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A review of garlic mustard ( Alliaria petiolata , Brassicaceae) as an allelopathic plant

Abstract

Alliaria petiolata is a widespread biennial herb from Eurasia that is one of the most recognizable invasive plants of forests in the eastern United States and southern Canada. After two decades of intensive study on its physiology, ecology, and impacts, this plant has come to be known in both the scientific and gray literature as an allelopathic plant capable of exerting negative, chemically mediated effects on plants and microbes in its environment. A critical review of the literature reveals that there is evidence both supporting and failing to support this assertion, and that conclusions can be affected greatly by the experimental approaches taken, the target species examined, the sources of allelopathic inputs, and environmental factors. The objective of this review is to provide a history of allelopathy research in A. petiolata, describing the various approaches that have been taken and conclusions drawn, and to summarize the current standing of A. petiolata as an allelopathic plant using the most ecologically relevant data on this phenomenon. Finally, we discuss the degree to which allelopathy, versus other mechanisms, may contribute to the invasive success of this plant.
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A review of garlic mustard (Alliaria petiolata, Brassicaceae) as
an allelopathic plant
Author(s): Don Cipollini Kendra Cipollini
Source: The Journal of the Torrey Botanical Society, 143(4):339-348.
Published By: Torrey Botanical Society
DOI: http://dx.doi.org/10.3159/TORREY-D-15-00059
URL: http://www.bioone.org/doi/full/10.3159/TORREY-D-15-00059
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Journal of the Torrey Botanical Society 143(4): 339–348, 2016.
A review of garlic mustard (Alliaria petiolata, Brassicaceae)
as an allelopathic plant
Don Cipollini
1
Department of Biological Sciences, Wright State University, Dayton, OH 45435
Kendra Cipollini
Department of Biology, Wilmington College, Wilmington, OH 45177
Abstract. Alliaria petiolata is a widespread biennial herb from Eurasia that is one of the most recognizable
invasive plants of forests in the eastern United States and southern Canada. After two decades of intensive study on its
physiology, ecology, and impacts, this plant has come to be known in both the scientific and gray literature as an
allelopathic plant capable of exerting negative, chemically mediated effects on plants and microbes in its
environment. A critical review of the literature reveals that there is evidence both supporting and failing to support
this assertion, and that conclusions can be affected greatly by the experimental approaches taken, the target species
examined, the sources of allelopathic inputs, and environmental factors. The objective of this review is to provide a
history of allelopathy research in A. petiolata, describing the various approaches that have been taken and conclusions
drawn, and to summarize the current standing of A. petiolata as an allelopathic plant using the most ecologically
relevant data on this phenomenon. Finally, we discuss the degree to which allelopathy, versus other mechanisms, may
contribute to the invasive success of this plant.
Key words: allelochemicals, bacteria, glucosinolates, invasive plants, mycorrhizae
Alliaria petiolata as a Model Invasive Plant.
Alliaria petiolata (Cavara and Grande, Brassica-
ceae) ‘‘Alliaria’’ is a biennial herb from Eurasia
that was first identified in North America in the
1860s (Nuzzo 1993). Alliaria has since become
known as one of the most notorious invasive plants
of forest understories and edges in the eastern
United States and Canada, but it is also a common,
but less intrusive, plant in its native range (Blossey
et al. 2001). Studies of the ecology of this plant in
North America first arose in the 1970s and many of
the early studies were on the life history and
reproductive ecology of this plant (e.g., Cavers et
al. 1979). As Alliaria was increasingly viewed as a
problematic plant, attention turned to mechanisms
of impact, including its ability to compete for
resources (e.g., Meekins and McCarthy 1999).
Another mechanism of impact, for which Alliaria
has become a ‘‘poster child,’’ is the production of
secondary metabolites that can exert negative
effects on native plants, insects, and microbes
(e.g., Cipollini et al. 2012a), collectively referred
to here as allelopathy. After two decades of
intensive study on this phenomenon, support for
the assertion that allelopathy is an important
invasive mechanism in this plant remains mixed.
In this review, we examine the history of
allelopathy research in Alliaria, highlighting
studies of its direct and indirect effects on plants
and beneficial microbes, and summarize the
current standing of this plant as an allelopathic
plant using the most ecologically relevant data on
this phenomenon.
Allelochemicals From Alliaria petiolata and
Their Direct Effects on Plants. In order for a
plant to exert allelopathic effects, it must produce
compounds with bioactive effects that are capable
of entering the environment around the plant and
persisting long enough to have effects on neigh-
boring organisms. Attesting to the bioactive
properties of its tissues, Alliaria has long been
used as a spice, having been found in 6,000-yr-old
Neolithic cooking pots from northern Europe (Saul
et al. 2013). It was likely originally brought to
North America in the late 1800s for culinary or
medicinal uses, but accidental introductions have
likely occurred as well (Nuzzo 1993). Its utility as
a spice is largely due to the presence of
glucosinolates, a class of compounds characteristic
of the Brassicaceae that provide the pungency to
mustard and other food products from this family
of plants (Drewnowski and Gomez-Carneros
2000). Glucosinolates and their derivatives have
1
Author for correspondence: don.cipollini@wright.
edu
doi: 10.3159/TORREY-D-15-00059
ÓCopyright 2016 by The Torrey Botanical Society
Received for publication September 19, 2015, and in
revised form December 2, 2015; first published August
1, 2016.
339
been long investigated for their weed-suppressive
and antimicrobial activities (Shreiner and Koide
1993). By as early as 1845, the bioactive chemistry
of roots and leaves of Alliaria was being
investigated, revealing that Alliaria produced a
‘‘mustard oil’’ in its roots similar in nature to that
of black mustard, Brassica nigra (Wertheim 1845).
