2. Allelopathy of S. canadensis
Allelopathy is the chemical interaction between donor plants and recipient plants through allelochemicals. Allelochemicals are produced in some plant parts and released into the vicinity of the donor plants, including their rhizosphere soil either by the root exudation, rainfall leachates, volatilization from the plant parts or decomposition processes of plant residues
[46,47,48,49][46][47][48][49]. Several investigations in field conditions showed that
S. canadensis reduced the number and biodiversity of the native plant community in its invaded ranges
[50]. The invasion level of
S. canadensis correlated negatively with the taxonomic diversity of the native plant community, and positively with the invasibility of the community
[51]. Those observations may imply the involvement of allelopathy in the interaction between
S. canadensis and native plant species to some extent. Many researchers have evaluated the allelopathic activity of the root exudates, rhizosphere soil, residues and plant extracts of
S. canadensis (
Table 1).
Table 1. Allelopathic activities of exudates, rhizosphere soil, residues and plant extracts of S. canadensis.
2.1. Allelopathy of Root Exudate and Plant Residue
Root exudates of
S. canadensis, which were obtained from its aeroponic culture, significantly suppressed the growth of two Asian original plant species;
Gnaphalium affine D.Don and
Xanthium sibiricum Patrin ex Widder., two of America origin;
Conyza canadensis (L.) Cronquist and
Celosia argentea L., two of tropical origin;
Aster subulatus Michx. and
Sesbania cannabina (Renz.) Poir. and a cosmopolitan species;
Eclipta prostrata (L.) L. The suppression rate was similar in all plant species
[52]. Root exudates of
S. canadensis also showed the growth inhibition of
Arabidopsis thaliana (L.) Heynh.
[53]. When the seeds of seven European native plant species were sown into the
S. canadensis cultivated soils with or without activated carbon, the germination of five species such as
Dactylis glomerata L.,
Lythrum salicaria L.,
Stachys officinalis (L.) Trevis. and
Trifolium pratense L. were significantly suppressed in activated carbon-free plots than in activated carbon plots. Although the germination rate was not significantly different between both plots, the biomass of
Arrhenatherum elatius (L.) P.Beauv. ex J. et C.Presl in the activated carbon plots after three months of sowing was two times greater than that in the activated carbon-free plots
[53]. Activated carbon is a widely used material to investigate allelopathy because it adsorbs allelochemicals in the plant rhizosphere soil
[34,67][34][67]. In addition, aqueous extracts of the rhizosphere soil of
S. canadensis inhibited the germination and growth of
Digitaria sanguinalis (L.) Scop. and
Amaranthus retroflexus L., and the inhibitory activity was greater in the extracts of the soil obtained from the invasive ranges of
S. canadensis (China) than that from its native ranges (USA)
[54]. These observations suggest that certain allelochemicals, which may cause growth inhibition, would be released into the rhizosphere soil as root exudates of
S. canadensis, and the released allelochemicals in the soil may be greater in the invasive ranges than those in the native ranges.
Crushed stems, leaves and rhizomes of
S. canadensis were mixed with soil and water and kept at 20/15 °C (12/12 h light/dark condition), and the mixture was filtered after 45 days. The obtained filtrate suppressed the germination and growth of
Raphanus sativus L. and
Triticum aestivum L.
[55]. This observation also suggests that certain allelochemicals may be released into the rhizosphere soil during the decomposition process of plant residues of
S. canadensis.
2.2. Allelopathy of Plant Extract
Some plant tissues may contain allelochemicals, since allelochemicals are synthesized and stored in certain plant tissues until their release into the environment
[46,47,48,49][46][47][48][49]. Many investigations on the allelopathic activity of the extracts from different plant parts of
S. canadensis have been conducted. Aqueous extracts of the leaves of
S. canadensis inhibited the germination and root growth of
Raphanus sativus L. and
Lactuca sativa L.
[57], as well as those of
Triticum aestivum L. and
Setaria viridis (L.)P.Beauv.
[58]. The extracts also suppressed the germination, growth and chlorophyll content of
Trifolium pratense L. and
Raphanus sativus L., and increased their electrolyte leakage from the cell membrane of the seedlings
[59,60][59][60].
The fresh leaves and stems of
S. canadensis were soaked in water for 48 h, and the obtained soaking water showed the inhibitory activity on the germination and growth of
Raphanus sativus L. and
Triticum aestivum L.
[67]. Aqueous extracts of the above-ground parts of
S. canadensis suppressed the germination and growth of
Lactuca sativa L.
[61], as well as those of
Digitaria sanguinalis (L.) Scop. and
Amaranthus retroflexus L.
[54]. The inhibitory activity was greater in the plant extracts obtained from the heavily invaded stands than in those obtained from the lightly invaded stands
[62], and in the plant extracts obtained from the invasive ranges than those from the native ranges
[54].
Aqueous extracts of the above-ground parts and roots of
S. canadensis inhibited the germination and growth of
Zoysia japonica Steud, and the extracts of the above-ground parts significantly stimulated malondialdehyde and peroxidase activity
[62]. The extracts of the stems, roots, blossoms and seeds of
S. canadensis suppressed the germination and growth of
Brassica napus L. and
Lolium perenne L.
