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Allelopathic effects of invasive Solidago canadensis on germination and root growth of native Chinese plants

Xin Chen, Lingxiao Mei and Jianjun Tang

1 College of Life Sciences, Zhejiang University, www.zju.edu.cn Email chen-tang@zju.edu.cn;

may1982411@sina.com; chen-tang@zju.edu.cn

Abstract

The primary hypothesis for the astonishing success of Solidago canadensis in invading southern China is that it has developed its population by producing highly fertile seeds and propagule in its foreign land. However, in our preliminary experiment we found that S. canadensis grew in dense stands, essentially monospecific, without native coexistence. One hypothesis to explain this phenomenon is that S. canadensis release allelochemicals into the surrounding environment to inhibit other species. To test this hypothesis, experiments on the allelopathy of S. canadensis on local plant species were conducted under growth chamber. Ethanol extracts (50 g fresh tissue in 2000 ml 50% Ethanol) were prepared and a series of dilutions ranging from 0% to 100% of each extract was used for testing seed germination and root growth of 11 local species Trifolium repens, Trifolium pretense, Medicago lupulina, Lolium perenne, Suaeda glauca, Plantago virginica, Kummerowia stipulacea, Festuca arundinacea, Ageratum conyzoides, Portulaca oleracea, Amaranthus spinosus. Extracts both from root and rhizome inhibited the germination and root growth of local species, and the inhibition enhanced with the concentration of extracts increasing. With the concentration of 1:60(M: M), the extract from rhizome inhibited seed germination and root growth of a number of native Chinese plant species although the magnitude of inhibition varied among different species, following the order grasses > forbs > legumes. These results imply that allelopathy might be an important mechanism by which S. canadensis has successfully invaded southern China.

Media summary

Extracts from roots and rhizome of S. Canadensis, an invasive weed in southern China, inhibited seed germination and seedling growth of a number of native Chinese plant species.

Keywords

Solidago canadensis, allelopathic effects, extract, local species

Introduction

The primary hypothesis for the astonishing success of many exotics as community invaders relative to their importance in their native communities is that they have escaped the natural enemies that control their population growth - the 'natural enemies hypothesis'. However, the frequent failure of introduced biocontrols, weak consumer effects on the growth and reproduction of some invaders, and the lack of consistent strong top-down regulation in many natural ecological systems indicate that other mechanisms must be involved in the success of some exotic plants. One mechanism may be the release by the invader of chemical compounds that have harmful effects on the members of the recipient plant community (i.e., allelopathy) (Hierro, et al., 2003). Callaway and Aschebong (2000) reported that Centaurea diffusa, an invasive Eurasian forb in western North America, produced chemicals that long-term and familiar Eurasian neighbors have adapted to, but that C. diffusa's new North American neighbors have not. Bais (2003) present evidence that Centaurea maculosa (spotted knapweed) displaces native plant species by exuding the phytotoxin (-)-catechin from its roots. Vivanco et al., (2004) also identified a chemical from the root exudates of C. diffusa, 8-hydroxyquinoline that presented allelopathic effects on native plants.

Solidago canadensis is one of the most destructive invasive weeds in the south -western China. The primary hypothesis for the success of S. canadensis invading in southern China is that it has developed its population by producing highly fertile seeds and propagule in its foreign land (Guo et al., 2003). However, in our preliminary field observation we found that S. canadensis grew in dense stands, essentially monospecific, without native coexistence. Our hypothesis to explain this phenomenon is that S. canadensis release allelochemicals into the surrounding environment to inhibit other species. To test this hypothesis, we conducted experiments on extracts of S. canadensis effects on seeds germination and seedling growth of 11 local plant species.

Methods

Plant materials and extract preparation

Plant samples of S. candensis were obtained from field. Roots and rhizome were separated from shoots and washed by distilled water. Ethanol extracts (50 g fresh tissue of root or rhizome in 2000 ml 50% Ethanol) were prepared by soaking tissues of root or rhizome in a solution of 75% alcohol for 24 hours. The solution was filtered and subsequently concentrated in an oven at 40C. The concentrated extract was dissolved in 75% alcohol again, and was filtered with filter paper. After the extract was concentrated, dissolved and filtered three times, the filtrate was put in the oven at 40C to dry until the liquid appeared sticky. All the extracts were stored for use at 4C.

