3.3.3  Costs and benefits of a rhizosphere

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Root function and overall plant performance can benefit conspicuously from processes in the rhizosphere. Infection by rhizobia (Section 3.5) and mycorrhizal fungi (Section 3.4) improve the nutritional status of many species and rhizobial strains have even been used to manipulate rhizosphere biology. Under natural conditions, intense microbial competition occurs in the rhizosphere, as seen by the success of Pseudomonas spp. discussed in Section 3.3.2, but the variability of soils, plant species and environmental conditions make it impossible to predict rhizosphere composition.

A significant proportion of photoassimilate is used to support a rhizosphere, reflecting the high cost of microbial activity and polymer exudation. This pattern is repeated in many species with up to 20% of plant carbon consistently lost by roots. Relative rates of microbial and root respiration are almost impossible to estimate in roots growing in undisturbed soils because of the intimacy of roots and microbes. In addition to consuming large amounts of plant carbon, microbes produce phytotoxins which can impose further restrictions on root function.

Mechanisms describing how a rhizosphere benefits its host are even more elusive because of the diversity of reactions in such a small space. Chelation is identified as a major influence on nutrient acquisition and might also help ameliorate ion toxicities. Physical properties of the rhizosphere are even less well understood, with questions such as root lubrication, root–mucilage shrinkage and interfacial water transport not yet resolved. Limited data do not support earlier notions of mucilage as a water-holding matrix and while secretions might help roots advance through soil, most friction is thought to be between root axes and cap cells (Bengough and McKenzie 1997). Physical properties of mucilage do not suggest it is an ideal lubricant. Whether the dynamic properties of a rhizosphere bring constant benefits to a plant or simply passively coexist with growing roots remains a critical question.