Interestingly, Wertheim indicated that Alliaria was
a common weedy plant of gardens in Germany at
the time. The chemical similarity between Alliaria
and B. nigra is due to the presence of allyl
isothiocyanate (AITC), a volatile compound that is
liberated when the glucosinolate sinigrin is hydro-
lyzed by the enzyme myrosinase. Allyl isothiocy-
anate accounts for nearly half of the volatile
content of fresh Alliaria leaves (Blazevic and
Mastelic 2008). While present in many weedy
mustards (Brassica spp., in particular) that have
invaded open habitats in North America, sinigrin
and AITC appear to be novel in the native North
American flora (Agerbirk et al. 2010, Barto et al.
2010a). Later studies driven primarily by a desire
to understand negative effects of this plant on
some North American native insects have revealed
a suite of other compounds in roots and leaves of
Alliaria, including novel hydroxynitrile gluco-
sides, such as alliarinoside, as well as a variety
of flavonoids, flavones and their glycosidic
derivatives, and cyanide and other volatile prod-
ucts (e.g., Haribal and Renwick 2001, Cipollini
and Gruner 2007, Blazevic and Mastelic 2008,
Frisch et al. 2014).
Despite its use as a spice, some understanding
of its bioactive chemistry, and its recognition as a
weedy plant, no studies of allelochemically
mediated ecological impacts of Alliaria are found
in the literature until the 1990s. The first published
study on allelopathy of Alliaria examined the
effects of aqueous extracts of leaf and root tissue
that were macerated in a vegetable juicer on
germination and seedling growth of four nonnative
commercial plant species (McCarthy and Hanson
1998). Little evidence of allelopathic inhibition of
germination or growth was found, but it was likely
that the vigorous tissue disruption and mixing of
plant enzymes with their substrates resulted in a
loss of bioactive allelochemicals from extracts,
including toxic and volatile degradation products
of glucosinolates that result from the action of
myrosinase on parent glucosinolates. Soon after,
Vaughan and Berhow (1999) reported strong
effects of organic solvent extracts of whole Alliaria
tissues containing allyl isothiocyanate (AITC) and
benzyl isothiocyanate (BzITC), toxic degradation
products of the two major glucosinolates that
Alliaria produces, sinigrin and glucotropaeolin.
Both compounds significantly inhibited radicle
elongation of wheat (Triticum aestivum)and
garden cress (Lepidium sativum).
Effects of either extracts of Alliaria or the
purified compounds it contains have been found to
vary with experimental conditions and target
species. In tests of purified AITC and BzITC and
their parent glucosinolates, Lepidium sativum (a
member of the Brassicaceae) was more susceptible
than Triticum aestivum to the effects of the parent
glucosinolates (Vaughan and Berhow 1999), which
may have been due to the possession of endoge-
nous myrosinase by L. sativum that yielded
bioactive degradation products from the parent
glucosinolates. A subsequent study with L. sativum
revealed that it was more tolerant to aqueous
extracts of dried and ground Alliaria leaves than
was lettuce, but it was more negatively affected
by extracts of Alliaria than by extracts from a
weedy member of its own genus, Lepidium
perfoliatum (Aminidehaghi et al. 2006). Later
studies (Cipollini et al. 2008, Cipollini et al. 2012)
showed that Arabidopsis thaliana (a member of
the Brassicaceae) suffered no negative growth and/
or fitness effects from exposure to aqueous extracts
of fresh Alliaria leaves, although it did from
exposure to extracts of another putatively allelo-
pathic plant, Lonicera maackii. Similarly, germi-
nation of Brassica rapa was reduced by extracts of
fresh leaves of L. maackii, but not of leaves of
Alliaria (Cipollini et al. 2012b). Conversely, Pisula
and Meiners (2010) found that extracts from dried
Alliaria leaves could inhibit germination of the
seeds of radish, Raphanus sativus (a member of
the Brassicaceae) at relatively low concentrations,
but that extracts from dried Lonicera japonica
leaves could not. Finally, in a study using aqueous
extracts of seeds, germination percentage of B.
rapa seeds was unaffected by exposure to Alliaria
seed extracts, but germination was delayed (Bar-
num and Franks 2013). However, exposure of B.
rapa seeds to its own seed extracts both reduced
germination percentage and delayed germination
to a greater degree than exposure to Alliaria
extracts. Only one study has compared the relative
impact of root versus leaf extracts, and found
milder effects of root extracts than leaf extracts on
340 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 143
germination of three target species (Cipollini and
Flint 2013).
One interesting commonality in the studies cited
so far is that many of them used model plants from
the same family as Alliaria as a target for potential
allelopathic effects, which could either enhance or
obscure the appearance of effects. Few of these
plants are particularly relevant to the ecology of
Alliaria in introduced habitats. While still using
extracts, Cipollini and Flint (2013) and Cipollini
and Greenawalt (2016) extended earlier work by
using ecologically relevant species. Leaf extracts
of Alliaria significantly reduced germination of the
native target species Anemone virginiana,Ble-
philia hirsuta and Elymus hystrix, while root
extracts significantly affected A. virginiana, but
not B. hirsuta and E. hystrix (Cipollini and Flint
2013). In Cipollini and Greenawalt (2016),
however, germination of E. hystrix and another
native, Chamaecrista fasciculata were not affected
by leaf extracts of Alliaria. These disparate
findings raise important questions about tissue
handling (dried versus fresh tissues), extract
preparation (maceration versus soaking, solvent
versus aqueous extractions), and source of inputs
(seeds versus leaves versus roots). Each of these
choices can determine the relative ‘‘toxicity’’ of an
extract preparation, but it is unclear in most studies
how closely laboratory-prepared extracts represent
natural allelochemical inputs from live and decay-
ing roots and leaves in the field. In addition,
relatively few extract studies have examined direct
allelopathy with ecologically relevant target spe-
cies, and those that have found that effects can
vary by target species or experimental conditions
(Cipollini and Flint 2013, Cipollini and Green-
awalt 2016). For example, while Cipollini and
Flint (2013) found direct effects of Alliaria leaf
extracts on germination of E. hystrix, a grass native
to invaded habitats, a subsequent study by
Cipollini and Greenawalt (2016) found minimal
effects on the same species. Differences may be
attributable to the use of Alliaria collected from
different sites and during different seasons, as the
bioactive chemistry of Alliaria can vary with
environmental conditions (Cipollini 2002, Hill-
strom and Cipollini 2011, Cipollini and Lieurance
2012, Smith 2015, Smith and Reynolds 2015), as
well as with population (Cipollini et al. 2005,
Lankau et al. 2009, Hillstrom and Cipollini 2011)
and season (Haribal and Renwick 2001). There is
also increasing recognition of pathogenic microbes
that may inhibit growth of Alliaria above- or
belowground, which may in turn reduce allelo-
pathic effects (e.g., Cipollini and Enright 2009).
