Interactions of Deleterious and Beneficial Rhizosphere Microorganisms and the Effect of Cropping Practices

Abstract

Evidence is increasing that the saprophytic microflora of the rhizosphere includes both deleterious and beneficial elements that have the potential to influence plant growth and crop yields significantly (13, 16, 25, 27, 57, 72, 77, 78). The deleterious microorganisms affect plant growth negatively, but they do not necessarily parasitize the plant tissue. Their deleterious activities include alterations of the supply of water, ions, and plant growth substances by changing root functions and/or by limiting root growth (1,57, 67, 77). The mechanisms by which beneficial nonsymbiotic microorganisms are considered to affect plant growth positively include promotion of the availability and uptake of mineral nutrients (6, 7, 27), provision of plant-growth substances (27, 56, 57), and, most of all, suppression of deleterious rhizosphere microorganisms (13, 27, 28, 72, 77). Beneficial rhizobacteria that promote plant growth, supposedly by competing for iron with deleterious rhizosphere microorganisms, have recently become known as plant growth-promoting rhizobacteria (PGPR) (13, 41, 67, 77). This paper reviews recent knowledge on deleterious and beneficial rhizosphere bacteria, their interactions, and their effect on yield as related to cropping practices.