Associative and Antagonistic Activities in the Rhizosphere

Rhizosphere and the Phyllosphere
Rhizosphere Introduction
Rhizospher Effect
Nitrogen Fixation in the Rhizosphere
Alteration of Rhizosphere Microflora
Associative and Antagonistic Activities in the Rhizosphere
Root Exudates
Organic Compounds Detected in Plant Root Exudates
Fungistasis
Techniques of Rhizosphere
The Soil Dilution and Plate Count Method
The Soil Plate
The Soil Box
The Contact Slide Technique
Artificial Rhizosphere
Model System

Associative and Antagonistic Activities in the Rhizosphere

	Associative and Antagonistic Activities in the Rhizosphere The dependence of one microorganism upon another for extracellular products, chiefly amino acids and growth promoting factors, can be regarded as an associative effect. Many reports indicate that cellular extracts of certain bacteria, fungi and algae increase the growth of other microorganisms in pure culture. Such findings are purely of academic interest unless actual benefits can be demonstrated in soil under field conditions.
Russian workers have demonstrated an increase in amino acid content in plants grown in soil inoculated with specific microorganisms. Such observations have also been made with regard to B-vitamins, auxins, gibberellins and antibiotics. Gibberellins and gibberellin-like substances are known to be produced by bacterial genera commonly occurring in the rhizosphere such as Azotobacter, Arthrobacter, Pseudomonas and Agrobacterium. The commonly observed increased germination due to Azotobacter inoculation may be attributed to growth promoting substances excreted by the bacterium. There is an increase in the exudation of organic acids, amino acids and monosaccharides by plant roots in the presence of microorganisms.
Microorganisms also influence root hair development, mucilage secretion and lateral root development of several plants. The fungi inhabiting the surface of roots influence the amount of substances absorbed into the root system. These attributes point out the existence of a two-way movement of metabolites between plants and microorganisms. Secretion of antibiotics by microorganisms and the resultant biological inhibition of growth of other susceptible microorganisms are demonstrable in soil as well as in pure cultures.
Such antagonistic effects are natural to expect even in uncultivated soil and from the agronomic point of view excessive inhibition of Azotobacter or Rhizobium in the root region may lead to decreased nitrogen fixation or nodulation. On the other hand, coinoculation of nitrogen fixing Azotobacter and Azospirillum isolates with Rhizobium appears to have beneficial influence in increasing nodule number, nitrogen fixation and yield of soybean, pea and clover.

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