Plant roots and microorganisms

The cooperation of higher plants with living microorganisms, if it can be considered as such, occurs most intensively and strikingly in the root zone. The nature of this root-microorganism association, the reasons for it, and the effects on the higher plants are considered below. 

The area immediately surrounding a root, commonly referred to as the rhizosphere, is the seat of intense biological activity. Most kinds of microorganisms thrive in this region but usually it is the bacteria that are most responsive. Certain species are affected more than others which means that the composition of the microbial population in the rhizosphere may be markedly different from that of similar soil in which no plants are growing. So far as known, all plants have their rhizosphere population that may differ with kind and age of the plant, as well as with conditions under which the plants are grown. 

The effect of the mass of microorganisms in contact with, or in extremely close proximity to, the plant roots and root hairs, varies widely. The products produced may be either beneficial or directly toxic (Bomer, 1960 Woods, 1960). The organisms themselves may have little or no effect or they may be parasitic in or on the roots. At any one time a condition of biological equilibrium or balance between the various groups of organisms present is likely to prevail. This equilibrium is, however, rather unstable 

In pure-culture inoculation experiments, made by Rovira (1963), it was found that Azotobacter did not colonize the roots of lucerne, maize, tomato, or wheat to any great extent. Clostridium and a facultative nitrogen-fixing bacillus were moderate colonizers. In mixed-culture, or unsterilized soils, where competition is likely to be very intense, one would expect even less success in attempts to influence the rhizosphere population by adding foreign organisms. 

The microorganisms surrounding the roots are active in the breakdown of any crop residues or other energy sources, and in the overall humification process. Since the rhizosphere organisms are known to be more active than are those located elsewhere in the soil mass (Katznelson, 1960), it follows that this humification process in the rhizosphere is accelerated at least to the extent that the numbers are increased. Starkey (1931a) observed such a correlation. 

---

J. C. Tarafdar and N. Claassen, Organic pho.sphorus compounds as a phosphorus source for higher plants through the activity of phosphatases produced by plants roots and microorganisms. Biol. Fertil. Soil.t 5 308 (1988). 

Sources Excretion from plant roots and microorganisms. Anaerobic degradation of organic litter Metal mobilization. Complexation of A1 (toxic for plants at certain concentrations)... 

Tarafdar JC, Claasen N (1988) Organic phosphoms compounds as a phosphoms source for higher plants throngh the activity of phosphatase prodnced by plant roots and microorganisms. Biol Fertil Soils 5 308-312... 

Oxygen deprivation in soil is caused by an imbalance between the 10" times slower diffusion of gases in water as compared with air, and the oxygen rate consumption by plant roots and microorganisms. To reduce the oxygen deprivation stress impact, plants have evolved a... 

---