Rhizosphere changes

Certain factors controlling metal fractionation in the soil rhizosphere are time-dependent. This is one of the reasons explaining some different results of root-induced rhizospheric changes in metal fractionation. For instance, using a rhizobox system, Chino et al. (1999) found that soluble copper increased near the root, while its total content showed little or no change. Cherrey et al (1999), however, reported a depletion of copper extracted with EDTA, DTPA or CaCl2 in the rhizosphere soil. 

Figure 7 Mixld for iron (Fe) deficiency induced changes in root physiology and rhizo-sphere chemistry associated with Fc acquisition in strategy I plants. (Modified froin Ref. 1.) A. Stimulation of proton extru.sion by enhanced activity of the plasnialemma ATPase —> Felll solubilization in the rhizospherc. B. Enhanced exudation of reductanls and chela-tors (carhoxylates. phenolics) mediated by diffusion or anion channels Pe solubilization by Fein complexation and Felll reduction. C. Enhanced activity of plasma membrane (PM)-bound Felll reductase further stimulated by rhizosphere acidificalion (A). Reduction of FolII chelates, liberation of Fell. D. Uptake of Fell by a PM-bound Fell transporter.

H. Mar.schner, Root-induced changes in the availability of micronutrients in the rhizosphere. Plant Roots The Hidden Half (Y. Waisel, A. Eshel, and U. Kakafi, eds.), Marcel Dekker, New York, 1991, p. 503. 

This suggests that it may be possible to manipulate the rhizosphere flora through genetic changes in the host plant. Of particular interest is whether different varieties, by exuding different compounds, can influence the rhizosphere flora in a way that would benefit the plant. 

R. Merckx, J. H. Van Ginkel, J. Sinnaeve, and A. Cremers, Plant induced changes in the rhizosphere of maize and wheat. Plant and Soil 96 85 (1986). 

H. Marschner, V. Romheld, W. J. Horst, and P. Martin, Root-induced changes in the rhizosphere importance for the mineral nutrition of plants. Z. Pflanzenernarhr. Boclenk. /49 441 (1986). 

As already noted by Campbell and Greaves (16), the rhizosphere lacks physically precise delimitations and its boundary is hard to demarcate. Dimensions may vary with plant species and cullivar, stage of development, and type of soil. Soil moisture may affect the measurable size of the rhizosphere as well wetter soils may stick better to roots than drier soils (Fig, 1). This will change the volume of soil regarded as rhizosphere soil upon separation of rhizosphere from bulk soil and thus alter the measured concentration in rhizosphere and non-rhizosphere soil of a response variable in exudate concentration or microbial production. 

There are indications that the variety of C substrates in the rhizosphere soil is basically too wide to be significantly affected by changes in quality of plant residues from the previous crop. For example, legumes as preceding crop were shown to increase significantly microbial diversity in the bulk soil, as estimated by Biolog assay, whereas in the rhizosphere soil this effect of legumes could not be detected (145). 

In conclusion, there is very little evidence that even if changes are found in the rhizosphere, the.se changes will remain and actually change the wide biological potential of. soils. A major concern is the lack of reference samples—i.e. what is the natural variance in biodiversity and what are the rates of change as they occur, for example, under intensification of agricultural land u.se. 

E. Paterson, J. M. Hall, E, A. S. Rattray, B. S. Griffiths, K. Ritz, and K. Killham, Effect of elevated COj on rhizosphere carbon flow and soil microbial processes. Glob. Change Biol. 5 363 (1997). 

Siderophore production by rhizosphere bacteria that are culturable on agar media has been estimated by plating out colonies on an indicator agar medium containing chrome azurol, a weak iron-chelating complex that changes color upon... 

Other species choose instead to bet on timing, on the rapid exploitation of nutrient abundance, as for example when a full river of nutrients is passing by, under the form of the growing root of a higher plant. Indeed in the proximity of roots, in the rhizosphere, the underground landscape radically changes. 

To describe the diffusion of solutes in the rhizosphere, where concentration gradients change with time, /, as well as space, mass conservation is invoked with the spatial geometry appropriate for the cylindrical root (8) ... 

Figure 3 Root-induced changes in pH in an agarose sheet obtained 2 h after embedding a maize root in the gel. Note that while the root lip is making its local rhizosphere more alkaline, the more basal portion is acidifying the rhizosphere. (From Ref. 102.1...

---