Transport roots, channels

Kinetics may play an important role in pesticide adsorption (28, 39). Rapid transport of pesticide in large soil pores (rocks, void root channels, worm holes, etc.) could take place, thus exposing the pesticide to only a fraction of the adsorption sites. Since the organic matter content decreases with depth, the occurrence of macropore flow could result in pesticide movement beyond the surface layers where most of the CM resides. 

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.

As noted in Sect. 10.1, heterogeneities play a dominant role in the migration of contaminants in the subsurface. Nonuniform, preferential patterns of flow and transport are ubiquitous. It is important to recognize that, at the field scale, contaminant movement generally is very difficult to anticipate. In natural soils and aquifer materials, macropores, soil cracks and aggregates, fissures, solution channels, root paths, and wormholes, as well as variable mineral composition (e.g., clay aggregates... 

The results of this calculation and comparison to a single-channel recording are presented in Figure 7. The root-mean-square level, which is required to explain 1/f noise in membrane current by transport phenomena through the open channel, is much higher than the actual level. The conclusion is that 1/f noise in an open channel (if it exists) cannot account for 1/f noise in a multichannel membrane. 

Dordas C, Chrispeels MJ and Brown PH (2000) Permeability and channel-mediated transport of boric acid across membrane vesicles isolated from squash roots. Plant Physiol 124 1349-1361. 

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