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Herbicidal effect of Ailanthus altissima leaves water extracts on Medicago sativa seeds germination

Authors:
  • Institut za poljoprivredu i turizam Porec
  • Institute of Agriculture and Tourism Porec, Croatia

Abstract

Ailanthus altissima (Mill.) Swingle is a deciduous tree native to Southeast Asia, and one of the worst invasive plant species in Europe and North America. A feature probably contributing to its invasiveness is a production of a secondary metabolites, one of which ailanthone, is shown in several studies to have, amongst other, a herbicidal effect on many plant species. In this study we have tested the herbicidal effect of A. altissima leaves water extracts on Medicago sativaL. seed germination. M. sativa has been shown in previous studies to be sensitive to A. altissima extracts. The main phytotoxic compound in A. altissima was previously shown to be ailanthone, although probably it is not the only one. Water extracts of leaves have been prepared and diluted to multiple concentrations in order to assess the relation between the extract concentration and the intensity of herbicidal activity. The obtained data showed that there was a significant difference between the emergence of treated and untreated seeds, precisely, the emergence of seeds treated with the highest concentration was on average 30% lower than control. These results were expected and consistent with the previous observations of ailanthone being phytotoxic to a wide variety of plants, causing germination inhibition and injuries in older plants. The data available so far show great promise for the possible future applications of ailanthone as a natural product herbicide.
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476
Herbicidal effect of Ailanthus altissima leaves water
extracts on Medicago sativa seeds germination
Barbara Sladonja
Institute of Agriculture and Tourism Poreč, Croatia, barbara@iptpo.hr
Danijela Poljuha
Institute of Agriculture and Tourism Poreč, Croatia, danijela@iptpo.hr
Marta Sušek
Institute of Agriculture and Tourism Poreč, Croatia, marta@iptpo.hr
Slavica Dudaš
Polytechnic of Rijeka, Agricultural Department Poreč, sdudas@veleri.hr
Abstract
Ailanthus altissima (Mill.) Swingle is a deciduous tree native to Southeast Asia, and one of the worst
invasive plant species in Europe and North America. A feature probably contributing to its
invasiveness is a production of a secondary metabolites, one of which ailanthone, is shown in several
studies to have, amongst other, a herbicidal effect on many plant species. In this study we have tested
the herbicidal effect of A. altissima leaves water extracts on Medicago sativaL. seed germination. M.
sativa has been shown in previous studies to be sensitive to A. altissima extracts. The main phytotoxic
compound in A. altissima was previously shown to be ailanthone, although probably it is not the only
one. Water extracts of leaves have been prepared and diluted to multiple concentrations in order to
assess the relation between the extract concentration and the intensity of herbicidal activity. The
obtained data showed that there was a significant difference between the emergence of treated and
untreated seeds, precisely, the emergence of seeds treated with the highest concentration was on
average 30% lower than control. These results were expected and consistent with the previous
observations of ailanthone being phytotoxic to a wide variety of plants, causing germination inhibition
and injuries in older plants. The data available so far show great promise for the possible future
applications of ailanthone as a natural product herbicide.
Key words: Ailanthus altissima, invasive species, herbicide effect, germination, ailanthone
1 Introduction
Ailanthus altissima (Mill.) Swingle is a deciduous tree native to Southeast Asia (Hu, 1979), and today
one of the most widespread invasive plant species in Europe and North America. It was introduced
into Europe in 1740s and North America in 1780s (Hu, 1979) primarily as an ornamental tree in cities,
due to its high esthetic value and a resistance to pollution and herbivory. Later its uses have been
extended to other areas, such as reforestation, erosion control and a food source for silk worms and
honey bees (Kowarik and Säumel, 2007). Wide usage in the past has resulted in diffusion of A.
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477
altissima in natural environments where it became established, widespread and uncontrollable. Today
it is present on all continents except Antarctica, being the most widespread in the meridional part of
the Northern temperate zone (Kowarik and Säumel, 2007). The areas it occupies have similar climatic
conditions as its native areal, of which most important being moderate winter cold. As it is limited by
cold, it's highly widespread in southern areas, such as the Mediterranean, where it's present in a wide
variety of habitat types (Vilà et al., 2008). On a south to north gradient its distribution is getting more
confined to urban areas which provide milder winter temperatures. Due to its tolerance to pollution
(Gatti, 2008; Gravano et al., 2003), A. altissima is widespread in urban environments and other
disturbed sites such as transportation corridors, abandoned lots and agricultural fields.
