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The Rhizosphere
The region in the vicinity of roots can be distinguished into many microhabitats as shown in. The term 'rhizosphere' was introduced in 1904 by the German scientist Hiltner to denote that region of the soil which is subject to the influence of plant roots. Rhizosphere is characterized by greater microbiological activity than the soil away from plant roots.
The intensity of such activity depends on the distance to which exudations from the root system can migrate. The term 'rhizosphere-effect' indicates the overall influence of plant roots on soil microorganisms.
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It is now clearly established that greater number of bacteria, fungi and actinomycetes are present in the rhizosphere soil than in non-rhizosphere soil and there are innumerable reports in literature to substantiate this fact.
Several factors such as soil type, its moisture, pH and temperature and the age and condition of plants are known to influence the rhizosphere effect. Apart from the numerical preponderance of microorganisms in the rhizosphere, the rhizosphere effect is also manifest in the occurrence and distribution of bacteria characterized by specific requirements of amino acids, B-vitamins and specialized growth factors (nutritional groups).
It has also been demonstrated that the rates of metabolic activity of the rhizosphere microorganisms are different from those of the nonrhizosphere soil. A wide range of enzymes of plant and microbial origin present in the rhizosphere catalyze the breakdown of organic materials. These enzymes include oxidoreductases, hydrolases, lyases and transferases besides cellulose, dehydrogenases and urease.
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Rhizosphere soil can be separated and a soil suspension obtained by shaking roots in. aliquots of sterile water from which subsequent dilutions are made. One millilitre of the appropriate dilutions is plated on suitable agar media for enumerating bacteria, actinomycetes and fungi.
Likewise, aliquots are transferred to liquid media for counting the numbers of algae, protozoa and bacteria responsible for ammonification, nitrification, denitrification, carbohydrate utilization, cellulose decomposition and nitrogen fixation. Total counts are then expressed on a dry weight basis per gram of soil. Root-free soil or soil samples from uncultivated plots are used as controls to judge the changes in the microbial population due to plant growth.
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A generalized diagramatic view of the root region of a growing plant
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The rhizosphere to soil ratio (R : S) can be calculated by dividing the number of microorganisms in the rhizospehre soil by the number in the soil free from plant growth. Results can also be expressed based on the weight of roots.
In many studies, the root sample along with the adhering soil have been subjected to the action of a waring blender with a known volume of sterile water and aliquots plated to determine the microflora from both inside and outside the root cortex.
In many reports another specialized microhabitat has been recognized and defined as the 'rhizoplane' or the 'root surface'. In sampling the root.
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Number of bacteria, actinomycetes and fungi in the non-rhizosphere and
rhizosphere soils of Dolichos lab lab (Lakshmi Kumari, 1961)
Age in days |
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1 |
5 |
10 |
15 |
20 |
Bacteria (× 107) |
Rhizosphere
Non-rhizosphere |
15.0
2.0 |
95.5
2.0 |
260.0
1.1 |
310.8
2.0 |
677.8
2.5 |
Actinomytes
(x 106) |
Rhizosphere
Non-rhizosphere |
5.5
4.5 |
3.5
6.0 |
34.5
1.3 |
95.8
1.0 |
83.3
1.0 |
Fungi
(x 104) |
Rhizosphere
Non-rhizosphere |
3.3
0.9 |
2.0
1.6 |
26.0
1.5 |
68.0
1.7 |
91.8
6.8 |
Percentage incidence of nutritional groups of bacteria in the non-rhizosphere and
rhizosphere soils of six week old maize (Zea mays) and gram (Cicer arietinum) (Dey, 1967)
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Basal medium |
Basal medium |
Basal medium |
Basal medium |
Basal medium |
Basal medium |
Basal medium |
Basal medium |
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+ |
+ |
+ |
+ |
+ |
+ |
+ yeast |
|
amino acids |
vitamins without vitamin B12 |
vitamins without vitamin B12 |
amino acids + vitamin |
yeast extract |
soil extract |
extract + soil extract |
Maize (zea mays)
Non-rhizosphere soil |
3.5 |
1.5 |
1.5 |
2.3 |
2.5 |
9.3 |
41.4 |
38.0 |
Rhizosphere soil |
22.5 |
14.4 |
7.6 |
13.1 |
3.5 |
14.4 |
12.3 |
12.2 |
Bengal Gram(Cicer arietinum) |
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Non-rhizosphere soil |
4.8 |
1.5 |
1.5 |
3.1 |
4.8 |
7.5 |
37.5 |
39.3 |
Non-rhizosphere soil |
4.8 |
1.5 |
1.5 |
3.1 |
4.8 |
7.5 |
37.5 |
39.3 |
Oxygen uptake ( l) by three composite samples of rhizosphere and nonrhizosphere soils* (from Katznelson and Rouatt, 1957)
Rhizosphere soil |
Composite sample |
Barley |
Rye |
Control soil |
1 |
359 |
295 |
101 |
2 |
363 |
327 |
101 |
3 |
392 |
294 |
96 |
Average |
371 |
305 |
99 |
After nine hours at 30°C; each figure is an average of duplicate determinations.
system for rhizoplane studies, soil adhering to roots is removed and roots subjected to serial washing by sterilized water (10 -12 times) until the clean root surface is exposed. When such washed roots are plated, characteristic fungi and bacteria appear on agar plates, thereby indicating that there are certain microorganisms intimately associated with the root surface.
Some fungi inhabit the root surface in a mycelial state. They belong to the genera Mortierella, Cephalosporium, Trichoderma, Penicillium, Gliocladium, Gliomastix, Fusarium, Cylindrocarpon, Botrytis, Coniothyrium, Mucor, Phoma, Pythium and Aspergillus. Fine structure studies on the epithelial layer of plant roots after inoculation with specific bacteria have shown that bacteria get embedded on the surface of the root with the help of the mucilagenous external layer or the 'mucigel' normally present on actively growing root system.
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