Ground beneath a forest of Casuarina equisetifolia trees showing allelopathy. Photographed at Moloa‘a, Kaua‘i, Hawai‘i by Eric Guinther. Used under Creative Commons licence.
Allelopathy – biological warfare
We might not see them in action, but many plants are engaged in active warfare with other plants. Because they can’t move around, plants must find other ways of evading enemies, finding mates and battling each other for resources.
Some plants win the battle by growing taller than others, catching the light and shading the competition. Others have aggressive roots that capture the water and nutrients before slower-rooted plants can.
As is becoming increasingly clear, many plant species engage in chemical warfare, excreting chemicals that suppress (or sometimes enhance) the growth of other plants. Eucalypts and casuarinas (sheoaks) are common examples. By suppressing potential competitors, the plants can gain access to all of the water and nutrients in their rootzone.
As early as about 300 BCE, the Greek philosopher Theophrastus recognised that “pigweed” (probably Portulaca) suppressed the growth of lucerne. Since then, many hundreds of plant species have been found to have allelopathic effects on other plants, due to a wide variety of chemicals. Notable examples that can cause problems for gardeners and landscapers include:
- couch grass (Cynodon dactylon)
- walnut (Juglans spp.)
- Casuarina spp.
- some eucalypt species
- summer grasses (Digitaria spp.)
- pigweeds (Portulaca spp.)
- Oats (Avena spp.)
- pines (Pinus spp).
Beneficial uses of allelopathy
Although allelopathy can be seen as detrimental to plants, allowing weeds such as lantana to grow at the expense of natives, it can also be used to our benefit. For example, rice has been shown to be allelopathic, and cultivars with a high capacity have been identified. Some of the genes responsible for allelopathy in rice have been identified, and work to incorporate enough of these genes into preferred rice cultivars is under way. If this can be achieved, the result will be better crop growth with less application of herbicides.
Allelopathy or competition?
It is not always clear whether chemically mediated allelopathy or physical competition is the reason for one plant’s suppression of another. It could even be a combination of both. In the example of casuarinas, we can observe ourselves that almost nothing grows beneath a casuarina grove. This could be due solely to the release of suppressant chemicals from the trees’ needles or bark, or it could simply be due to the thick root mat the casuarinas produce at the soil surface, which soaks up every drop of rain. Either way, it works to the trees’ advantage, giving us a possible new feature to incorporate into crop plants for natural weed control.
Allelopathy should not be confused with the production of toxic intermediate breakdown products from poorly composted organic matter. Compost that has gone anaerobic (sour or lacking oxygen) will produce a range of organic acids that can kill plant roots, for example.
Further reading
Wikipedia: http://en.wikipedia.org/wiki/Allelopathy
Kebede Z. 1994. Allelopathic chemicals: their potential uses for weed control in agroecosystems. Dept Plant Pathology and Weed Science, Colorado State University, Fort Collins, CO, USA.
Smith W et al. 2001. Allelopathy of bermudagrass, tall fescue, redroot pigweed, and cutleaf evening primrose on pecan. HortScience 36(6):1047–1048.
Khanh TD et al. 2007. Rice allelopathy and the possibility for weed management. Annals of Applied Biology 151: 325–339.