effect of root competition was higher than
that of toxic compounds because seedlings from
root-exclusion tubes with no activated carbon (a)
were affected less than seedlings from the treatment
with activated carbon to adsorb the allelochemicals
but without reducing the competition (b). The
results of the experiment led to conclude that both
competition (below-ground) and allelopathy has
caused inhibition of shoot elongation and failure to
gain weight in P. sylvestris seedlings.
Wardle et al. [50] have also selected six
species of grass: Dactylis glomerata (cocksfoot),
Phalaris aquatica (phalaris), Bromus wildenowii
(prairie grass), Lolium perenne (perennial ryegrass),
Festuca arundinacea (tall fescue) and Holcus
lanatus (Yorkshire fog) as donor species and
Carduus nutans or nodding thistle (a major weed in
many temperate regions) as an acceptor species.
The collected root and shoot leachates from the
donor species were analysed for their nutrient
content (i.e. ammonium, phosphate and nitrate) and
were tested for their effect on the germination and
growth of C. nutans. The collected root leachates
from the six grass species had no significant effect
on the germination and growth of C. nutans.
However, both hypocotyl and root growth of the C.
nutans were significantly reduced by the shoot
leachates of P. aquatica, L. perenne, H. lanata and
F. arundiancea. As the nutrient concentration in
both root and shoot leachates of the six species was
not significantly different, it was concluded that the
differential effect was purely due to allelochemicals
present in the shoots of the four donor species and

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not to competition.
Factors interacting with allelochemicals in
soil. Many factors interact with allelochemicals
such as soil pH, organic matter content, nutrient and
moisture content [51]. An allelochemical can either
retain or lose its effectiveness at a given
concentration according to the availability or non-
availability of more readily available carbon
sources to soil microbes. Soil microbes may
preferentially metabolize readily available carbon
sources other than organic allelopathic compounds
and thus leave the soil relatively enriched in
allelochemicals, which are available for the uptake
by plant roots [52-54]. The concentration of
released allelochemicals in the rhizosphere also
depends upon the age of the allelopathic plant. A
young plant releases more allelochemicals into
surroundings, which might help it in establishing
themself among well-established plants [55].
The transformation of allelochemicals in the
soil can result in more or less toxic compounds.
Some allelochemicals are labile while others such
as some alkaloids resist changes because of their
anti-microbial activity (Wink et al. 1998). Some of
the transformed products from allelochemicals
become more potent than the original compound:
for example the hydroxamic acids DIMBOA (2,4-
dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and
MBOA (6-methoxy-benzoxazolin-2-one) from
maize and wheat become more active when
transformed into the corresponding derivatives,
DIBOA (2,4-dihydroxyl-1,4-benzoxazin-3-one) and
BOA (2-benzoxazolinone) by removal of
formaldehyde from the parent molecule [56].
Allelopathy also has a close relationship with
plant stresses; because most stressed plants have the
ability to release allelochemicals into the
environment to a greater concentration than normal.
By doing so they are targeting the stress-producing
competitor plants, nematodes or microorganisms
with allelochemicals. Measuring the effects of
allelochemicals and the concentration released
might aid to establish association between
allelopathy and stress [57]. Other factors that are
responsible for the release or effectiveness of
allelochemicals are soil nutrient and water
availability, light, pesticide treatment and diseases
[58]. Actually allelopathy is a complex
phenomenon which can be affected by various
factors including soil pH, moisture, organic matter,
and micro-organisms), age (young vs. old plant) of
donor and receiver plants and climatic factors
(temperature and rain) (Fig. 2). The released
allelochemicals adsorbed on soil solids and further
metabolized biologically (micro-organisms) and
chemically (oxidation or reduction) with time.
Allelochemicals from trees. The study of
compounds that are released by some plants to
affect the environment is of great importance to
understand the mechanism of ecological interaction
[59]. In most studies allelopathy played essential
role in forests development by effecting the
composition of the flora. Juglone is a known
allelopathic compound release by walnut (Juglans
nigra), that can negatively effect the growth and
development of other plants species [60]. Under
natural conditions juglone enters the soil through
various process including root exudation, litter
decay and rain through fall. Since root exudation is
a continuous process, it is assumed that juglone is
FIGURE 2
Factors affecting behaviour and the phytotoxic activity of allelochemicals in soil
Modified from Kobayashi [51].

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continuously added to the soil, affecting the
neighboring plants [61]. Although Davis [60]
observed that plants grown in the vicinity of black
walnut died because of the presence of juglone, the
physiological action of juglone was not well
understood until the study conducted by [62]. This
study showed that juglone at a concentration of 10
μM inhibited the growth of duckweed (Lemna
minor) by decreasing the chlorophyll content and
net photosynthesis rate. The results were further
confirmed by an experiment in which leaf discs of
soya bean (Glycine max) were placed in 10 μM
juglone: the rate of photosynthesis was reduced.
