Root Directory
Besides the absorption of water and nutrients, virtually all roots have symbiotic associations with soil microorganisms. The term "Rhizosphere" is used to indicate the interface of the root Epidermis and its immediate zone of soil contact. It has been estimated that 90% of land plants have mycorrhizal associations with soil fungi. This is probably an underestimate. Those who want to raise endangered species for out-planting to nature, achieve good results when the propagules have been grown in medium containing the soil microflora of the out-planting sites. This is very pertinent for Hawaiian species which are hard to reestablish in nature. Dr. Wong has detailed information concerning this!!!!!
There are a few famous symbiotic relationships. The most well known is that between legumes and soil bacteria in the genus Rhizobium. The Rhizobia can fix nitrogen gas when the symbiosis is achieved. They can't fix nitrogen in the free-living state, however! Fixed nitrogen is obtained by the legume and the Rhizobium gets a nice place to live with all of the amenities!. This includes photosynthate, water and minerals. This is an extremely important relationship because Nitrogen is usually the most limiting element is terrestrial ecosystems. Furthermore, many legumes form the basis for agriculture on a world-scale. Some legume seeds, like soybeans, contain high levels of protein. These are the most important agricultural sources of protein in the world.
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Two basic types of root nodules are produced by legumes. One type is ephemeral and lasts days or a few weeks. This is called a determinate structure. It has a short, predestined life-span. Consequently, new nodules are being formed as the root grows in the soil and others are being lost on older parts of the root system. Soybean nodules are like this. The nodule is a spherical elaboration of the ground tissue system in the root cortex and has a specialized anatomy. The outer part of the nodule becomes sclerotic as parenchyma cells are converted into sclereids. This tissue blocks gas exchange to some extent. More internal Vascular Bundles surround a large central mass of parenchyma which contains cells infected with bacteroids. that fix nitrogen. Uninfected Parenchyma cells are also present.
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Part of a Legume/Rhizobium Nodule seen with Dark-Field Optics. |
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Cells from a Legume/Rhizobium Nodule seen with Bright-Field Optics. The densely stained cells contain Rhizobium bacteroids. This is where atmospheric N is fixed. |
The second nodule type is illustrated by alfalfa (Medicago sp.). In this case the nodule has an Apical Meristem which functions for many months. It is called indeterminate in that meristematic activity is theoretically unlimited. It is lengthy compared to the determinate nodules, however. These are tumescent, elongate structures. The meristem produces new cells which become infected with bacteriods from older cells. These nodules have a much more extensive vascular system which surrounds the nitrogen-fixing parenchyma which occupy the center of the nodule. This central location is not a coincidence. The enzyme which fixes nitrogen needs an anaerobic environment. Consequently, the location of the bacteroids. inside living, non-photosynthetic cells which are in the center of the root favors N-fixation. Lignified external layers may also limit gas exchange. Furthermore, the bacteroids. produce leghaemoglobin which acts like hemoglobin and binds oxygen, thus reducing oxygen levels.
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Long section through an Alfalfa Nodule The Apical meristem is at the upper right. The red cells are the most recently infected cells. |
Apical region of an Alfalfa Nodule. The Meristematic cells are small and stain densely. The newly infected cells are enlarged and isodiametric. |
Some plants have nitrogen-fixing symbioses with Actinomycetes. Many of these are "pioneer" species which colonize barren sites. Some alien species which are displacing native species in Hawaii have this type of symbiotic relationship. Consequently, it is hard to get rid of them because they grow much faster than native competitors, and they alter the soil nitrogen levels markedly compared to more native stands. This may favor the growth of other "alien" species at the expense of native plants.