High invasive potential is caused by many properties, such as tolerance of a wide span of ecological
conditions and pollution, high reproduction, growth and regeneration rate and a production of
secondary metabolites with herbicidal and insecticidal activities. The main component responsible for
a herbicidal effect in A. altissima is shown to be ailanthone, a chemical in the group of quassinoids
(Heisey, 1996), mainly present in the Simaroubaceae family. Ailanthone is in several studies shown to
be toxic for many plant species, including weeds, crops and trees (Mergen, 1959; Heisey, 1990a;
Lawrence et al., 1991; Heisey, 1996; Heisey and Heisey, 2003). It's believed that, by producing and
releasing ailanthone trough it's tissues, in largest part by roots, A. altissima has an allelopathic effect
on nearby plant species, slowing their growth and in such way outcompeting them (Heisey, 1990b). In
addition, insecticidal activities of ailanthone probably serve as a deterrent of herbivores (Caboni et al.,
2012; Lü and Shi, 2012), limiting the number of species able to control its growth.
A. altissima had a great importance in Chinese folk medicine, which induced the research of the active
properties of quassinoids. Today, they are shown to have antiviral (Chang and Woo, 2003; Tamura et
al., 2003), antitubercular (Rahman et al., 1997), antimalarial (Okunade et al., 2003), antifungal,
antibacterial (Rahman et al., 1997; Zhao et al., 2005; Huo et al., 2012) and anticancer properties
(Tamura et al., 2003; DeFeo et al., 2005). Considering this, A. altissima extracts could pose an
important source of compounds with a wide variety of possible applications in medicine and
agriculture.
The purpose of this study was to test the herbicidal effects of A. altissima leaves water extracts on
emergence of Medicago sativa seeds. Water extracts of dry leaves have been diluted to multiple
concentrations in order to determine the relation between herbicidal activity and concentration of
extracts. M. sativa has previously been shown to be susceptible to ailanthone (Tsao et al., 2002), so the
goal here was to repeat and confirm the toxicity, and possibly describe its correlation to the extract
concentration.
2 Materials and methods
2.1 Extract preparation
Leaves of A. altissima were collected around the city of Poreč and were left to dry in a warehouse.
Water content was determined by weighing the air-dried leaves for several days, until their weight
remained constant and gravimetrically by sample drying on 105°C. A 100 g of dry leaves were soaked
in 1L of water and after 48 hours the mixture was filtered through Whatman filter paper (Grade 5).
The approximate ratio of tissue weight and solvent volume was taken from Lawrence et al. (1991).
The initial solution was used as a 100% treatment and dilutions of 80% and 60 % were prepared by
adding water. A volume of 100 ml of each concentration was prepared for treatments.
2.2 Treatment
M. sativa seeds have been planted in 12 planting pots on depth of approximately 0.5 cm. In each pot
20 seeds have been planted. The seeds were watered with tap water and left to dry for 2 hours before
the application of extracts. Pots have been divided in 4 groups and treated with following treatments:
33 ml of 100%, 80% and 60% initial extract concentration for test groups (1-3) and 33 ml of tap water
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14
88
4
612 12 16
0
2
4
6
8
10
12
14
16
18
20
C 60% 80% 100%
germinated not germinated
a
bb c
chi-square (χ2)=10,435
for control group (C). They were incubated in an indoor laboratory on room temperature. The results
were observed 2 and 6 days after the treatment by counting emerged seedlings.
2.3 Statistical analysis
The obtained data were analyzed by chi-square (χ2), p≤0,05 test to determine if there was any
significant difference between data groups.
3 Results
The number and percentage of germinated seeds are presented in Table 1. Two days after the
treatment only seeds from the control group and one seed from the 60% group have germinated. Six
days after the treatment all of the pots contained emerged seeds although in different quantities. The
quantities of emerged seeds were being reduced going from the control group towards the 100% group
(Figure 1, Table 1). On average, 68% of seeds in the control group (C) have germinated 6 days after
the treatment, in comparison to 42% in groups 2 and 3 and 20% in group 1. The significant difference
is shown to exist between all data groups except between groups 2 and 3 which had the same average
percentage of emerged seedlings.
Table 1: Number of germinated seeds and a corresponding percentage of initial seed number in 3
treated groups and control group, 0, 2 and 6 days after the treatment.
0 days 2 days after 6 days after
Containers 1 2 3 1 2 3 1 2 3
Treatments
1. 100%
0 0 0 0 0 0 4
(20%)
7
(35%)
1
(5%)
2. 80 %
0 0 0 0 0 0 13
(65%)
5
(25%)
7
(35%)
3. 60 %
0 0 0 0 0 1
(5%)
5
(25%)
17
(85%)
3
(15%)
C 0 0 0 3
(15%)
1
(5%)
7
(35%)
10
(50%)
11
(55%)
20
(100%)
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Figure 1: Emergence of M. sativa seeds in relation to the extract concentration, 6 days after the
treatment.
