3.3.2  Rhizosphere biology

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Figure 3.12 Mature rhizosphere from roots of clover (Trifolium subterraneum L.). The outer cortex has been crushed and epidermal cells (EP) have become distorted, leading to leakage of substrates into the rhizosphere. The rhizosphere is rich in microorganisms with bacteria (B) clearly visible. Soil (Q) and clay (CL) particles are held together in the inner rhizosphere by a mucilage of polysaccharides. Sustained losses of carbon required to maintain this microflora are thought to come from exudation and senescence of root cells. (× 10 000) (Foster et al. 1983; reproduced with permission of Springer-Verlag)

Microbial activity, sustained by photoassimilates secreted from roots, contributes substantially to rhizosphere properties. The level of microbial activity is also influenced by availability of nitrogen as a substrate for microbial growth. Soils with high fertility and biological activity have microbial densities 5–50 times greater in the rhizosphere than in bulk soil. The diversity of rhizosphere microflora is spectacular (Figure 3.12) and still incompletely described. Early studies in South Australia showed that Pseudomonas and Arthrobacter species are present on roots in ratios which reflect the rapid doubling times and competitive dominance of Pseudomonas (Bowen and Rovira 1976). Within genera, species of Pseudomonas have frequencies varying up to 500-fold. In general, gram-negative bacteria, fungi and ascomycetes (Section 3.4) are most abundant.

Rhizosphere microorganisms are not uniformly distributed along roots. Apices are almost free of microbes but densities can increase dramatically in subapical zones. Very mature root axes with lateral branches are sparsely populated with microbes. In maize roots, the area occupied by microorganisms is only 4% nearest the root apex, rising to 20% in subapical zones (Schönwitz and Ziegler 1986). Even within these zones, there are large variations in distribution, with radial epidermal walls of roots secreting exudates which can support huge microbial populations, up to 2 × 1011 microbes cm–3.

Roots do of course influence adjacent soil throughout their length by setting up gradients of water, gases and ions. For example, in waterlogged soils leakage of O2 from aerenchymatous roots leads to oxidation of metal ions and local build up of aerobic microflora around roots of agricultural plants (Chapter 18). In general, however, the most active microbial populations and rates of chemical transformation in the rhizosphere occur in the subapical zones of the root. In supporting these processes, root-associated microbes metabolise inorganic nitrogen, depositing protein nitrogen in the process of immobilisation. Microbial activity also produces plant growth regulators such as auxin, cytokinins and gibberellins, possibly in amounts sufficient to influence root morphogenesis. Ethylene can also be produced by rhizospheric fungi, potentially influencing root morphological changes such as lateral root initiation.

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