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). Here, we provide an abbreviated compilation of evidence for allelopathy in general, present a detailed case study for Centaurea diffusa, an invasive Eurasian forb in western North America, and review general evidence for allelopathic effects of invasive plants in native communities. The primary rationale for considering allelopathy as a mechanism for the success of invaders is based on two premises. First, invaders often establish virtual monocultures where diverse communities once flourished, a phenomenon unusual in natural communities. Second, allelopathy may be more important in recipient than in origin communities because the former are more likely to be naive to the chemicals possessed by newly arrived species. Indeed, results from experiments on C. diffusa suggest that this invader produces chemicals that long-term and familiar Eurasian neighbors have adapted to, but that C. diffusa's new North American neighbors have not. A large number of early studies demonstrated strong potential allelopathic effects of exotic invasive plants; however, most of this work rests on controversial methodology. Nevertheless, during the last 15 years, methodological approaches have improved. Allelopathic effects have been tested on native species, allelochemicals have been tested in varying resource conditions, models have been used to estimate comparisons of resource and allelopathic effects, and experimental techniques have been used to ameliorate chemical effects. We do not recommend allelopathy as a `unifying theory' for plant interactions, nor do we espouse the view that allelopathy is the dominant way that plants interact, but we argue that non-resource mechanisms should be returned to the discussion table as a potential mechanism for explaining the remarkable success of some invasive species. Ecologists should consider the possibility that resource and non-resource mechanisms may work simultaneously, but vary in their relative importance depending on the ecological context in which they are studied. One such context might be exotic plant invasion.

Allelopathy and exotic plant invasion

Studies investigating the allelopathic effect of root, stem, leaf, and fruit water extracts and infested soil of Euphorbia helioscopia L on the seed germination and seedling growth of wheat, chickpea, and lentil were conducted in a completely randomized design with 4 replications Water extracts of root, stem, leaf, and fruit were prepared by soaking dried plant parts of E helioscopia in water (1:20 w/v) for a period of 24 h Seedling emergence, seedling vigor index, and total dry weight of wheat, chickpea, and lentil seedlings were significantly reduced when these crops were grown in soil taken from an E helioscopia infested field compared to soil collected from an area free of any vegetation E helioscopia infested soil also significantly decreased the root length of wheat and lentil, and shoot length of lentil compared to the control soil Water extracts of various organs of E helioscopia significantly decreased the seedling vigor index and growth of test crops Leaf extract had a greater inhibitory effect than the other extracts Water extracts from the root, stem, leaf, and fruit of E helioscopia resulted in a reduction in the seed germination (chickpea and lentil only) and germination index but the leaf extract increased the mean germination time in all test crops

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Allelopathic potential of Euphorbia helioscopia L. against wheat (Triticum aestivum L.), chickpea (Cicer arietinum L.) and lentil (Lens culinaris Medic.)

SUMMARY During the 2006-2007 period, the allelopathic effect of cold water extracts from Amaranthus retroflexus L., Chenopodium album L., Erigeron canadensis L. and Solanum nigrum L. on seed germination and initial development of Glycine max L., Pisum sativum L. and Vicia sativa L. was studied under laboratory conditions in the Institute of Forage Crops, Pleven. It was found that: water extracts from fresh and dry biomass of A. retroflexus, Ch. album, E. canadensis and S. nigrum had an inhibitory effect on seed germination of G. max, P. sativum and V. sativa, the inhibition rate for the extracts from fresh biomass varying from 28.8 to 81.5% and for the extracts from dry weed biomass it was from 26.8 tо 89.2%; The values of LC50 varied from 13.5 tо 72.2 g l -1 for the extracts from fresh biomass and from 7.0 tо 84.1 g l -1 for the extracts from dry weed biomass and they could be conditionally grouped in the following ascending order: A. retroflexus < S. nigrum < E. canadensis <Ch. album and for extracts from dry biomass: A. retroflexus < E. сanadensis < Ch. album < S. nigrum; P. sativum was the most sensitive to the allelopathic effect of the extracts from fresh and dry weed biomass - LC50 varied from 13.5 tо 21.6 g l -1 , followed by V. sativa - LC50 from 26.0 tо 11.7 g l -1 and G. max had relatively the lowest sensitivity - LC50 was from 46.6 tо 56.7 g l -1 .

