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a IRRI, Philippines and Weed Sci., Royal Veterinary and Agric. Univ., Agrovej Toastrup, Denmark
b Dep. of Biol., Lakehead Univ., Thunder Bay, Canada P7B 5E1
Corresponding author (Mol{at}kvl.dk)
Received for publication November 30, 1999.
THE PAPERS in this special section were presented at a symposium, "Allelopathy in Natural and Managed Ecosystems," of the Second World Congress on Allelopathy, held at Lakehead University, Thunder Bay, Canada, 9–13 Aug. 1999. The symposium aimed to provide a forum to present and discuss the latest results of allelopathy research related to agriculture, forestry, and other forms of vegetation management. A total of 44 oral communications were presented in two sessions; of these, 16 are presented in this issue. In addition to the oral presentations in all of the sessions, 56 posters were presented on different aspects of allelopathy along with a CD-ROM demonstration on the history of allelopathy. Two hundred scientists from 34 countries representing five continents attended the congress. The Second World Congress on Allelopathy was organized under the auspices of the International Allelopathy Society, and Lakehead University provided excellent conference facilities.
The history of allelopathy research is mired in speculation, faulty experimentation, and unreasonable conclusions. The discipline has gone through many ups and downs in generating interest among scientists in soil science, agriculture, ecology, and forestry. However, real progress in allelopathy began in the 1960s with the work of Muller et al. (1964), and subsequently with the publication of the book Allelopathy by E.L. Rice in 1974. Substantial progress has since been made in understanding allelopathy in an overall ecological perspective as well as in how we can manipulate allelopathic interactions to our advantage in agricultural practices (e.g., weed control).
It is difficult to demonstrate allelopathy in nature due to the complex interrelations between the different, often genetically determined, characteristics of plant interference. Proving allelopathy also requires a cause-and-effect relationship between identifiable exuded chemical compounds and their effects on plants. Methodological concerns have been the major obstacle in the study of allelopathy. However, this is becoming easier to overcome with our increasing understanding of the chemical processes occurring in both natural and managed ecosystems and with our ability to identify allelochemicals. Progress in chemistry, biotechnology, and digital technology as well as an increased understanding of ecological processes help explain the rapid progress in allelopathy research over the last four decades.
Breakthroughs in understanding the genetic control of allelopathic interactions closely depend on research tools in molecular biology. Progress in this area can lead to the development of highly competitive plant varieties, which might become a sustainable additional tool for weed management in agroecosystems. Similarly, digital imaging techniques and information technology have the potential to play an increasingly important role in ecological studies and ecosystem-level studies of allelopathy (e.g., succession studies in forests). They can also help in in-depth studies of structural changes at a cellular level caused by allelopathy. Having said that, we must also recognize that there is still a tendency to use inappropriate, oversimplified methods such as plant extract studies and the lettuce seedling bioassay in the quest to prove allelopathy. This creates a bad image of the subject and provides fuel for most of its critics—justifiably so.
Traditionally, farmers have realized the benefits of modifying the rhizosphere ecology of crops through crop rotation, mulching, burning, application of green manure, etc. Allelopathy research today is trying to unravel the mechanisms underlying the effects of these practices. The real benefits of allelopathic research can be realized only if our knowledge can be used to improve the sustainability of our production systems in agriculture and forestry and the conservation of natural and seminatural vegetation. In this regard, we see progress in allelopathy research on two fronts: one in applied research on the cultural techniques of cropping systems, silviculture, and nature conservation and the second in crop improvement. To be effective, allelopathy research must involve a range of scientists of many different specializations with the overall objective of understanding the mechanisms of allelopathy. The future of allelopathy research also lies in understanding the ecosystem perspective of the allelopathic phenomenon. Demonstrating allelopathy and understanding its interconnections with other ecological processes influencing ecosystem functions will make a significant contribution to the sustainable management of our natural resources.
The allelopathy papers in this issue of Agronomy Journal touch on all of the above mentioned facets of allelopathy research. Five papers deal with the ability of allelopathic rice (Oryza sativa L.) to suppress weeds, starting with an overview of the progress made in using allelopathy in rice for weed control. A prerequisite for the use of allelopathy in rice is to be able to screen efficiently and to select breeding lines that are weed suppressive. High-performance liquid chromatography techniques have been developed that show promising results in the correlation between the chemical content of rice plants and their ability to suppress weeds under laboratory conditions. These results are presented in the second paper on rice allelopathy. The third paper likewise deals with using rice extracts to screen rice lines for weed-suppressing ability. To be able to evaluate the consequences of the widespread use of allelopathic rice and to facilitate the understanding of the mechanisms of rice allelopathy, it is important to identify the allelochemicals responsible for weed suppression. Different approaches can be taken to identify these allelochemicals, and the fourth paper on rice discusses bioassay-guided isolation as the method of choice for its ability to identify unknown chemicals. Finally, the last paper on rice reveals the identification of four quantitative trait loci that are correlated with rice allelopathy. This paper proves for the first time that allelopathy is a genetically driven phenomenon.
Potential weed suppression from allelopathic field crops is also discussed in several other papers. In Mexico, a range of allelopathic legume cover crops has been studied to understand the best management strategies for efficient weed suppression. One paper makes a special effort to address the weed problems of resource-poor farmers who might not have alternatives to hand weeding. Allelopathy in smallholder fields is also discussed in relation to sorghum [Sorghum bicolor (L.) Moerch] production. The mechanisms of weed-suppressive activity from turnip rape (Brassica rapa L.) mulch are discussed in a paper pointing out that isothiocyanates might play an important role. Finally, the methodology in allelopathy studies is highlighted and discussed in a paper on the allelopathic effects of wheat (Triticum aestivum L.) straw on perennial ryegrass (Lolium perenne L.).
The success of some weeds as weeds can also be understood by understanding their allelopathic characteristics, and one paper attempts to reveal the effects of the noxious weeds Cardaria draba and Salvia syriaca on vegetable crops in Jordan.
Imbalances in any ecosystem might increase or reduce the effects of allelopathic plants. Phytopathogenic microorganisms causing rust in ryegrass are given as an example of enhancement of the allelopathic potential against white clover (Trifolium repens L.). Microorganisms by themselves can also cause allelopathic effects, and this is illustrated by two papers on the allelopathic effects of Aspargillus japonica and its allelochemical, secaloic acid F.
Most allelopathic plants are thought to exert their action through soil. Understanding soil chemistry and soil processes is therefore of great importance in understanding allelopathy. Therefore, a review paper is included in this issue on soil as a growth medium and an active component of allelopathy through interaction, decomposition, or chemical alterations of allelochemicals. Last but not least, two papers deal with examples of allelopathy and nutrient competition in forest ecosystems.
Our final comment is an enormous thank you to all of the authors who have been most cooperative in the creation of this special section in the issue. A special thank you also goes to all the reviewers who reviewed the manuscripts, an often time-consuming task with little reward. Thank you to all.
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