In North America both maize (Zea mays) and
soya bean (G. max) are often planted with black
walnut but had not been studied for the adverse
effect of black walnut until [61]. They studied the
effect of juglone on these crops in detail. Seedlings
of both species (maize and soya) when exposed to
juglone had stunted root and shoot growth, low leaf
photosynthesis, and leaf and root respiration of both
species, but soya was found more sensitive than
maize [61].
Another good example of an allelopathic tree
is tree of heaven (Ailanthus altissima), a native
Chinese tree that was introduced in Europe around
the 18th century. This tree is now widely distributed
in the world and can thrive among a wide variety of
floras, which might be due to its ability to produce
several classes of allelochmicals [63]. The first
investigation on the phytotoxic activity of A.
altissima extract was that of Mergen [64]; it was
confirmed by Voigt and Mergen [65]. In both
studies the authors found that aqueous extracts of
A. altissima were toxic to nearby plants.
A bioassay experiment to study the
allelopathic effect of A. altissima extracts on
germination and subsequent radicle growth was
done by De Feo et al. [59]. The surface-sterilized
seeds of radish (Raphanus sativus), garden cress
(Lepidium sativum) and purslane (Portulaca
oleracea) were incubated in Petri dishes containing
filter paper, impregnated with aqueous extract from
A. altissima or distilled water as a control. The A.
altissima extract inhibited the root growth of radish,
garden cress and purslane by 100, 80 and 72%
respectively. The active compounds were isolated
from the crude extract and identified as terpenoids:
ailanthone, ailanthinone, chaparrine and ailanthinol
B. [59] suggested that the active principle from A.
altissima could be used as a possible natural
herbicide. Phytochemical studies of A. altissima
revealed triterpenoids [66], lipids and fatty acids
[67], phenolics [68] and volatile compounds [69]
are the main allelopathic compounds.
Allelopathic crop species. Wheat. Wheat
(Triticum aestivum), one of the essential staple
food-crops, has been studied extensively for its
allelopathic potential. Aqueous extracts of wheat
residue (dried leaves and stem) and seedlings are
autotoxic [70] and showed an allelopathic effect
against weeds [71]. The inhibitory effect of an
aqueous extract from wheat straw on the
germination and growth of rye grass (Lolium
rigidum) increased interest in separating and
identifying the active compounds [71]. The
phytotoxic compounds identified in extracts were
mainly phenolic acids and benzoxazinone [72].
Rice. Various staple crops have been screened
for their weed suppressive field performances
and/or laboratory allelopathic potential, with rice
being the most thoroughly studied staple crop spe-
cies [73]. Various classes of compounds have been
identified as allelochemicals from rice exudates
including phenolics, fatty acids, benzoxazinoids
and terpenoids. Compounds putatively responsible
for growth inhibition isolated from rice root exu-
dates are the diterpenoid, momilactone B (3,20-
epoxy-3α-hydroxy-9β-primara-7,15-dien-16,6β-
olide); a flavone (5,7,4′-trihydroxy-3′,5′-
dimethoxyflavone); and a cyclohexenone (3-
isopropyl-5-acetoxycyclohex-2-enone) [73]. Gener-
ally, different classes of allelochemicals are distrib-
uted among different tissues of rice seedling with
root tissues containing large quantities of phenolic
acids while momilactone B, the flavone and the
cyclohexenone are abundant in both root and shoot
tissues [74].
Barley. Barley (Hordeum vulgare) is a
smother crop that possesses allelopathic potential to
inhibit weed growth [75]. The secondary
metabolites (alkaloids: gramine and hordenine) that
are released by the barley root are responsible for
the inhibition of Sinapis alba, a weed plant. The
alkaloids from barley root exudates were separated
and quantified by HLPC. Hordenine was a major
component of barley root exudates with a
production rate of 2 μg/plant/day in hydroponic
solution. Purified barley alkaloids were tested for
allelopathy in a laboratory bioassay. Hordenine and
gramine were applied at three different
concentrations (0, 15 and 50 ppm) to surface-
sterilized Sinapis alba seeds on filter paper and
incubated in the dark for three days. Both hordenine
and gramine inhibited radicle growth of Sinapis
alba by increasing both size and number of
vacuoles, disorganizing organelles and damaging
the cell wall. The cell wall from the control
treatment was uniform while cell wall from alkaloid
treatment was irregular (i.e. in some part it was
thinner while in other it was very thick) [76].