4 Discussion
The obtained results clearly showed the reduction in plant germination rate by the increased
concentration of extracts. Although between groups 2 and 3 there was no significant difference in
seedling emergence, the difference between other groups, especially group 1 and control group
showed the strong inhibitory effect A. altissima extracts had on germination of M. sativa seeds. This
effect on M. sativa was previously observed in the work of Tsao et al. (2002) where it was tested by
measuring the radicle elongation of treated and untreated seeds. The results showed a strong inhibitory
effect, especially on seeds incubated under the presence of light, in which case the activity of the
extracts was shown to be 2 to 3 fold higher than those of seeds incubated in the dark. This enhanced
phytotoxic activity of ailanthone in the presence of light was previously observed by Lin et al. (1995).
Although ailanthone may not be the only phytotoxic compound in A. altissima tissues it is the most
active as shown by Heisey (1993) who identified it as a component responsible for the phytotoxic
effect, and Heisey (1996) who compared the toxicity of pure ailanthone to those of root bark extracts.
One of the obtained fractions of solutions extracted with dichloromethane and etyl-acetate had nearly
identical level of toxicity as pure ailanthone (0,7 mgL-1 causes 50% inhibition of radicle elongation of
Lepidium sativum L. seeds). Ailanthone is a polar compound easily extracted by polar solvents like
water, although the extraction is more efficient with the increase of the solvent polarity as shown in
mentioned study. In this study water was used as a solvent, so the solution contained lower levels of
ailanthone although sufficient to cause inhibition of germination. This effect of aqueous extracts of A.
altissima tissues was first observed by Mergen (1959) who tested leaves extracts on 46 tree species.
The extracts caused different levels of damage in all species except Fraxinus americana L.
The difference between the quantities of extracts needed for inducing a phytotoxic effect can be
caused by the different tissues used in the extraction. Heisey (1990a) showed that the highest
concentration of active compounds is present in the root bark (inner), followed by stem bark, leaves,
wood and flowers as tissue containing the lowest concentration. In this study leaves were used as they
are the most accessible and easily collected and the toxicity of their extracts is sufficient.
To date several studies have been made concerning the toxicity of A. altissima, with different tissues,
extraction methods and testing species used. In Heisey and Heisey (2003) phytotoxic activity was
tested on 17 plant species in field condition, using methanol extracts of inner bark. The lowest
application rate, which was equivalent to 0,3 kg ha-1 of pure ailanthone caused mortality and damage
of more than 50% in 9 species and a significant reduction in shoot biomass in 13 species. In the second
trial extracts were applied on 4 crops in field to determine the effect on natural growing weeds. The
treatments provided partial weed control (highest reduction of biomass was 40%) but also caused
significant crop injuries. Altogether all of the tested species except cotton (Gossipium hirsutum L.)
and yellow nutsedge (Cyperus esculentus L.) were affected by the treatment in different intensities.
Lawrence et al. (1991) tested the effect on A. altissima leaves and stem water extracts on Lactuca
sativa L. seeds germination rate and radicle elongation and showed by the obtained results that both
processes were significantly inhibited by applied extracts.
Although it is highly phytotoxic, the effects of ailanthone are short-lasting, as it is easily degradable by
soil microorganisms. This was shown by Heisey (1996) who observed the elongation of cress radicle
in sterile and non-sterile soil. In non-sterile soil the toxicity was lost after 3 - 5 days, depending on the
application rate, in contrast to the sterile soil where the toxicity persisted for 21 days of the experiment
duration.
Considering its high phytotoxicity, ailanthone shows potential as a possible future natural product
herbicide, although its nonselectivity, observed in multiple studies, would present an obstacle if not
resolved in some way. In addition, its rapid biodegradability could be a positive feature from the
conservational aspect as it has a short lasting effect in the environment, but a negative one if possible
applications as a herbicidal compound would be taken into account. Its usage in such way would
require repeated applications every few days for the toxicity to persist.
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Article
Full-text available
Compounds inhibitory to the growth of neighboring plant species were found in significant concentrations in the leaves and stems of young Ailanthus altissima ramets. The surrounding soil also contained appreciable concentrations of similarly acting toxins. Individuals of neighboring plant species have either incorporated active portions of inhibitory compounds or responded to Ailanthus by producing growth-inhibiting substances. Under greenhouse conditions, individuals of neighboring plant species previously unexposed to Ailanthus in the field were found to be more susceptible to the Ailanthus toxins than individuals previously exposed. Moreover, seeds produced by unexposed populations were also more susceptible to Ailanthus toxins than seeds produced by previously exposed populations. These differences demonstrated that the allelochemicals of Ailanthus altissima exhibited a measurable impact upon neighboring plant species. Since the progeny of these populations displayed a differential response to Ailanthus toxin, this phenotypic difference between the two populations may have a heritable basis.
