Soil-plant-microbes relationships

Understanding of the soil-plant-microbe relationship probably remains one of the most difficult challenge for life sciences due to the integration of observations at many scales. New efforts relying more on holistic approaches are needed. 

Another upcoming area of solid-phase bioremediation enhancement entails the managed use of the plant-microbe relationship (Bell, 1992 Kingsley etal., 1994). The use of plants in solid-phase bioremediation offers a number of potential advantages associated with the support of rhizosphere microbial communities that may be active towards soil contaminants (Bolton et al., 1992). 

Molecular analysis of the interaction between plants, microbes, and soil components may help us understand the causal relationships of events taking plaee in the rhizosphere. Nevertheless, due to the necessity to simplify the experimental approaches, we still do not have the complete picture that takes into ae-count the relative weight of each factor. 

Rhizodegradation is a symbiotic relationship that has evolved between plants and soil microbes. The plants provide the nutrients necessary for the microbes to thrive, and the microbes provide a healthier soil environment in which the plant roots can proliferate. 

The differences between clones may depend on a combined effect of plant exudate and microbial effects on the exudate (Marschner, 1995). In studies under nonsterile conditions, rhizosphere microbes may alter the chemical composition of root exudates. Therefore, the differences between high and low metal soil condition as well as different metals in spiked soil can be due to toxic metal effects or effects resulting from an excess of chloride on microbes. When comparing various clones the differences in exudate composition could have been due to various microbe-clone relationships. One should, however, keep in mind that a microbe-plant relationship is present in real environment where we also find these metal-accumulation differences between clones. Whether the differences in rhizosphere processes are due to plants alone or a combination with microbial interactions has to be further investigated. 

What follows this introduction to plant-plant interactions (Chapter 1) are three additional chapters. The first chapter (Chapter 2) describes the behavior of allelopathic agents in nutrient culture and soil-microbe-seedling systems under laboratory conditions. Simple phenolic acids were chosen as the allelopathic agents for study in these model systems (see justifications in Section 2.2.6). The next chapter (Chapter 3) describes the relationships or lack of relationships between weed seedling behavior and the physicochemical environment in cover crop no-till fields and in laboratory bioassays. Here as well the emphasis is on the potential role of phenolic acids. The final chapter (Chapter 4) restates the central objectives of Chapters 2 and 3 in the form of testable hypotheses, addresses several central questions raised in these chapters, outlines why a holistic approach is required when studying allelopathic plant-plant interactions, and suggests some ways by which this may be achieved. 

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