[63], and the extracts of the roots and rhizomes of
S. canadensis also inhibited the root growth of
Raphanus sativus L. and
Lactuca sativa L.
[57].
Aqueous ethanol extracts of the roots and rhizomes of
S. canadensis inhibited the germination and growth of
Trifolium repens L.,
Trifolium pratense L.,
Medicago lupulina L.,
Suaeda glauca (Brunge) Brunge,
Plantago virginica L.,
Kummerowia stipulacea (Maxium.) Makino,
Festuca arundinacea Schreb.,
Ageratum conyzoides L.,
Portulaca oleracea L. and
Amaranthus spinosus L.
[68]. Aqueous ethanol extracts of the above- and below-ground parts of
S. canadensis suppressed the germination of
Kummerowia striata (Thunb.) Schindl., and the inhibitory activity was greater in the plant extracts collected from the invasive ranges of
S. canadensis than those from its native ranges
[56]. Aqueous and ethanol extracts of the leaves, stems and rhizomes of
S. canadensis inhibited the germination and growth of
Morus alba L.,
Pharbitis nil (L.) Roth,
Triticum aestivum L. and
Brassica campestris L., and the inhibition was grater in the ethanol extracts than in the aqueous extracts
[64].
Investigations on the aqueous and ethanol extracts of every part of
S. canadensis showed the allelopathic activity on the germination, growth, chlorophyll content, electrolyte leakage and/or some enzyme activities of several plant species, including the native plant species. The inhibitory activity was greater in the plant extracts obtained from the invasive ranges of
S. canadensis than in those from its native ranges, and in the extracts collected from the heavily invaded stands than in those collected from the lightly invaded stands. These observations suggest that whole parts of
S. canadensis may contain water and ethanol extractable allelochemicals, which may cause the inhibition. In addition, the plants grown in the invasive ranges and heavily invaded stands may contain more allelochemicals than the plants in the native ranges and lightly invaded stands.
2.3. Effects of the Extract on Arbuscular Mycorrhizal Fungi
The rhizomes of
S. canadensis were soaked in water for 24 h, and the obtained soaking water caused the suppression of the arbuscular mycorrhizal colonization of
Echinochloa crus-galli (L.) P.Beauv.,
Kummerowia striata (Thnb.) Schindl. and
Ageratum conyzoides L.
[66]. The field and greenhouse investigations also showed that
S. canadensis altered the composition of the arbuscular mycorrhizal fungal population in its rhizosphere soil through the inhibition of some dominant species and the stimulation of other species. The established arbuscular mycorrhizal community increased the competitive ability and the biomass of
S. canadensis [65,66,69,70,71][65][66][69][70][71]. This altered arbuscular mycorrhizal community also increased the mycorrhizal-mediated
15N uptake in
S. canadensis, as well as decreased the
15N uptake in the native species
Kummerowia statrica (Thunb.) Schindl.
[72]. In addition, the aqueous ethanol extract of the roots and rhizomes of
S. canadensis also suppressed the population of the soilborne pathogens, namely
Pythium ultimum Trow and
Rhizoctonia solani J.G. Kühn
[73]. These observations indicate that the aqueous extracts
of S. canadensis may alter the arbuscular mycorrhizal population and suppress the colonization of the native plant species. The established arbuscular mycorrhizal community enhanced the competitive ability of
S. canadensis. Certain compounds in the extracts may be involved in the alteration of the arbuscular mycorrhizal community.
2.4. Allelochemicals
As described above, the inhibitory activity of the extracts of the plants and rhizosphere soil of
S. canadensis obtained from the invasive ranges was greater than that obtained from the native ranges
[54,56][54][56]. The concentrations of total phenolics, total flavones and total saponins in
S. canadensis and its rhizosphere soil obtained from the invasive ranges were also greater than those from the native ranges
[54,56][54][56]. These concentrations in the soil obtained from
S. canadensis-infested stands were also greater than those in the soil obtained from
S. canadensis-free stands
[74].
Major compounds identified in the aqueous methanol extracts of the leaves and inflorescences of
S. canadensis were chlorogenic acid, quercitrin and rutin (quercetin-3-
O-β-rutinoside) (
Figure 2)
[75]. A fatty acid,
n-hexadecanonic acid, was isolated from the aqueous ethanol extract of the stems and leaves of
S. canadensis as an allelopathic agent.
n-Hexadecanonic acid significantly inhibited the growth of
Triticum aestivum L.
[55]. A flavonoid, kaempferol-3-
O-D-glucoside, was isolated from the aqueous ethanol extract of the
S. canadensis straw, and the compound inhibited the growth of
Echinochloa colona (L.) Link
[75]. In addition, the concentration of rutin in the leaves of
S. canadensis was greater than that of other
Solidago species
[76,77][76][77]. Some flavonoids were also identified in the aerial parts of
S. canadensis [78].
Figure 2. Allelochemicals identified in S. canadensis and S. altissima.