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Determination of allelopathic effect of some invasive weed species on germination and initial development of grain legume crops

A bicyclic sesquiterpene dialdehyde, polygodial did not inhibit root elongation up to a concentration of 12.5 μg/ml in a lettuce seedling assay: trans-Cinnamic acid inhibited the elongation by 50% at 1.2 μg/ml (8.1 μM). The inhibitory activity of trans-cinnamic acid was enhanced 17-fold when used in combination with 6.25 μg/ml (26.5 μM) of polygodial. A decrease in the pH of the medium was observed during normal seedling growth, indicating transport of protons from the cells by a plasma membrane H+-ATPase. The inhibitory effect of trans-cinnamic acid on the elongation was reduced to some extent in 2 mM phosphate buffer (pH 7.0) during seedling growth. Although polygodial did not inhibit the activity of H+-ATPase in the plasma membrane fraction of roots in normally growing seedlings, a decrease in activity was found in the fraction obtained from seedlings incubated with 20 μg/ml of polygodial. These results suggest that polygodial functions synergistically with trans-cinnamic acid in the inhibition of root elongation via restriction of proton transport from the cytoplasm of germinated cells.

Synergism of polygodial and trans-cinnamic acid on inhibition of root elongation in lettuce seedling growth bioassays.

The field of allelopathy is one of the most fascinating but controversial processes in plant ecology that offers an exciting, interdisciplinary, complex, and challenging study. In spite of the established role of soil microbes in plant health, their role has also been consolidated in studies of allelopathy. Moreover, allelopathy can be better understood by incorporating soil microbial ecology that determines the relevance of allelopathy phenomenon. Therefore, while discussing the role of allelochemicals in plant-plant interactions, the dynamic nature of soil microbes should not be overlooked. The occurrence and toxicity of allelochemicals in soil depend on various factors, but the type of microflora in the surroundings plays a crucial role because it can interfere with its allelopathic nature. Such microbes could be of prime importance for biological control management of weeds reducing the cost and ill effects of chemical herbicides. Among microbes, our main focus is on bacteria--as they are dominant among other microbes and are being used for enhancing crop production for decades--and fungi. Hence, to refer to both bacteria and fungi, we have used the term microbes. This review discusses the beneficial role of microbes in reducing the allelopathic effects of weeds. The review is mainly focused on various functions of bacteria in (1) reducing allelopathic inhibition caused by weeds to reduce crop yield loss, (2) building inherent defense capacity in plants against allelopathic weed, and (3) deciphering beneficial rhizospheric process such as chemotaxis/biofilm, degradation of toxic allelochemicals, and induced gene expression.

Unravelling the beneficial role of microbial contributors in reducing the allelopathic effects of weeds.

Allelopathic effect ofAvena fatua L., Cyperus rotundus L., Polygonum hydropiper L., and Solanum nigrum L. were examined on seedling growth of certain commonly used varieties of wheat (Triticum aestivum L.) in the Tarai region of U.P. state. The weed extracts inhibited the length of plumule in all the varieties (100%) with Solanum and it was in 12 (92%), 10 (77%) and 06 (46%) varieties with Polygonum, Avena and Cyperus, respectively. In radicle length, it was in 92% with both Polygonum and Solanum; and 85% and 69% of the varieties with Avena and Cyperus, respectively. However, all the four weed extracts reduced the dry weight of plumule, radicle and total seedling in all the varieties (100%) of wheat except in HD--2329 with Cyperus, in which it was positive. The percent reduction (percentage of control) was more than 50% in 92%, 77%, 54% and 39% of the varieties, respectively with Solanum, Polygonum, Avena and Cyperus. Among the weed extracts, the inhibitory effect on seedling growth in different varieties followed the order: Solanum > Polygonum > Avena and > Cyperus. On the basis of the present results, UP--2003 and WH--542 followed by PBW--226, Sangam and HD--248 were more susceptible to all the four weed extracts compared to the rest of the varieties of wheat.

Gahlot A

Papers

Effect of weed extracts on seedling growth of some varieties of wheat.