Increasing attention has been given to the role and potential of allelopathy as a management strategy for crop protection against weeds and other pests. Incorporating allelopathy into natural and agricultural management systems may reduce the use of herbicides, fungicides, nematicides, and insecticides, cause less pollution and diminish autotoxicity hazards. There is a great demand for compounds with selective toxic-ity that can be readily degraded by either the plant or by the soil microorganisms. Plant, microorganisms, other soil organisms and insects can produce allelochemicals which provide new strategies for maintaining and increasing agricultural production in the future. Compounds with allelopathic activity may provide novel chemistry for the synthesis of herbicides, insecticides, nematicides, and fungicides that are not based on the persistent petroleum derived compounds which are such a public health concern (Waller and Chou, 1989; Waller, 1999).
Several crops (some of which can be used as cover crops) have been proved to release allelopathic compounds in the soil (Jimenez-Osornio and Gliessman, 1987; Blum et al., 1997; Inderjit and Keating, 1999; Anaya, 1999), many of which have been chemically characterized (Pereda-Miranda et al., 1996; Inderjit, 1996; Seigler, 1996; Waller et al., 1999). The idea of exploiting these compounds as natural herbicides is therefore very attractive (Putnam, 1988; Weston, 1996; Duke et al., 2000).
However, the large majority of the studies carried out on this topic have referred to reductionist trials carried out in controlled environments, often with the only aim to extract and characterize allelochemicals or, at the most, to test the effect of these compounds on the germination of selected sensitive species in bioassays. In the case of crop-weed interactions, absolute evidence of the occurrence of allelopathy in the field is difficult to obtain, mainly because allelopathic effects are difficult to disentangle from resource competition and other biotic effects (Weidenhamer, 1996; Inderjit and del Moral, 1997). Additionally, the production and release of allelochemicals depend largely upon environmental conditions, usually being higher when plants are under stress, e.g. extreme temperatures, drought, soil nutrient deficiency, high pest incidence (Einhellig, 1987); also, the range and concentration of chemicals that a given species can produce can vary with environment conditions (Anaya, 1999). Other effects that need to be examined are allelopathy-mediated weed-weed, weed-crop and crop-following (or companion) crop interactions. It is therefore questionable whether allelopathy management per se would ever represent a consistently effective weed management tool; however, a better understanding of allelopathic occurrence in field situations, and of how it is influenced by cultural practices, would make it possible to include allelopathic crops in organic cropping systems and use them as a complementary tactic in a weed management strategy (Barberi, 2002).
The extracts of many dominant plants in Taiwan, such as Delonix regia, Digitaria decumbens, Leucaena leucocephala, and Vitex negundo, contain allelopathic compounds, including phenolic acids, alkaloids, and flavonoids that can be used as natural herbicides, fungicides, etc. which are less disruptive of the global ecosystem than are synthetic agrochemicals (Chou, 1995). Many important crops, such as rice, sugarcane, and mungbean, are affected by their own toxic exudates or by phytotoxins produced when their residues decompose in the soil. For example, in Taiwan the yield of the second annual rice crop is typically 25% lower than that of the first, due to phytotoxins produced during the fallowing period between crops. Autointoxication can be minimized by eliminating, or preventing the formation of the phytotoxins through field treatments such as crop rotation, water draining, water flooding, and the polymerization of phytotoxic phenolics into a humic complex (Chou, 1995).
Wetland soils provide anoxia-tolerant plants with access to ample light, water, and nutrients. Intense competition, involving chemical strategies, ensues among the plants. The roots of wetland plants are prime targets for root-eating pests, and the wetland rhizosphere is an ideal environment for many other organisms and communities because it provides water, oxygen, organic food, and physical protection. Consequently, the rhizosphere of wetland plants is densely populated by many specialized organisms, which considerably influence its biogeochemical functioning. The roots protect themselves against pests and control their rhizosphere organisms by bioactive chemicals, which often also have medicinal properties. Anaerobic metabolites, alkaloids, phenolics, terpenoids, and steroids are bioactive chemicals abundant in roots and rhizospheres in wetlands. Bioactivities include allelopathy, growth regulation, extraorganismal enzymatic activities, metal manipulation by phytosiderophores and phytochelatines, various pest-control effects, and poisoning. Complex biological-biochemical interactions among roots, rhizosphere organisms, and the rhizosphere solution determine the overall biogeochemical processes in the wetland rhizosphere and in the vegetated wetlands. in order to comprehend how wetlands really function and to understand these interactions it is necessary to implement long-term collaborative research (Neori et al., 2000). We can find promising allelochemicals and useful interactions in the rich biodiversity of these particular ecoystems, but without doubt, in all type of ecosystems.
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