The term allelopathy denotes the production of specific biomolecules by one plant that can induce suffering in, or give benefit to, another plant. However, it is most commonly used in the former sense - an interaction in which one plant causes suffering to another plant.

Conceptually, biomolecules (specifically termed allelochemicals) produced by a plant are released into the environment and subsequently influence the growth and development of neighbouring plants. It is important to keep in mind that allelopathy involves the addition of a chemical compound or compounds (secondary metabolites) into the environment, while "competition" involves the removal or reduction of some factor or factors in the environment (such as nutrients, water, or light).

Although allelopathic science is a relatively new field of study, there exists convincing evidence that allelopathic interactions between plants play a crucial role in both natural and manipulated ecosystems. These interactions are undoubtedly an important factor in determining species distribution and abundance within some plant communities. Allelopathic interactions are also thought to be an important factor in the success of many invasive plants. For specific examples, see Spotted Knapweed and conifers.

Mechanisms of action

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

Demonstrating allelopathy in nature

The vast majority of allelopathy research attempts to focus on direct negative plant-plant interactions caused by 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. nutrient availability; anon., 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.

Proving that allelopathy is occurring is difficult for the reason that it is difficult to separate the effects of allelopathy from those due to resource competition (e.g., 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 the factors do not vary independently in nature. Willis (1985) required that six criteria be met, and even when maximally relaxed to just three, proving allelopathy is rarely if ever accomplished (Blum et al., 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

One of the most worked out aspects of allelopathy in manipulated ecosystems is the role of allelopathy in agriculture. Current research is focused on the effects of weeds on crops, crops on weeds, and crops on crops. This research furthers the possibility of using allelochemicals as growth regulators and natural pesticides (a number of them are either commercially available or in the process of large-scale manufacture) to promote sustainable agriculture.

Several thoroughly 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). However, in other studies allelopathy has been demonstrated to play a crucial role in forests, influencing the composition of the vegetation growth, while also providing an explanation for the patterns of forest regeneration.

The Black Walnut (Juglans nigra) produces juglone, an allelopathic substance that interferes with the growth of other plants. Juglone is somewhat selective, with certain species greatly affected by it and others not affected at all. Eucalyptus leaf litter and root exudates are allelopathic for certain soil microbes and plant species. The Tree of Heaven, (Ailanthus altissima) produces allelopathic substances in its roots which inhibit the growth of many plants.

References

• anon. (Inderjit). 2002. Multifaceted approach to study allelochemicals in an ecosystem. In: Allelopathy, from Molecules to Ecosystems, M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
• Blum U., S. R. Shafer, and M. E. Lehman. 1999. Evidence for inhibitory allelopathic interactions involving phenolic acids in field soils: concepts vs. an experimental model. Critical Reviews in Plant Sciences, 18(5):673-693.
• Einhellig, F.A. 2002. The physiology of allelochemical action: clues and views. In: Allelopathy, from Molecules to Ecosystems, M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
• Harper, J. L. 1977. Population Biology of Plants. Academic Press, London.
• Jose S. 2002. Black walnut allelopathy: current state of the science. In: Chemical Ecology of Plants: Allelopathy in aquatic and terrestrial ecosystems, A. U. Mallik and anon. (Inderjit), Eds. Birkhauser Verlag, Basel, Switzerland.
• Mallik, A. U. and anon. (Inderjit). 2002. Problems and prospects in the study of plant allelochemicals: a brief introduction. In: Chemical Ecology of Plants: Allelopathy in aquatic and terrestrial ecosystems, Mallik, A.U. and anon., Eds. Birkhauser Verlag, Basel, Switzerland.
• Muller C. H. 1966. The role of chemical inhibition (allelopathy) in vegetational composition. Bull. Torrey Botanical Club, 93:332-351.
• Reigosa, M. J., N. Pedrol, A. M. Sanchez-Moreiras, and L. Gonzales. 2002. Stress and allelopathy. In: Allelopathy, from Molecules to Ecosystems, M.J. Reigosa and N. Pedrol, Eds. Science Publishers, Enfield, New Hampshire.
• Rice, E.L. 1974. Allelopathy. Academic Press, New York.
• Webster 1983. Webster's Ninth New Collegiate Dictionary. Merriam-Webster, Inc., Springfield, Mass.
• Willis, R. J. 1985. The historical basis of the concept of allelopathy. J. Hist. Bio., 18: 71-102.
• Willis, R. J. 1999. Australian studies on allelopathy in Eucalyptus: a review. In: Principles and practices in plant ecology: Allelochemical interactions, anon. (Inderjit), K.M.M. Dakshini, and C.L. Foy, Eds. CRC Press, Boca Raton, FL.

Botany

Allelopati | Allelopathie | Allelopatia | Allelopati