Due to a number of limitations, extract bioas-
says may have a relatively limited value in
predicting what might happen in field environ-
ments (Inderjit and Nielsen 2003); for example,
allelochemicals rarely enter the environment in the
form of a concentrated fresh leaf extract. However,
because extract studies are often optimized to see
effects, extract studies can reveal a species’
potential to have allelopathic effects. While the
observation of effects in the laboratory does not
mean that effects will be important in the field, the
lack of effects observed in the laboratory likely
means that effects will be unimportant in the field.
Extract bioassays can also more easily allow the
comparison of invasive species to determine
relative potential for phytotoxic effects. For
example, Pisula and Meiners (2010), Cipollini et
al. (2012b), Cipollini and Flint (2013), and
Cipollini and Greenawalt (2016) compared the
effects of 11, 3, 3, and 5 invasive species,
respectively, in this manner. In most of the above
studies, Alliaria generally was one of the strongest
inhibitors, though effects were weaker in Cipollini
and Greenawalt (2016). However, extracts are
usually complex mixtures of plant secondary
compounds, proteins, sugars, mineral nutrients,
and microbes, among other things, which are
variables that are infrequently controlled. Thus,
even when effects are seen in extract studies, it can
be difficult to pinpoint the mechanism of impact.
Evidence for Allelopathic Effects From Soil
Conditioning and Field Studies. To add ecolog-
ical realism, Prati and Bossdorf (2004) introduced
a soil conditioning approach to allelopathy studies
in Alliaria by growing this plant in soil and
subsequently using the soil alone (presumably
enriched with root exudates and likely an altered
microbial community) in pot studies. This ap-
proach does not isolate allelopathic effects that are
due specifically to secondary compounds, but it
better reflects the way in which Alliaria roots
affect the soil as it is growing in nature. It largely
removes the potential allelochemical impact of
leaves, however, which may be important in the
field due to leaf leaching by precipitation and
degradation of senescing leaves. Prati and Boss-
dorf (2004) also used activated carbon in their
study, which had become a popular control in
2016] CIPOLLINI AND CIPOLLINI: ALLELOPATHY IN ALLIARIA PETIOLATA 341
studies of allelopathy due to its ability to absorb
organic compounds from soils and to ameliorate
allelopathic effects (Ridenour and Callaway 2001).
While putative allelochemicals were not measured,
nor were effects mediated by nutrients or microbes
considered, seed germination of two Geum species
was generally lower in soils conditioned by
Alliaria than in unconditioned soils, and it
generally increased with the addition of activated
carbon (activated carbon alone had no direct effect
on seed germination in unconditioned soils). By
using one Geum species from Europe and one from
North America, and Alliaria populations collected
from both continents, this study also added a
biogeographical perspective. In particular, the
North American Geum species appeared to benefit
more from activated carbon treatment overall than
the European species, suggesting the increased
sensitivity to Alliaria in a na¨ıve competitor. In
addition, the European Geum species only re-
sponded positively to activated carbon treatment in
soil conditioned with European Alliaria, suggest-
ing that there were weaker allelopathic effects from
populations of Alliaria found in North America.
Soil conditioning and field studies have indicat-
ed that allelopathic effects of Alliaria can vary
across populations or with the degree of compe-
tition experienced by Alliaria or its neighbors.
Lankau et al. (2009) showed that the allelopathic
effects of soil conditioned by Alliaria growth on
biomass of three tree species generally decreased
with the age of the Alliaria population used to
condition the soil, which correlated with declining
root glucosinolate levels with population age. They
suggested that the costs and benefits of allelo-
chemical production favored the loss of allelo-
pathic potential in older populations of Alliaria
where Alliaria was the dominant species and
mostly competing with itself. While the negative
correlation between Alliaria root glucosinolates
and population age was relatively strong in this
study, correlations between either of these factors
and target tree biomass were weak, suggesting that
soil concentrations would have been important to
measure in order to implicate them in age-
dependent allelopathic effects. Interestingly, the
growth of Platanus occidentalis appeared to
decline in this study in response to increasing
age (and root glucosinolate level) of the Alliaria
populations, but this same tree species did not
respond significantly to the presence of Alliaria in
live soils in another pot study (Lankau 2009), in
which root glucosinolate levels also did not
correlate with tree growth. Barto et al. (2010b)
showed that root and leaf extracts of Alliaria could
reduce seed germination and growth of Impatiens
pallida, an effect that could be ameliorated by
activated carbon. When applied at high doses to
the soil, effects of these extracts also followed a
plant density-dependent pattern of phytotoxicity on
growth of I. pallida in a pot study, in which plants
growing at low densities suffered greater per capita
negative effects than plants growing at higher
densities. However, when applied at concentra-
tions expected in field soils, effects of Alliaria
extracts on I. pallida growth were not distinguish-
able from the effect of resource competition alone
(Barto and Cipollini 2009a). Lankau (2012)
showed that Alliaria root glucosinolate levels were
higher in plants from areas with high competitor
densities, but that a native competitor, Pilea
pumila, appeared to be evolving resistance to
allelopathic effects of Alliaria. These results
concur with results from the first study to suggest
the evolution of resistance of a native plant,
Impatiens capensis, to the presence of Alliaria in
its invasive range, though allelopathy was not
explicitly considered (Cipollini and Hurley 2009).
Such evolution would be expected if allelopathy
by Alliaria has exerted selective pressure on
competing plant populations for a sufficient
amount of time.
