Processes in Roots and the Rhizosphere

Root products, as defined by Uren and Reisenauer (17), represent a wide range of compounds. Only secretions are deemed to have a direct and immediate functional role in the rhizosphere. Carbon dioxide, although labeled an excretion, may play a role in rhizosphere processes such as hyphal elongation of vesicular-arbuscular mycorrhiza (39). Also, root-derived CO2 may have an effect on nonphotosynthetic fixation of CO2 by roots subject to P deficiency and thus contribute to exudation of large amounts of citrate and malate, as observed in white lupins (40). The amounts utilized are very small and, in any case, are extremely difficult to distinguish from endogenous CO2 derived from soil and rhizosphere respiration. 

In this chapter we review the current literature available on the influence of root exudates on rhizosphere microbial populations and the effects of plant, microbial and soil factors on the processes of rhizodeposition and microbial colonization and activity. We first give a brief overview and definitions of some of the main concepts relating to the rhizosphere and rhizodeposition. 

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