Introduction

The rhizosphere

The rhizosphere encloses the zone of soil around a plant root in which the plant root exerts an influence on the growth and distribution of microorganisms. An important source of microbial growth limiting nutrients are the products of rhizodeposition, which include exudates, secretions, lysates and gases (Whipps & Lynch 1985). The increase in the specific growth rate of microorganisms in response to increased organic carbon input has been shown to result in a 5 to 10-fold increase in the number of bacteria when compared with that of the population in the bulk soil (Rouatt et al. 1960; Rovira & Davey 1974). Rhizosphere microorganisms largely depend on root products for their carbon and energy supply (Merckx et al. 1986), and maximum microbial population density occurs at the root surface or rhizoplane (Clarke 1949). This is a consequence of the presence of the highest concentration of growth limiting nutrient at the rhizoplane. Rhizosphere microorganisms do not form a continuous layer on the root surface, but occur in microcolonies (Newman & Bowen 1974; Rovira & Campbell 1975). This microbial cover has been estimated to be below 10% (Rovira et al. 1974; Bowen & Rovira 1976; Bowen & Theodorou 1979). The concentrations of growth limiting nutrient and microbial numbers decrease as a function of radial distance from the root, and under optimal conditions, with adequate growth limiting nutrient supply, the total bacterial numbers would be limited by space. Chemotaxis towards plant root exudates and extracts is a well established phenomenon (Morris et al. 1992), enabling motile bacteria to migrate towards the root through chemotaxis. Non-motile bacteria accumulate in response to an elevated specific growth rate.