Ion Transport in Roots

Taiz and Zeiger (2002) give a full account of this topic. Mineral ions absorbed from solution outside the root surface must be transported across the root to the main long-distance transport vessels in the xylem, through which they reach the shoot. This process is highly specific for different ions and molecules and is closely regulated. The regulation is in part a fnnction of the anatomy of the varions root tissues and in part a fnnction of active transport processes in root cells. The pathways and transport processes are affected by root adaptations to anoxia. 

An ion entering a root may immediately enter the symplast by crossing the plasma membrane of an epidermal cell, or it may remain in the apoplasm and diffnse throngh cell walls. It may snbseqnently enter the symplasm by crossing 

Finally the ion mnst leave the symplast of the xylem and be loaded into the xylem s long-distance conducting vessels. The mechanism of xylem loading apparently involves both passive and active transfer from the xylem parenchymal cells. 

The immediate means of regulating ion transport and hence absorption into the root is throngh the control of active uptake across plasma membranes. Changes in root anatomy in response to changes in nntritional or other external conditions are necessarily slower. 

The cell plasma membrane separates the cell cytoplasm from the external medium. The composition of the cytoplasm must be tightly controlled to optimize cellular processes, but the composition of the external medium is highly variable. The membrane is hydrophobic and impedes solute diffusion. But it also facilitates and regulates solute transfers as the cell absorbs nutrients, expels wastes and maintains turgour. 

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