Other phytotoxic compounds were also
released by barley including vanillic acid,
chlorogenic acid, p-coumaric acid and ferulic acid.
These phenolic acids significantly inhibited seed
germination and radicle and shoot growth of green
foxtail (Setaria viridis), which is a typical grass

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weed of arable land [77]. The evidence of
morphological changes caused by barley
allelochemicals suggests that the presence and
release of those biologically active metabolites
from barley may contribute to the control of weeds
on arable land [76, 77].
Rye. Natural compounds from rye (Secale ce-
reale) with high allelopathic activity can also be
used as a good source of natural herbicides [78].
The weed- suppressive ability of rye is due to the
presence of the benzoxazinoid, 2,4-dihydroxy-1,4-
benzoxazin-3-one (DIBOA) and its breakdown
product, 2-benzoxazolinone (BOA) [79]. The effec-
tiveness and mode of action of DIBOA and BOA
on cucumber (Cucumis sativus) seedlings was
checked by electron and light microscopy. BOA
inhibited the root growth and reduced the number
of lateral roots by 77-100%. DIBOA also slowed
root growth but had no effect on the number of
lateral roots. Both compounds reduced the regener-
ation of root cap cells, increased cytoplasmic vacu-
olation, reduced ribosome density and number of
mitochondria and interfered with lipid catabolism.
Disturbance in cell ultrastructure revealed that both
DIBOA and BOA reduce root growth as a result of
disrupting lipid metabolism, reducing protein syn-
thesis and reducing transport or secretory capabili-
ties because of underdeveloped dictyosomes [78].
Sorghum. The allelopathic proerties of
sorghum have been studied for decades [80]
because of its inhibitory effect on weed growth by
releasing hydrophobic compounds into the
rhizosphere [81]. The bioactive compounds isolated
from sorghum root exudates mainly consist of a
dihydroquinone that was quickly oxidised to a p-
benzoquinone known as sorgoleone [80].
Sorgoleone when tested at 50 μM against a
wide range of weeds (Abutilon theophrasti, Datura
stramonium, Amaranthus retroflexus, Setaria
viridis, Digitaria sanguinalis and Echinochloa
crusgalli) in a nutrient medium for 10 days caused
growth inhibition. The effect of 10 μM sorgoleone
on weeds in a laboratory bioassay suggested that
sorgoleone is a potential herbicide and contributes
to sorghum allelopathy [81]. Further studies by [82]
showed that the stunted growth of weeds in the
presence of sorgoleone is due to its interference
with photosynthesis.
Sunflower. Sunflower (Helianthus annuus)
has a high level of secondary metabolites that are of
commercial interest. The crude extract of shoots
showed an inhibitory effect on germination of
lettuce (Lactuca sativa) seeds [83]. In both
greenhouse and laboratory experiments the water-
soluble extract from sunflower leaf, stem, flower
and root inhibited seed germination, radicle
elongation, radicle weight increase, hypocotyl
elongation and hypocotyl weight increase of wild
barley (Hordeum spontaneum) [84]. Plant height
and weight of wild barley was also reduced
significantly when it was grown in soil previously
used by sunflower. Incorporation of fresh sunflower
roots and shoots into soil released allelopathic
compounds that inhibited the germination, and the
height and weight increase of barley [84]. [85]
observed similar effects when they treated bambara
groundnut (Vigna subterranea) seeds with
sunflower (shoot and root) extracts on a filter paper
in the dark for 5 d under laboratory conditions.
Both radicle and shoot lengths were negatively
affected by sunflower exudates. The water-soluble
allelochemicals of sunflower could be used in weed
management programmes [84].
Allelochemicals of other plant species. Gar-
lic has been cultivated as a food ingredient and as a
medicine for almost 4000 years. In the last few
decades researches has been done to confirm the
health benefits related to Garlic including microbial
infections and anticancer. Recently it has been ob-
served that most of the biologically active com-
pounds isolated from garlic contain sulphur [86].
The sulphur containing compounds were proved to
be good nematicides and insecticides [87].
Vulpia myros (silver grass), an annually
growing winter grass, is one of the best examples
that were extensively studied for an allelopathic
effect on other plant species. [88] started a series of
experiments, which provided support for the
concept of V. myros allelopathy. In the first set of
experiments, the aqueous extracts of decomposed
plant residue collected at various time-points from
V. myros were applied to wheat seeds. The water-
soluble extract at high concentrations significantly
inhibited germination while at low concentration it
delayed germination of wheat seeds. The residues
also showed an inhibitory effect on coleoptile and
especially on growth [88]. After that successful
experiment the authors tried to purify, quantify and
characterise the compounds of interest by gas
chromatography – MS (GC−MS). The extracts
from V. myros were fractioned by gas
chromatography (GC) through a wie-bore packed
column. The fractions showed bioactivity towards
wheat were collected and were identified by MS
[89]. In pursuit of identification they have
characterised 20 active compounds (Table 1), out of
which syringic acid, dihydroferulic acid, vanillic
acid, p-hydroxybenzenepropanoic acid and succinic
acid were present in much higher amounts than
dihydrocinnamic acid and catechol. The identified
compounds accounted for 0.05% of the dry weight
of V. myros residues.
