Rhizosphere Concept and It’s Historical Background
The root system of higher plants is associated not only with soil environment composed of inorganic and organic matter, but also with a vast community of metabolically active microorganisms. As living plants create a unique habitat around the roots, the microbial population on and around the roots is considerably higher than that of root free soil environment and the differences may be both quantitative and qualitative.
1. Rhizosphere: It is the zone/region of soil immediately surrounding the plant roots together with root surfaces, or it is the region where soil and plant roots make contact, or it is the soil region subjected to influence of plant roots and characterized by increased microbial.
2. Rhizoplane: Root surface along with the closely adhering soil particles is termed as rhizoplane.
Term "Rhizosphere" was introduced for the first time by the German scientist Hiltner (1904) to denote that region of soil which is subjected to the influence of plant roots. The concept of "Rhizosphere Phenomenon" which shows the mutual interaction of roots and microorganisms was came into existence with the work of Starkey et al (1929), Clark (1939) and Rauath and Katznelson (1957).
N. V. Krassinikov (1934) found that free living nitrogen-fixing bacteria, Azotobacter were unable to grow in the wheat rhizosphere.
Starkey (1938) examined the rhizosphere region of some plant species and demonstrated the effect of root exudates on the predominance of bacterial population in particular and other soil microorganisms in general in the rhizosphere region. Thus, he put forth the concept of "Rhizosphere effect / phenomenon" for the first time.
F E Clark (1949) introduced / coined the term "Rhizoplane” to denote the root surface together with the closely adhering soil particles.
R. I. Perotti (1925) suggested the boundaries of the rhizosphere region and showed that it was bounded on one side by the general soil region (called as Edaphosphere) and on the other side by the root tissues (called Histosphere).
G. Graf and S. Poschenrieder (1930) divided the rhizosphere region into two general areas i.e. outer rhizosphere and inner rhizosphere for the purpose of describing the same site of microbial action.
H. Katznelson (1946) suggested the R:S ratio i.e. the ratio between the microbial population in the rhizosphere (R) and in the soil (S) to find out the degree or extent of plant roots effect on soil microorganisms. R: S ratio gives a good picture of the relative stimulation of the microorganisms in the rhizosphere of different plant species.
R: S ratio is defined as the ratio of microbial population per unit weight of rhizosphere soil (R), to the microbial population per unit weight of the adjacent non-rhizosphere soil (S)
A. G. Lochhead and H. Katznelson (1940) examined in detail the qualitative differences between the microflora of the rhizosphere and microflora of the non-rhizosphere region and reported that gram-negative, rod shaped and non-spore forming bacteria are abundant in the rhizosphere than in the non-rhizosphere soil
C. Thom and H. Humfeld (1932) found that corn roots in acidic soils yielded predominantly Trichoderma while roots from alkaline soils mainly contained Penicillium.
M J. Timonin (1940) reported some differences in the fungal types and population in the rhizosphere of cereals and legumes. R: S ratio of fungal population was believed to be narrow in most of the plant species, usually not exceeding 10.
E. A. Peterson and others (1958) reported that the plant age and soil type influence the nature of fungal flora in the rhizosphere, and the number of fungal population gradually increases with the age of plant.
M. Adati (1932) studied many crops and found that though actinomycetes were relatively less stimulated than bacteria, but in some cases the R: S ratio of actinomycetes was as high as 62.
R. Venkatesan and G. Rangaswami (1965) studied the rhizosphere effect in rice plant on bacteria, actinomycetes and fungi and reported that (i) for actinornycetes R: S was more (ranging from 0 to 25) depending on the age of plant roots and the dominant genera reported were Nocardia, (ii) R:S ratio reduced with the depth of soil.
E. A. Gonsalves and V. S. Yalavigi (1960) reported the presence of greater number of algae in the rhizosphere
J. W. Rouatt et al reported positive rhizosphere effect on protozoa, but a negative effect on algae in wheat plants.