Allelopathy

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The suppression of growth of one plant species by another due to the release of toxic substances (Webster 1983).

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Chemical ecologists have defined allelopathy more broadly as any direct or indirect, harmful or beneficial effect of one plant on another through the production of chemical compounds that escape in the environment (Rice, 1984).  Others have narrowed this definition slightly by limiting the scope of allelopathic compounds to those that are secondary metabolites (Malik and Inderjit 2002).

Contents
1 Importance of allelopathy
2 Mechanisms of action
3 Demonstrating an allelopathy in nature
4 Role of plant stress
5 Examples of allelopathy
6 See also
7 References

Importance of allelopathy

Allelopathic interactions are undoubtably an important factor in determining some species distribution and abundance within plant communities.  Allelopathic interactions are also thought to be an important factor in the success of many invasive plants.  For a detailed example, see Spotted Knapweed or conifers.

 

Mechanisms of action

There are hundreds of secondary metabolites, and many are known to be phytotoxic (Einhellig 2002).  Allelopathic effects of these compounds are often observed early in the life cycle, i.e. via inhibition of seedling germination and/or growth.  They exhibit a wide range of mechanisms of action, from affects on DNA (alkaloids), to photosynthetic and mitochondrial function (quinones), to phytohormone activity, ion uptake and water balance (phenolics).  Interpretations of mechanisms of action are complicated by the fact that many individual compounds have multiple phytotoxic effects (Einhellig 2002).

Demonstrating an allelopathy in nature

In practice, the vast majority of allelopathy research attempts to focus on direct negative plant-plant interactions via allelochemicals.  One of the greatest challenges of this approach is showing that the effect is direct, since allelochemicals can have indirect effects on plant species through interaction with biotic (e.g. mycorrhizae), and/or abiotic soil factors (e.g. nutrients) respectively (Inderjit 2002).  In terrestrial systems, the soil plays an important role as the matrix through which potential allelochemicals pass.  Both abiotic and microbial decomposition will have significant effects on the concentration of allelochemicals reaching other plants.

It is also quite difficult to separate the effects of allelopathy from those due to resource competition (i.e. for space, light, water, nutrients or CO2).  Controlled greenhouse studies that allow for examination of a single independently varying factor may be of little interest since they never vary independently in nature.

Thus proving that allelopathy is occurring is quite difficult.  Willis (1985) required that six criteria be met, and even when maximally relaxed to three, proving its existence is rarely accomplished (if ever, see Blum 1999).

  1. pattern of inhibition of one species by another
  2. putative aggressor must produce a toxin
  3. known mode of release of this toxin
  4. toxin transport or accumulation in the environment
  5. afflicted plant have means of uptake of toxin
  6. observed pattern of inhibition cannot be solely explained by physical competition or other biotic factors

Role of plant stress

Allelopathy also interacts with plant stress, because stressed source plants often release a greater array and concentration of allelochemicals, and stressed target plants may be more susceptible to allelochemicals (Reigosa et al. 2002).  Measurement of the effects of allelochemicals along stressor gradients should help to elucidate the relationship between allelopathy and stress.


Examples of allelopathy

The most thoroughly studied studied cases of allelopathic interaction are in desert shrubs and trees. Salvia leucophylla was one of the best early examples (Muller 66), but even its strong evidence for allelopathy may be of limited importance as suggested by caging experiments (Harper 1977). Others argue that allelopathy plays an important role even in the face of this evidence.

The Black Walnut (Juglans nigra) produces juglone, an allelopathic substance that interferes with the growth of other plants. Eucalyptus leaf litter and root exudates are allelopathic for certain soil microbes and plant species.

See also

References

See also: Allelopathy, Alkaloid, Biological life cycle, Black Walnut, Carbon dioxide, Chemical, Conifer, DNA, Desert