In the first study to address allelopathic effects
of Alliaria in the field with the use of activated
carbon as a tool to ameliorate allelopathy, Cipollini
et al. (2008b) demonstrated a beneficial effect of
activated carbon placed in the root zone on growth
of the annual, I. capensis, transplanted near
Alliaria. Lankau (2009) later used activated carbon
to reveal that direct allelopathic effects of Alliaria
on P. occidentalis growth in a pot study were only
apparent in sterilized soils, suggesting that the
microbial community of live soils played an
important role in degrading allelochemicals. It
was also only in sterilized soils where negative
correlations between glucosinolate concentrations
of Alliaria roots and biomass production of P.
occidentalis were observed. However, it is impor-
tant to note that studies using activated carbon as a
manipulative tool must be interpreted with caution
due to potentially confounding effects, including
alterations in nutrient dynamics, that activated
carbon addition may produce (Lau et al. 2008).
Like studies using tissue extracts, most studies
342 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 143
using either soil conditioning approaches or that
were performed in the field and coupled with
activated carbon addition have indicated the
potential for allelopathic effects of Alliaria to be
important. However, these studies were not able to
isolate the mechanism(s) of impact and whether
effects were direct or indirect.
Allelochemical Effects of Alliaria on
Mycorrhizal Fungi. Since the Brassicaceae gen-
erally lack associations with mycorrhizal fungi
(Shreiner and Koide 1993), a great deal of
attention has been given to the ability of Alliaria
to allelopathically inhibit mycorrhizal fungi of
competing plants. This would allow Alliaria to
inhibit mutualistic microbes of its neighbors,
possibly gaining a competitive advantage with no
negative effects on itself, a possibility first raised
by Vaughn and Berhow (1999). A simple labora-
tory study demonstrated that leaf extracts of
Alliaria were able to inhibit spore germination of
Gigaspora rosea, an arbuscular mycorrhizal fungal
(AMF) species (Roberts and Anderson 2001). In
this study, the density of Alliaria plants growing
naturally in the field was also negatively correlated
to mycorrhizal inoculum potential of the soils,
which could have been due to (unmeasured)
increases in allelochemical concentrations in soils,
or simply due to a decline in suitable hosts with
increases in Alliaria density caused by resource
competition.
Stinson et al. (2006) subsequently demonstrated
lowered mycorrhizal infection rates in several
North American tree species exposed to both
aqueous Alliaria extracts and to soils conditioned
by Alliaria, with a resultant indirect negative effect
on tree growth. Wolfe et al. (2008) similarly found
lowered ectomycorrhizal fungal (EMF) infection
rates of pine trees in areas invaded by Alliaria in
the field. Callaway et al. (2008) used a combina-
tion of soil conditioning, leaf extracts, and a range
of naturally co-occurring plant species to show that
allelopathic effects of Alliaria increased with the
extent of mycorrhizal dependence of the target
plant species, and that North American species
generally suffered greater negative effects than
their congeneric European counterparts. Impor-
tantly, this study used the ratio of tissue concen-
trations to soil concentrations of glucosinolates
reported in studies of Brassica green manures
(e.g., Brown and Morra 1997) to estimate field-
relevant soil concentrations of Alliaria allelochem-
icals to utilize in experiments. Using this approach,
and the ability to produce leaf extracts with
different classes of metabolites, AMF from North
American soils lacking an evolutionary history
with Alliaria were shown to be more susceptible to
negative effects of ecologically relevant levels of
Alliaria extracts than AMF from European soils.
Both glucosinolate and flavonoid extracts of
Alliaria leaves were partly responsible for allelo-
pathic effects on AMF spores, and the mixture of
the two worked synergistically to inhibit the
germination of spores present in soils with no
history of Alliaria. This study provided some of
the best support for the novel weapons hypothesis
(Callaway and Ridenour 2004) to date for this or
any other plant, and was the first to implicate
metabolites other than glucosinolates in allelopath-
ic effects of Alliaria.
Koch et al. (2011) showed that extracts of
Alliaria leaves containing both glucosinolates and
flavonoids could inhibit colony growth of Glomus
intraradices, an AMF species, in carrot root
cultures. However, Barto et al. (2010b) found that
if I. pallida plants had established a relationship
with AMF prior to contact with Alliaria extracts,
then vegetative growth was unaffected by exposure
to extracts. This was true even though some
negative effects on AMF colonization could be
seen in glass root-viewing chambers where AMF
colonization could be monitored using epifluo-
rescence microscopy of live roots through time.
This indicates that spore germination and the early
development of mycorrhizal symbioses are critical
stages that may be affected by Alliaria, but that
healthy mycorrhizal relationships may protect
against allelopathic effects of Alliaria. Hale et al.
(2011) found that soil respiration was reduced in
the presence of Alliaria in a pot study, ostensibly
due to a reduction in mycorrhizal community
function. There was a concomitant reduction in
physiological function in Maianthemum racemo-
sum, potentially reflecting the cost of disruption of
mycorrhizal mutualisms. This effect may have
been amplified in this study, however, due to the
direct incorporation of fresh leaves to soils, which
is unlikely to be a common route of entry of
allelochemicals to soils in the field.
Barto et al. (2011) showed that AMF infection
rates of sugar maple growing near Alliaria plants
were reduced more greatly in Ohio than in
Massachusetts, with populations of Alliaria being
presumably younger in Ohio. Arbuscular mycor-
2016] CIPOLLINI AND CIPOLLINI: ALLELOPATHY IN ALLIARIA PETIOLATA 343
rhizal fungal community composition in the
rhizosphere of sugar maple also changed in
response to the presence of Alliaria in this study.
Lankau (2010a) later showed that effects of
Alliaria populations on mycorrhizal fungal rich-
ness and community structure in the rhizosphere of
Quercus rubra were correlated with concentrations
of glucosinolates and alliarinoside in the roots of
Alliaria. Importantly, as Alliaria increasingly
altered mycorrhizal community structure, growth
of Q. rubra and another native tree increasingly
declined. However, correlations between the effect
of Alliaria and population age were not as apparent
as in the 2009 study. While links between
mycorrhizal community structure and native plant
performance may be unclear, alterations in mycor-
rhizal community structure induced by the pres-
ence of Alliaria persisted for at least six years after
its removal from a forested habitat (Lankau et al.
