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thesis to the fungi and receiving mineral nutrients and other benefits from the fungi.
However, like any relationship, this one is not inevitably benign. One partner (either the plant or the fungus) can become parasitic on the other, receiving benefits without supplying them. Sometimes mycorrhizal fungi do not provide any minerals or other detectable benefits to their hosts, yet continue to receive nutrients. Sometimes only minor amounts of nutrients are provided by the fungus, the nutrients made available are not needed or do not enhance plant growth, or the benefits provided are vastly outweighed by the carbon and energy drain on the plant. Sometimes the plant provides little benefit to the fungus, yet receives benefits from it, as in the case of the orchid mycorrhizal association (at least some of the time).
In some cases, the plant may be capable of discarding the fungus when phosphorus is available, accepting the fungus only when it is of benefit to the plant. The fungus may, on the other hand, be the controlling member of the pair, not only determining the association but controlling the plant’s growth, development, and allocation patterns by producing hormones that override the plant’s own hormonal signals. Nonmycorrhizal plants have been observed to use a variety of mechanisms to actively reject fungal infection.
Plant Interconnections via Mycorrhizal Fungi
Among the host of strange and interesting phenomena associated with mycorrhizae, one that has fascinated many plant ecologists is the potential interconnection between different individual plants via mycorrhizal hyphae. Two plants that are infected with the same mycorrhizal fungus can be connected by a hyphal strand running through the soil. Even plants of completely unrelated species or genera can be connected in this way. Phosphorus, nitrogen, water, and even carbohydrates have been found to move between plants, apparently through these hyphal connections. However, it is difficult in practice to distinguish such a “pipeline” effect from that of substances leaking out of the roots or hyphae of one plant and being immediately picked up by the roots or hyphae of another.
The major unresolved question is, to what extent is this kind of transfer important in natural ecosystems? Some ecologists have argued that nutrients may be transferred from dominant individuals to seedlings, to understory species in forests, or to competitively inferior plants, allowing them to survive and persist in the plant community. These plants would then be, at least to some extent, parasitizing the plants from which they were drawing carbon or mineral nutrients. We do not yet know whether this phenomenon is a rare curiosity or a common occurrence, or how important it is in determining plant community composition.
A closely related issue is the potential for such interconnections to alter competitive relationships among individuals. Mycorrhizal nutrient transfer could potentially intensify such competitive interactions or ameliorate them. It could alter the competitive hierarchy, making inferior competitors more successful, or it could help the dominant competitor. We do not yet know how to predict the effects of such interconnections on competition, nor do we have any idea how common or how important such effects are in nature. Future research on mycorrhizae is certain to come up with many surprises.
Summary
Soil is a complex product of the interaction between living organisms and their terrestrial substrate. It includes physically and chemically altered products derived from rock and from organic materials, along with air and water. Soils vary from one region to another, and within local environments, in many different aspects. There is a tremendous diversity of soil properties and characteristics, and these differences affect the vegetation found growing in the soil as well as affecting the growth of individual plants.
Soil texture describes the relative proportions of clay, silt, and sand particles in the soil. These particles differ from one another in size, shape, and mineral composition, and they impart different characteristics to soils. The pH of soil also has an important influence on plant growth and on what plants are found growing in the soil. One of the important factors in characterizing local soils is the soil profile, which consists of often distinct soil horizons, or layers at different depths. In addition to the physical factors responsible for soil properties, soil organic matter is a critical component in determining soil structure.
Plants depend on the soil to obtain water. Different soils hold different amounts of water in the macropores and micropores of the soil structure. As a soil dries after a rain, water first drains out of the macropores, and then moves by capillary action through the micropores to plant roots. Eventually, as the soil water potential declines, water becomes less and less available to plants, causing wilting and ultimately death unless more water enters the soil.
Plants also depend on the soil for the mineral nutrients that are essential to their survival and growth. Nitrogen and phosphorus are generally the most limiting mineral nutrients for plants. Both are needed by plants in fairly large quantities. Nitrogen availability, in particular, depends on bacterial activity, including symbiotic nitrogen fixation. The availability of these two nutrients varies greatly in soils because nitrogen is easily lost from soils, while phosphorus is often present in unavailable forms.
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Plants may depend on a symbiotic relationships with fungi living on or in their roots, called mycorrhizae, to obtain phosphorus and other minerals from the soil. Plants may obtain other benefits from these associations as well. Most terrestrial plants have mycorrhizae and depend on them to survive, grow, and reproduce in their
Additional Readings
Classic References
Lyon, T. L. and H. O. Buckman. 1922. The Nature and Properties of Soils.
Macmillan, New York. [Subsequent editions, H. O. Buckman and N. C. Brady 1952, 1960, 1969.]
Current version:
Brady, N. C. and R. R. Weil. 2001. The Nature and Properties of Soils. 13th ed. Prentice-Hall, Upper Saddle River, NJ.
Contemporary Research
Batty, A. L., K. W. Dixon, M. Brundrett and K. Sivasithamparam. 2001. Constraints to symbiotic germination of terrestrial orchid seed in a mediterranean bushland. New Phytologist 152: 511–520.
Brundrett, M. 1991. Mycorrhizas in natural ecosystems. Adv. Ecol. Res. 21: 171–313.
natural habitats. Two of the most important types of mycorrhizae are ectomycorrhizae (ECM) and endomycorrhizae, particularly vesicular-arbuscular mycorrhizae (VAM). The relationships between plants and mycorrhizal fungi are complex and variable, and are important in the function of many ecosystems.
Newsham, K. K., A. H. Fitter and A. R. Watkinson. 1995. Multi-func- tionality and biodiversity in arbuscular mycorrhizas. Trends Ecol. Evol. 10: 407–411.
Fitzhugh, R. D., C. T. Driscoll, P. M. Groffman, G. L. Tierney, T. J. Fahey and J. P. Hardy. 2001. Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem. Biogeochemistry 56: 215–238.
Additional Resources
Jeffrey, D. W. 1987. Soil-Plant Relationships: An Ecological Approach.
Croom Helm, London, and Timber Press, Portland, OR.
Pimentel, D. (ed.). 1993. World Soil Erosion and Conservation. Cambridge University Press, Cambridge.
Rendig, V. V. and H. M. Taylor. 1989. Principles of Soil-Plant Interrelationships. McGraw Hill, New York.