Rhizosphere Subject

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. 

Since his early pioneering work on the rhizosphere (a term Hiltner used to describe specifically the interaction between bacteria and legume roots), our knowledge of the subject has greatly increased, and today perhaps a more appropriate definition of the rhizosphere is the field of action or influence of a root (1). The rhizosphere is generally considered to be a narrow zone of soil subject to the influence of living roots, where root exudates stimulate or inhibit microbial populations and their activities. The rhizoplane or root surface also provides a... 

I. PRODUCTION OF ANTIBIOTICS. The production of secondary metabolites with antimicrobial properties has long been recognized as an important factor in disease suppression (see Chap. 7). Metabolites with biocontrol properties have been isolated from a large number of rhizosphere microorganisms, including the fluorescent pseudomonads (Table 2). Further discussion is not given here since this is the subject of recent reviews (122,123). 

Fresh organic matter plays a fundamental role in plant nutrition by supplying nutrients released through degradation processes however, humified organic substances also become a source of nutrients when subjected to mineralization processes. The main aspects of the cycle of organic matter at the rhizosphere soil are reported in Chap. 6. 

A. A. Meharg and K. Killham, Carbon distribution within the plant and rhizosphere for Lolium perenne subjected to anaerobic soil conditions. Soil Biol. Biochem. 22 643 (1990). 

These results were obtained for oilseed rape grown in the same acidic soil as above but which had been subjected to various rates of lime application, in order to investigate a broader range of pH values. A substantial decrease in Cu solubility as a consequence of rhizosphere alkalinization was also found consistently for tomato grown in two acidic Cu-contaminated vineyard soils, including the one described above, in another study where soil solution Cu was sampled by Rhizon microsamplers (Cornu et al., 2007). Similarly, a decrease in... 

Siderophores are low molecular weight compounds that chelate Fe3+ and transport it into the cell. Competition for iron appears to occur between pseudomonads and the rhizosphere bacteria and fungi. Three unusual siderophores with antifungal activity have been described from fluorescent pseudomonads, pyochelin (9), S-dihydroaeruginoic acid (10) and aeruginoic acid [13]. The importance of siderophores in biological control has been the subject of some debate [14]. 

Rhizoboxes. As also recognized by Helal and Sauerbeck (1981), in all of the procedures dealing with plants grown in pots, the main problem in obtaining a rhizosphere soil sample is the subjectivity of the separation. The use of pots filled with artificial substrates, such as calcined sand and clay (Warembourg et al, 2003), facilitates separation of the rhizosphere soil from the bulk, but it is not very real to life. 

To determine the contents of REEs (La, Ce, Pr, Nd, Sm, Eu) associated with different chemical compartments of the soil, both bulk and rhizosphere soil were subjected to selective sequential dissolution according to the following procedure, modified from Berna et al. (2000) ... 

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