The identified compounds were then utilised
in bioassays to test for their biological role. Each
individual compound exhibited characteristic
behaviour towards the test plant, wheat. Each of the

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TABLE 1
Dried yield and phytotoxic strength of allelochemicals isolated from ether extract of V. myros plant
residue [modified table from An et al. [89] and An et al. [90]]
Threshold
concentration (ppm)
I50 (ppm)
Chemical name
Dry weight (%)
Coleoptie
Root
Coleoptie
Root
Protocatechuic acid
0.0018
500
250
> 1000
712
Succinic acid
0.0080
500
100
> 1000
631
3,4-Dimethoxyphenol
0.0002
250
250
> 1000
585
Dihydrocaffeic acid
0.0004
250
100
941
576
Syringic acid
0.0140
500
10
932
602
Hydroquinone
0.0002
500
250
924
542
p-Hydroxybenzoic acid
0.0018
250
500
886
463
Vanillic acid
0.0081
500
100
800
373
p-Hydroxyphenyl acetic
acid
0.0008
10
1
886
97
3-(4-Hydroxyphenyl)
propanoic acid
0.0044
1
1
729
458
Gentisic acid
0.0011
1
100
712
441
p-Coumaric acid
0.0005
100
250
678
373
Pyrogallol
0.0007
100
100
729
275
Ferulic acid
0.0008
250
1
534
352
Catechol
0.0002
100
1
644
200
2-Hydroxy-3-phenyl
propanoic acid
0.0017
10
250
483
246
Dihydrocinnamic acid
0.0001
100
1
288
85
Salicylic acid
0.0037
10
100
273
169
Benzoic acid
0.0012
100
100
309
200
Total dry weight of ether extract was 306.1 mg from 100 g of V. myros,wholeplant residue. % dry weight: dry weight of a
compound relative to total dry weight of whole plant residue.
Threshold concentration: the concentration at which a compound started growth inhibition of wheat coleoptiles and roots.
I50 is the concentration of compound at which it caused 50% inhibition of coleoptile and root growth in the test plant.
compounds caused greater reduction of root
elongation than coleoptile elongation.
Dihydrocinnamic acid and catechol had high
allelopathic effects even at very low concentrations
[90, 91].
Plants in the genus Ophiopogon are also
famous for a wide variety of biologically active
compounds; one of the best-studied is Ophiopogon
japonicus commonly known as dwarf lily turf. O.
japonicus was initially known for its medicinal
value but was latterly discovered to be an
allelopathic plant. The methanolic extract from the
roots of O. japonicus reduced root and hypocotyl
growth in lettuce. Salicyclic acid and p-
hydroxybenzoic acid were identified as an
allelochemicals in O. japonicus by NMR ( [92]
Root exudates of buckwheat (Fagopyrum
esculentum) are also a good source of allelopathic
compounds including vanillic acid, gallic acid 4-
hydroxyacetophenone. All three compounds have
inhibitory effects on the growth and development of
other plant species [93]. Vanillic acid from
germinating watermelon seeds has also inhibited
the shoot growth of lettuce and tomato but
stimulated the shoot growth of amaranth and
barnyard grass in a laboratory bioassay [94].
CONCLUSION
In allelopathy one species releases bioactive
secondary metabolites to affect the growth and
development of other species. Various plant species
are capable of releasing biologically potent
compounds that can be actively used as
allelochemicals against weeds, herbs,
microorganisms and nematodes. The compounds
(phenolics, alkaloids, oligosaccharides, etc.) that
are released from various plant organs under
favourable conditions have the ability to contribute
towards a complex set of interactions in the
rhizosphere. For example, terpenoids,
isothiocynates and phenolic compounds from
various crop species can be used as weedicide,
herbicides and against seed borne fungi. Also, the
leaf extracts from Betula pubescens and
polysulphides from garlic can be used as a potential
nematocide against parasitic neamatodes in the
infected soils.

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Received: 14.05.2018
Accepted: 06.12.2018
CORRESPONDING AUTHOR
Amjad Iqbal
Associate Professor, Department of Agriculture,
Garden Campus, Abdul Wali Khan University
Mardan, 23200, Pakistan
e-mail: amjadiqbal147@yahoo.com