2014).
While some studies have attempted to expose
competing plants or AMF to ecologically realistic
concentrations of putative allelochemicals from
Alliaria through soil conditioning or extract
dilution, only two studies have assessed concen-
trations of putative allelochemicals in soils
affected by Alliaria. Barto and Cipollini (2009b)
were unable to extract glucosinolates from field
soils under Alliaria, but they were able to capture
some potentially bioactive flavonoid derivatives
through biomimetic extraction using polydi-
methylsiloxane (PDMS) tubing, a technique
developed to capture allelochemicals in soils by
Weidenhamer (2005). However, compounds were
only detectable at a few time points during the
season and half-lives of most of these compounds
were on the order of hours when they were
applied to nonsterile field soils. Thus, the
potential impact of these metabolites would vary
seasonally, and they would likely need regular
replenishment in the field to be influential. Frisch
et al. (2014) has shown that most of the bioactive
glucosinolates and alliarinoside in Alliaria leaves
are metabolized by endogenous plant enzymes
(e.g., glucosidases, such as myrosinase) within
hours in leaf homogenates, and many of the
products are volatile. This has implications for the
expected longevity of bioactive compounds in the
environment, as well as implications for the
stability of extracts used in allelopathy studies.
Despite these issues, Cantor et al. (2011) were
able to detect AITC (the volatile derivative of
sinigrin) in field soils at levels that were sufficient
to inhibit spore germination of Glomus clarum,an
AMF species, in laboratory bioassays. This was
important because no studies implicating gluco-
sinolates in allelopathic effects of Alliaria in
either field or pot studies had confirmed the
presence of the parent compounds or their
derivatives in the soil. Nonetheless, the transient
nature of their detection and their degradation
rates indicate that if glucosinolates or other
metabolites are partly responsible for allelopathic
effects of Alliaria,thentheymustgenerallywork
in low concentrations and would be expected to
vary in their contribution to allelopathic effects
throughout the season. To that end, only minor
reductions on fungal hyphal abundance were
noted in Alliaria-invaded plots in the field where
AITC could be detected.
Despite the evidence that Alliaria or its extracts
can have either direct or indirect allelopathic
effects, a number of studies have not found major
allelopathic effects of Alliaria on plants or their
fungal partners. In some cases, the negative effects
of Alliaria seen on some variables in a study have
been emphasized over the lack of effect on others.
For example, Burke (2008) found little effect of
Alliaria presence on either AMF infection rates of
three forest herbs or AMF community structure in
a field study. Barto and Cipollini (2009a) and
Barto et al. (2010b) showed that Alliaria leaf
extracts can have direct negative effects on
germination and growth of I. pallida in the
laboratory. Extracts had no effect on growth or
AMF infection rates of I. pallida, however, if the
plants were colonized by AMF before exposure to
Alliaria extracts (Barto et al. (2010b). Koch et al.
(2011) found little effect of Alliaria-conditioned
soils or extracts on AMF species richness or
community structure in a pot study, despite
showing that Alliaria extracts could reduce AMF
colony growth in culture. Lankau (2010b) showed
that effects of Alliaria on AMF community
composition in the field appear to change with
the age of Alliaria populations. In particular, AMF
species richness declined and community structure
changed when moving from young to medium-
aged populations, but AMF species richness
rebounded in older populations of Alliaria.
Different conclusions about the effect of Alliaria
on mycorrhizae and subsequent indirect effects on
plant competitors could be related to variation in
allelochemical quantities and profiles of Alliaria
344 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 143
populations at different field sites (Lankau et al.
2009, Lankau 2010a). In most studies of the direct
and indirect effects of Alliaria on mycorrhizae, the
relative impact of Alliaria versus other interacting
species has also not been determined. Cipollini and
Greenawalt (2016) found fewer effects of Alliaria
leaf extracts on mycorrhizal infection of E. hystrix
than extracts of four other invasive species.
However, Brouwer, Hale, and Kalisz (2015) found
more negative effects on carbon storage in tissues
of Maianthemum racemosum from treatment by
leaves of Alliaria than by leaves of Hesperis
matronalis, a related nonnative species considered
to be less invasive than Alliaria. They also found
that vital rates improved for this plant in the field
in areas where Alliaria had been removed for
several years. While implicating negative effects
on mycorrhizae, infection rates and community
structure were not examined. So, while the
negative impact of Alliaria on mycorrhizal rela-
tionships has been often invoked, its effects, and
their importance, may be no greater than that of
some other (nonrelated) invasive species. No
studies have directly compared the effect of
Alliaria on mycorrhizae with that of related, but
native, mustard species.
Allelochemical Effects of Alliaria on Bacteria
and Bacterial Communities. Given its nonmy-
corrhizal status, potential impacts of Alliaria on
mycorrhizal fungi have been a worthy target of
attention. In contrast, impacts of Alliaria on soil
bacteria have been much less well studied and can
be difficult to interpret as an allelopathic mecha-
nism, in part because negative impacts of Alliaria
on some beneficial bacteria could harm its own
growth. Nonetheless, a few studies have now
examined impacts of Alliaria on either individual
bacteria or bacterial communities. Burke and Chan
(2010) found that while seasonal differences
existed, bacterial richness, evenness, and diversity
were similar in soils under Alliaria and under two
native forest herbs. Instead, they suggested the
physicochemical properties of the soil, and chang-
es therein across seasons, were more important
than plant identity in determining bacterial com-
munity structure. In contrast, Lankau (2010b)
found that bacterial communities experienced
significant shifts in richness and in the abundance
of species sensitive to its effects after Alliaria
invasion, but they tend to recover to the native
condition in areas where Alliaria has been present
for a long time, including the restoration of
sensitive species.
