Allelopathy

Casuarina equisetifolia litter completely suppresses germination of understory plants as shown here despite the relative openness of the canopy and ample rainfall (>120 cm/yr) at the location

Allelopathy is the inhibition of growth of a plant due to biomolecules released by another. It is the opposite of symbiotic mutualism. The biomolecules are called allelochemicals and are produced by some plants as secondary metabolites. When the allelochemicals are released into the environment, they inhibit the development of neighbouring plants.

By contrast, resource competition involves the reduction of growth factors (such as nutrients, water or light) from the environment.

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 (Centaurea maculosa), Garlic Mustard (Alliaria petiolata), and Nutsedge.

Examples of allelopathy

One of the most studied aspects of allelopathy 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 herbicides, to promote sustainable agriculture. A number of such allelochemicals are commercially available or in the process of large-scale manufacture. For example, Leptospermone is a purported allelochemical in lemon bottlebrush (Callistemon citrinus). Although it was found to be too weak as a commercial herbicide, a chemical analog of it, mesotrione (tradename Callisto), was found to be effective.[1] It is sold to control broadleaf weeds in corn but also seems to be an effective control for crabgrass in lawns.

A famous case of purported allelopathy is in desert shrubs. One of the most widely known early examples was Salvia leucophylla, because it was on the cover of the journal Science in 1964.[2] Bare zones around the shrubs were hypothesized to be caused by volatile terpenes emitted by the shrubs. However, like many allelopathy studies, it was based on artificial lab experiments and unwarranted extrapolations to natural ecosystems. In 1970, Science published a study where caging the shrubs to exclude rodents and birds allowed grass to grow in the bare zones.[3] A detailed history of this interesting story can be found in Halsey 2004.[4]

Allelopathy has been shown to play a crucial role in forests, influencing the composition of the vegetation growth, and also provides an explanation for the patterns of forest regeneration. The black walnut (Juglans nigra) produces the allelochemical juglone, which affects some species greatly while others not at all. Eucalyptus leaf litter and root exudates are allelopathic for certain soil microbes and plant species. The tree of heaven, (Ailanthus altissima) produces allelochemicals in its roots that inhibit the growth of many plants. The pace of evaluating allelochemicals released by higher plants in nature has greatly accelerated, with promising results in field screening.[5]

Many crop cultivars show strong allelopathic properties, of which rice (Oryza sativa) has been most studied. Rice allelopathy depends on variety and origin: Japonica rice is more allelopathic than Indica and Japonica-Indica hybrid. More recently, critical review on rice allelopathy and the possibility for weed management reported that allelopathic characteristics in rice are quantitatively inherited and several allelopathy-involved traits have been identified.[6]

Garlic mustard is an invasive plant species in North American temperate forests. Its success may be partly due to its excretion of an unidentified allelochemical that interferes with mutualisms between native tree roots and their mycorrhizal fungi.[7]

A study of kochia (Kochia scoparia) in northern Montana by two high school students[8] showed that when kochia precedes spring wheat (Triticum aestivum), it reduces the spring wheat's performance. Effects included delayed emergence, decreased rate of growth, decreased final height, and decreased average vegetative dry weight of spring wheat plants.[9] A larger study later showed that kochia seems to exhibit allelopathy on various crops in northern Montana. [10]

References

  1. ↑ Cornes, D. 2005. Callisto: a very successful maize herbicide inspired by allelochemistry. Proceedings of the Fourth World Congress on Allelopathy [1]
  2. ↑ Muller, C.H., Muller, W.H. and Haines, B.L. 1964. Volatile growth inhibitors produced by aromatic shrubs. Science 143: 471-473. [2]
  3. ↑ Bartholomew, B. 1970. Bare zone between California shrub and grassland communities: The role of animals. Science 170: 1210-1212. [3]
  4. ↑ Halsey, R.W. 2004. In search of allelopathy: An eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral. Journal of the Torrey Botanical Society 131: 343-367. The California Chaparral Institute also offers a PDF-format version of this paper. [4]
  5. ↑ Khanh, T.D, Hong, N.H., Xuan, T.D. Chung, I.M. 2005. Paddy weed control by medical and leguminous plants from Southeast Asia .Crop Protection [doi:10.1016/j.cropro.2004.09.020]
  6. ↑ Khanh, T.D, Xuan, T.D.and Chung, I.M.2007. Rice allelopathy and the possibility for weed management. Annals of Applied Biology [doi:10.1111/j.1744-7348.2007.00183.x]
  7. ↑ Stinson, K.A., Campbell, S.A., Powell, J.R., Wolfe, B.E., Callaway, R.M., Thelen, G.C., Hallett, S.G., Prati, D., and Klironomos, J.N. 2006. Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. PLoS Biology [5]
  8. ↑ For their work in this area, Overcast & Cox were awarded a first place team prize at the International Science and Engineering Fair (ISEF) in 2001.
  9. ↑ M.C. Overcast, J.J. Brimhall. 2000. Allelopathic Effects of Selected Weed Exudates on Germination and Early Growth of Triticum aestivum in Northern Toole County, Montana. [6]
  10. ↑ M.C. Overcast, D.R. Cox. 2001. Effects of Allelochemicals Produced by Kochia scoparia on Selected Crops Grown in North Toole County (NTC), Montana.