Article
Compounds inhibitory to the growth of neighboring plant species were found in significant concentrations in the leaves and stems of young Ailanthus altissima ramets. The surrounding soil also contained appreciable concentrations of similarly acting toxins. Individuals of neighboring plant species have either incorporated active portions of inhibitory compounds or responded to Ailanthus by producing growth-inhibiting substances. Under greenhouse conditions, individuals of neighboring plant species previously unexposed to Ailanthus in the field were found to be more susceptible to the Ailanthus toxins than individuals previously exposed. Moreover, seeds produced by unexposed populations were also more susceptible to Ailanthus toxins than seeds produced by previously exposed populations. These differences demonstrated that the allelochemicals of Ailanthus altissima exhibited a measurable impact upon neighboring plant species. Since the progeny of these populations displayed a differential response to Ailanthus toxin, this phenotypic difference between the two populations may have a heritable basis.
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Mature trees of Ailanthus altissima produce one or more potent inhibitors of seed germination and seedling growth. Inhibitor activity is highest in bark, especially of roots, intermediate in leaflets, and low in wood. Crude extracts of Ailanthus root bark and leaflets corresponding to 34 and 119 mg water extractable material/L, respectively, caused 50% inhibition of cress radicle growth. Ailanthus seeds also contain one or more inhibitors. These are bound within the seed by the pericarp but diffuse into water agar when the pericarp is removed. The inhibitor(s) could readily be extracted from Ailanthus tissues with methanol, but not dichloromethane, indicating polar characteristics. Ailanthus leaflets had highest inhibitory activity during expansion in spring, whereas activity of trunk bark peaked just before emergence of leaves. This pattern suggests transport of allelochemicals from bark into new leaves. A comparison of seven plant species for sensitivity to the inhibitor(s) from Ailanthus root bark showed little selectivity, although velvetleaf was somewhat more resistant. The inhibitor(s) from Ailanthus root bark exhibited strong herbicidal effects when sprayed pre- and postemergence on plants in soil in the greenhouse. Postemergence effects were striking, with nearly complete mortality of all species, except velvetleaf, at even the lowest doses tested. The results suggest the allelochemical(s) from Ailanthus may have potential for development as natural-product herbicides.
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Inhibitor activity is highest in bark, especially of roots, intermediate in leaflets, and low in wood. Crude extracts of Ailanthus root bark and leaflets corresponding to 34 and 119 mg water extractable material/L, respectively, caused 50% inhibition of cress radicle growth. Ailanthus seeds also contain one or more inhibitors. Inhibitor(s) from Ailanthus root bark exhibited strong herbicidal effects when sprayed pre- and postemergence on plants in soil in the greenhouse. Postemergence effects were striking, with nearly complete mortality of all species. -from Author
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1. The presence of a substance that was toxic to other tree seedlings was demonstrated in aqueous extracts of foliage from Ailanthus altissima (Mills) Swingle. 2. The effect of concentration of, and the response to, this extract was tested on thirty-five species of gymnosperms and on eleven species of angiosperms. Members of all species, with the exception of those of Fraxinus americana L., were adversely affected.
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Aqueous extracts of Ailanthus altissima bark and foliage were previously shown to be toxic to other plants. Using bioassay-directed fractionation, I isolated the phytotoxic compound from A. altissima root bark and identified it to be ailanthone, a quassinoid compound having molecular mass of 376. Ailanthone was highly phytotoxic, with concentrations of 0.7 ml/L causing 50% inhibition of radicle elongation in a standardized bioassay with garden cress (Lepidium sativum) seeds. Ailanthone exhibited potent pre- and postemergence herbicidal activity in greenhouse trials. Postemergence activity was especially striking; even the lowest application rate (0.5 kg/ha) caused complete mortality of five of the seven plant species tested within 5 d of treatment. In contrast, the highest application rate (8 kg/ha) did not cause any detectable injury to A. altissima seedlings, indicating the presence of a protective mechanism in the producer species to prevent autotoxicity. Ailanthone was rapidly detoxified in field soil as a result of microbial activity. Applications of ailanthone equivalent to 0.5 and 4.0 kg/ha completely lost their phytotoxicity within less than or equal to 5 d when incubated in the presence of nonsterile soil. When incubated with sterile soil under identical conditions, however, ailanthone remained highly phytotoxic throughout the 21-d duration of the investigation. The high level of postemergence herbicidal activity in conjunction with its rapid biodegradation in soil suggest ailanthone may have potential for development as a natural-product herbicide.