In a recent study, Portales-Reyes et al. (2015)
examined whether Alliaria extracts or synthetic
AITC and BzITC were capable of disrupting
interactions between a native legume, Amphicar-
paea bracteata, and its mutualistic rhizobia. They
found that BzITC applied at expected field
concentrations reduced rhizobia growth rate in
the laboratory, but its application had no effect on
nodulation in the greenhouse when rhizobia were
grown with their host plants. Allyl isothiocyanate
did not affect either the plants or rhizobia in
isolation, but plants grown with rhizobia in the
presence of AITC showed reduced nodulation.
Despite finding some impacts of synthetic gluco-
sinolate derivatives, they found no effects of
aqueous extracts of fresh leaves of Alliaria on
plant performance or nodulation. In addition, the
amount of biomass accumulated by plants exposed
to AITC (that suffered reduced nodulation) was not
significantly different from that accumulated by
control plants (with typical levels of nodulation).
This study should be commended for being the
first to consider the potential impact of Alliaria on
a native legume and its rhizobia. However, the lack
of effect of Alliaria extracts and lack of the effect
of reduced nodulation on plant growth are likely
more important results than the minor impact of
purified AITC on nodulation that was highlighted.
This finding clearly warrants more research into its
significance.
Placing Allelopathic Effects of Alliaria in
Context. A large number of studies of allelopathy
of Alliaria are limited by the fact that they are
conducted under greenhouse or laboratory condi-
tions, making the ability to extend results to the
field difficult. However, when taken together,
direct and indirect allelopathic effects of Alliaria
on plants and their associated microbial mutual-
isms seem possible under some circumstances,
but several studies have found little apparent
allelochemical impact of Alliaria on either plants
or their microbial partners. When allelopathic
effects have been found in the most ecologically
realistic scenarios, their magnitude depends on
plant density, age and allelopathic potential of the
Alliaria population, the evolutionary history of
the soils and their microbial communities with
Alliaria, and the dependence of target plants on
mutualistic association with microbes. Few stud-
2016] CIPOLLINI AND CIPOLLINI: ALLELOPATHY IN ALLIARIA PETIOLATA 345
ies have followed the response of plants and
microbial communities through the invasion
process of Alliaria, but one study has demon-
strated that changes in Alliaria densities and
responses of the native plant community appeared
to be largely independent (Davis et al. 2014).
Unfortunately, only a few studies have attempted
to place allelochemical-mediated impacts of
Alliaria in the context of other mechanisms of
competition or impact; thus, the relative impor-
tance of allelopathy as a mechanism of invasive
success in Alliaria has been difficult to gauge. In
one study that examined multiple impacts,
Cipollini et al. (2008) showed that adding
activated carbon to the rhizosphere improved
the growth of I. capensis plants that were
transplanted near Alliaria plants (presumably by
ameliorating allelopathic effects), but removing
the aboveground biomass of the competing
Alliaria plants had similar positive impacts on
growth of I. capensis. A recent study by Poon and
Maherali (2015) weighed the relative importance
of allelopathy and resource competition as
mechanisms by which Alliaria may influence
plants and their mycorrhizae. Specifically, they
tested the hypothesis that suppression of mycor-
rhizal relationships by Alliaria,ifitwere
important, would have more severe consequences
under low resource conditions. These authors
used the soil conditioning approach to produce
soils in which Alliaria hadbeenpreviouslygrown
or not. Then, they grew 27 mycorrhizal tree, forb,
and grass species from the introduced range of
Alliaria in those soils, with and without compe-
tition with Alliaria. Previous growth of Alliaria in
the soils reduced nitrogen and phosphorus
availability by .50% and 17%, respectively,
and reduced mycorrhizal colonization of compet-
itor species by .50%. However, competition
with Alliaria suppressed the biomass of 70% of
competing species in control soil, but only 26% of
competing species in soil with a previous history
of Alliaria growth. Interestingly, in an interaction
that has been rarely examined in the history of
allelopathy studies, biomass of Alliaria itself was
reduced by 56% in soils with a history of Alliaria
growth, while the average reduction in biomass in
competitor species was only 15% in soils with a
history of Alliaria growth. Since Alliaria is
nonmycorrhizal, this form of negative plant-soil
feedback was likely mainly due to nutrient
depletion. The authors concluded that, although
mycorrhizal colonization was clearly suppressed
by prior growth of Alliaria in most species in this
study, the negative effect of nutrient depletion by
Alliaria had larger impacts on competing species
than the negative effect of suppressing mycorrhi-
zal colonization. Importantly, this study was also
thefirsttoshowthatgrowthofAlliaria in field
soils clearly has more detrimental effects on itself
than on most other species, despite reducing
mycorrhizal colonization of those species and
depleting soil nutrients available to them.
The study of the direct and indirect allelopathic
effects of Alliaria on plants and beneficial
microbes is approaching its 20th year. A critical
review of the evidence that has accumulated from
laboratory and field studies suggests that this plant
could be allelopathic under some circumstances in
the field, but other mechanisms of impact, such as
competition for nutrients, are perhaps more
important. Studies isolating the role that allelopa-
thy plays in the ecology of this plant in the field are
lacking, and whether allelopathic effects, when
seen, benefit the fitness of garlic mustard is still
unclear. Opportunities exist to utilize variation in
allelochemical production that exists in Alliaria
populations, or changes induced by environmental
conditions, coupled with better chemical capture
and analytical techniques, to more clearly link
allelochemical production by Alliaria to negative
impacts on native communities and to the fitness of
Alliaria itself. However, there is a growing body of
literature on the relative lack of impact of Alliaria
on resident plant or microbial communities,
especially in areas that have been invaded for
some time (Lankau 2010b, Davis et al. 2012,
Davis et al. 2014, Davis et al. 2015). Impacts of
Alliaria, whether due to allelopathy or other
mechanisms, might be observed at the invasion
front of this plant before it negatively impacts its
own success, where soils and competing plants and
microbes are still na¨ıve to this plant, and before
natural controls on its populations take root.