Culture conditions

The extracts were diluted in distilled water to 1:15, 1:30, 1:60 (M:M). Seeds were surface sterilized using H2O2(10%v v-1) for 2 min, followed by washing in sterile distilled water. Surface sterilized seeds were inoculated on filter paper in Petri dishes (7cm in diameter) for germination. Petri dishes were kept in Growth chamber (Safe experimental Instrument Company, Haishu, Ningbo, China) with 16 h light and 8 h dark photoperiod, 251C temperature and 90% 0.8 relative humidity.

Allelopathy bioassay

Every Petri dish was filled with 5ml appropriate concentration solution of root or rhizome extract, while the control Petri dish was filled with 5ml distilled water. There were three replicates in each treatment.

Effects of root and rhizome extract with three concentrations: Con.1 (1:15), Con.2 (1:30), Con.3 (1:60) on seeds germination and root growth of white clover were examined. Thirty sterilized seeds were inoculated in a Petri dish.

Rhizome extract with concentration of 1:60 (M:M) was used to test the effects of S. canadensis on the seeds germination and root growth of the following 11species: Trifolium repens, Trifolium pretense, Medicago lupulina, Lolium perenne, Suaeda glauca, Plantago virginica, Kummerowia stipulacea, Festuca arundinacea, Ageratum conyzoides, Portulaca oleracea, Amaranthus spinosus. Thirty sterilized seeds of each species were inoculated in a Petri dish.

Statistical analysis

One-way ANOVA was performed on each dependent variable by using the general linear model (GLM) design in SPSS version 10.0 (SPSS Inc., Standard Version). Least significant difference (LSD) at 5% confidence level was used for comparisons of treatments.

Results

Effects of root and rhizome extracts with different concentrations on white clover

Extracts both from root and rhizome in different concentrations significantly inhibited the germination of white clover (Fig.1). The inhibition increased with increased concentration of extract. However, all the three concentrations of rhizome extracts showed stronger inhibition on germination than root extracts did (Fig.1).

Figure 1 Effects of extracts from root and rhizome of S. candensis on germination of white clover.

Growth of roots was significantly impacted by root and rhizome extracts of S.canadensis (Fig.2). However, the inhibiting effect on root growth of extracts from rhizome was much stronger than that from root of S.Canadensis. Root growth of white clover was completely inhibited at concentrations 1:15 and 1:30 of rhizome extracts.

Figure 2 Effects of extracts from roots and rhizome of S. candensis on root growth of white clover.

Effects of rhizome extract on 11 species

At concentration of 1:60, extracts from rhizome of of S.canadensis had significant affects on the germination and root growth of several, although not all local species (P<0.01) (Fig. 3). Further, the magnitude of effects varied among different species. The inhibitory effects were highest in grasses such as Lolium perenne and Festuca arundinacea, followed by forbs and legumes.

Figure 3 Effects of extractable from the rhizome of S. candensis on germination root growth of different species (A: Trifolium repens, B: Trifolium pretense, C: Medicago lupulina, D: Kummerowia stipulacea, E: Suaeda glauc, F: Ageratum conyzoides, G: Portulaca oleracea, H: Amaranthus spinosus, I: Lolium perenne, J: Festuca arundinacea, K: Plantago virginica)

Conclusion

Extracts from both roots and rhizomes of invasive weed S. candensis significantly impacted a number of local plant species from the orchard ecosystem. More specifically, the inhibitor effects of extracts from rhizomes were higher than that from roots. Our results suggest that allelopathy could be an important mechanism for the successful invasion of S. canadensis in southern China.

References

Bais HP, Vepachedu R, Gilroy S, et al (2003). Allelopathy and exotic plant invasion: from molecules and genes to species interactions. Science, 301: 1377-1380

Bais HP, Walker TS, Stermitz FR, et al (2002). Enantiomeric-dependent phytotoxic and antimicrobial activity of (+/-)-catechin: a rhizosecreted racemic mixture from spotted knapweed. Plant Physiology, 128: 1173-1179

Callaway R M, Aschehoug E T (2000). Invasive plants versus their new and old neighbors: a mechanism for exotic invasion. Science, 290: 521-523

Guo S L, Fang F (2003). Physiological adaptation of the invasive plant Salidago canadensis to environments. Acta Phytoecologica Sinica, 27:47-52

Hierro J L, Callaway R M (2003). Allelopathy and exotic plant invasion. Plant and Soil, 256: 29~39

Vivanco J M, Bais H P, Stermitz F R, et al (2004). Biogeographical variation in community response to root allelochemistry: novel weapons and exotic invasion. Ecology Letters, 7: 285-292

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