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348 JOURNAL OF THE TORREY BOTANICAL SOCIETY [VOL. 143
... Community changes accompanying invasion by A. petiolata include decreases in forest understory native plant species diversity and reduction in leaf litter arthropod richness [18,19]. Impacts by A. petiolata on neighboring plants may be the result of allelopathic activity by this species, however, the degree of impact from allelopathy may be less important compared to other interactions, such as competition [20]. Multiple introductions from multiple source populations added to a relatively high genetic diversity for A. petiolata [21]. ...
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Increasing costs in the agricultural sectors is nowadays with the use of herbicides on weeds control that need to use non chemical methods to reduce the environmental impact of chemical herbicide, insecticides and weedicide to prevent weed resistance, use of allelochemical natural herbicide for weed control to reduce the costs. In integrated weed management programs allelopathic chemicals as an alternative for weeds control. These chemicals inhibit the weeds growth and as a weapon to be used against these unwanted plants. Allelopathic crops species relationship, genetic diversity is very extreme and genetic control of these compounds to be seems. The main aims of this review paper are to find out the efficient allelopathic nonchemical control of weeds from crops and best way of controlling the noxious weeds with these plants extract.
... A well-studied example of allelopathy is that of the invasive Alliaria petiolata, which was reviewed in [208,209]. A field comparison of A. petiolata patches and control areas revealed that root tip biomass was lower in invaded soils, the effect was most prominent at the immediate neighborhood of the patches, and in conifer-dominated forests [210]. ...
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Chapter
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Garlic mustard (Alliaria petiolata), Amur honeysuckle (Lonicera maackii) and lesser celandine (Ranunculus ficaria) are three invasive species in US Midwestern forests. The comparative allelopathic effects of leaf extracts of these species on germination and reproduction of Arabidopsis thaliana were investigated in a growth room. Highest extract concentrations (0.3 and 0.2 g fresh leaf tissue/mL distilled water) of L. maackii delayed germination in potting soil compared to the control. Extracts of L. maackii also decreased the number of siliques in potting soil compared to the control and to A. petiolata extracts, with extracts of R. ficaria having intermediate effects. In field soil, extracts of L. maackii and R. ficaria significantly decreased the number of siliques compared to extracts of A. petiolata. In a third experiment, effects on germination of three agricultural species (Brassica oleracea, Lactuca sativa and Ocimum basilicum), were studied. Ranunculus ficaria and L. maackii extracts were least harmful to germination, while A. petiolata extracts were most harmful. Germination of L. sativa and O. basilicum was more sensitive to A. petiolata and R. ficaria extracts, while germination of B. oleracea was more sensitive to L. maackii extracts. These results showed differential allelopathic effects of these invasive species, which varied with test species and experimental conditions.
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Allelopathic potential of two weeds of the cruciferae family, namely garlic mustard (Alliaria petiolata (M. Bieb.) Cavara & Grande) and clasping pepperweed (Lepidium perfoliatum L.), on lettuce (Lactuca sativa L.) and garden cress (Lepidium sativum L.) was studied in germination assays applying donor plant extracts. Weed plant parts were dried individually at 60°C and then ground. Each plant's powder was used to produce aqueous extracts of 10% (w/v) concentration. After centrifugation and filtration, the extracts were diluted with double distilled water to concentrations of 2.5 and 5 %, whereby double distilled water was used as control. The experiment was carried out in a factorial design with two levels of weed, two levels of test plant and four levels of extract concentration. Results showed that weed extracts had a similar and significant inhibitory effect on germination percentage of each of test plant with increasing extract concentration. Radicle growth of both of test plants and hypocotyle growth of lettuce also were decreased with increasing extract concentration. However, garden cress hypocotyl growth was decreased only in response to garlic mustard extracts while extracts of the second weed at first caused an increase in hypocotyl growth at concentrations of 2.5 and 5 % and a decrease only at a concentration of 10 %. Overall negative allelopathic effects on lettuce growth parameters were greater than on garden cress.
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The widespread invasion of the nonmycorrhizal biennial plant, Alliaria petiolata in North America is hypothesized to be facilitated by the production of novel biochemical weapons that suppress the growth of mycorrhizal fungi. As a result, A. petiolata is expected to be a strong competitor against plant species that rely on mycorrhizal fungi for nutrient uptake services. If A. petiolata is also a strong competitor for soil resources, it should deplete nutrients to levels lower than can be tolerated by weaker competitors. Because the negative effect of losing the fungal symbiont for mycorrhizal plants is greatest when nutrients are low, the ability of A. petiolata to simultaneously suppress fungi and efficiently take up soil nutrients should further strengthen its competitive ability against mycorrhizal plants. To test this hypothesis, we grew 27 mycorrhizal tree, forb and grass species that are representative of invaded habitats in the absence or presence of competition with A. petiolata in soils that had previously been experimentally planted with the invader or left as a control. A history of A. petiolata in soil reduced plant available forms of nitrogen by >50% and phosphorus by 17% relative to control soil. Average mycorrhizal colonization of competitor species was reduced by >50% in A. petiolata history versus control soil. Contrary to expectations, competition between A. petiolata and other species was stronger in control than history soil. The invader suppressed the biomass of 70% of competitor species in control soil but only 26% of species in history soil. In addition, A. petiolata biomass was reduced by 56% in history versus control soil, whereas the average biomass of competitor species was reduced by 15%. Thus, our results suggest that the negative effect of nutrient depletion on A. petiolata was stronger than the negative effect of suppressing mycorrhizal colonization on competitor species. These findings indicate that the inhibitory potential of A. petiolata on competitor species via mycorrhizal suppression is not enhanced under nutrient limitation.
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QuestionTo what extent are species, including native and non-native species, influencing one another's distribution and abundance in the herb layer of a Minnesota oak woodland?LocationOak woodland succeeding into a more mesic forest, on bluffland of the Mississippi River, east-central Minnesota.Methods We collected plant composition and species cover data in 182 1.0 × 0.5 m quadrats regularly spaced on a 6-ha study grid in the oak woodland. We also recorded slope, slope position, aspect, elevation and photosynthetically active radiation (PAR) at each quadrat.ResultsPresence and abundance of other plant species, topographic variables and light availability explained only a small portion of the variation (5–19%) in the distribution and abundance of individual species. The most common strongest predictor of cover for the ten most common species was species richness, with the association being positive. The non-native species, garlic mustard (Alliaria petiolata) exhibited the strongest positive association with species richness. Only one of the 45 pair-wise comparisons of the ten species resulted in a negative relationship between the species. Abundance and distribution of two species were associated with topographic features, but this accounted for much less of the variation in abundance than did species richness.Conclusion We found little evidence that competition or any other interactions among common herb layer species, including the non-native Alliaria petiolata, play an important role in determining the abundance and the distribution of herb layer species in this oak woodland. Topographic factors may explain a small amount of the distribution and abundance patterns of a few species. But, for the most part, species are more likely to be present when other species are present, suggesting that they are simply establishing in microsites favourable to plants in general.
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Some introduced species become invasive by releasing novel allelochemicals into the soil, directly harming nearby plants and soil microbes. Alliaria petiolata (garlic mustard) is an invasive plant, well known to excrete a suite of phytotoxic and anti-microbial allelochemicals, including allyl isothio-cyanate (AITC) and benzyl isothiocyanate (BITC). While the effects of these chemicals on plant-mycor-rhizae mutualisms are well documented, the effects on other plant-soil microbe interactions, such as the legume-rhizobia mutualism, have not yet been tested. Here, we performed laboratory and greenhouse experiments with both synthetic chemicals and leaf extracts to investigate the effects of allelochemicals in A. petiolata on a native leguminous plant, Amphicarpaea bracteata, and its rhizobia mutualists. We found that BITC reduced rhizobia growth rate in the lab, but had no effect on nodulation in the greenhouse when rhizobia were grown in the presence of plants. AITC did not directly harm either plants or rhizobia, though plants and rhizobia grown in the presence of AITC showed reduced nodulation, indicating that it disrupted the formation of the mutualism itself. We found no effects of A. petiolata allelochemical leaf extracts on plant performance or nodulation. Our data suggest that AITC causes mutualism disruption in this system by preventing the formation of nodules, which reduces plant growth and could threaten the long-term performance of rhizobia. Our study shows that the allelo-chemicals in A. petiolata disrupt the legume-rhizobia resource mutualism, adding another impact of these novel weapons in addition to their well-documented role in disrupting plant-mycorrhizae symbioses.
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Alliaria petiolata (M. Bieb.) Cavara and Grande (garlic mustard) is a non-indigenous member of the Brassicaceae that is invading woodlands throughout eastern North America. Previous work has demonstrated that this species is having a negative effect on the diversity of understory communities and is actively displacing native species. The purpose of this study was to evaluate the extent to which allelopathy might be acting as the mechanism of interference. Extracts of garlic mustard root and shoot tissues were applied to seeds and seedlings of four target species used as bioassays: radish, winter rye, vetch, and lettuce. While seed germination rates varied by species and extract concentration, total germination after 5-7 d was largely unaffected by any extract concentration. Only radish seeds treated with the most concentrated root solution exhibited a depressed germination relative to the water control. Likewise, seedling biomass was generally unaffected by any extract treatment. Only shoot biomass for rye was significantly depressed with the highest concentration of leaf extract. Our data provide little evidence that allelopathy in involved in the invasive success or community interference of this non-indigenous species, even though the Brassicaceae are well known to possess potentially biological active compounds with alleleopathic effects.
Article
Garlic mustard is a well-known invader of deciduous forests of North America, yet the influence of environmental factors on garlic mustard allelochemical production is not well understood. Three experiments were conducted to detect interactions between one garlic mustard allelochemical (glucosinolate) production and light availability. First, to detect patterns of glucosinolate production across a natural light gradient, leaves and roots of mature plants and first-year rosettes were sampled in patches ranging from 100 to 2 % of full sun within an Indiana forest. Second, to determine whether genetic variation drives observed correlations between glucosinolate content and light, seed collected across light gradients within six sites was grown in a common garden and glucosinolate production was measured. Finally, to understand whether local adaptation occurred in garlic mustard's response to light, seed collected from defined light environments across six sites was grown under four light treatments. Results of the field sampling showed that mature plants' root glucosinolate content was elevated in high compared to low light. In the common garden experiment, however, there was no correlation between light availability at seed origin and constitutive glucosinolate content. Additionally, in the common light treatments, there was no evidence for local adaptation to light environment. Overall, the results indicate that plasticity in response to light, not genetic variation among plants growing in different light environments, generates correlations between glucosinolate content and light in the field. Since mature garlic mustard populations in high light may exhibit increased glucosinolate content, it makes them potential targets for management.
Article
Extended leaf phenology (ELP) may commonly drive invasion in Eastern deciduous forests of North America. ELP may confer an advantage in competition, and may interact with other invasion factors. For example, ELP may interact with allelopathy (release of toxins) if exposure to seasonal light influences allelochemical production. Here, we examine ELP and its interaction with glucosinolate (allelochemical) production in invasive garlic mustard (Alliaria petiolata). To test ELP’s role in invasion, garlic mustard was grown in monoculture or polyculture with native species under natural and extended shade regimes. Consistent with an ELP invasion mechanism, garlic mustard survival was higher in natural shade than extended shade, although invader biomass and native responses did not differ between light treatments. While garlic mustard leaf glucosinolate concentration was higher in natural than extended shade during September of its first year, this pattern did not hold at three other time points. Stronger support for the role of ELP in driving garlic mustard invasion emerged from direct manipulation of germination phenology, with higher garlic mustard survival and biomass resulting when germination occurred earlier in the season. Analysis of allelochemical production across eight populations of garlic mustard revealed significant inter-population variation in glucosinolate responses to light availability. Overall, results of these three experiments indicate that ELP may facilitate garlic mustard’s survival in invaded communities. We did not find strong evidence for a synergistic relationship between ELP and allelopathic potential, possibly due to high levels of inter-population variation in the relationship between allelopathy and light.