The first step in ion uptake by roots involves crossing the cell wall of the epidermis (Figure P5.1). Further radial movement across the root cortex may proceed along two pathways. The first one involves diffusion of ions in the continuum of cell walls called the free space or apoplast. The cortex, however, is separated from the vascular cylinder (the stele) by a layer of cells called the endodermis whose anticlinal walls are impregnated with suberin (Figure P5.1). These suberized walls, known as the Casparian strip, are impervious to water and nutrients. To bypass this barrier, ions must cross the plasma membrane of the endodermal cells to enter the stele. The second pathway involves transport across the plasma membrane of epidermal cells and subsequent diffusion across the cortex and endodermis to the stele in the continuum of cell cytoplasm called the symplast. The cytoplasms of adjacent cells are connected via plasmodesmata, which are tubular extensions of the plasma membrane that traverse the cell wall (see Figure W1.1, WATER RELATIONS). Irrespective of the pathway followed, once in the stele, ions diffuse in the symplast toward xylem conducting elements (tracheids and vessels) (Figure P5.1). To enter the xylem, ions must exit the symplast and reenter the apoplast because xylem elements are dead cells. Ions are carried in
the xylem by the transpiration stream to the aboveground plant tissues (again, see Figure W1.1).
Ions are transported across membranes (plasmalemma or tono-plast) with the aid of transport protein systems called pumps, carriers, or channels. Transport down an electrochemical potential gradient is termed passive, whereas that proceeding against the electrochemical gradient is termed active. The three transport systems are (1) the primary active transport system, (2) the secondary active transport system, and (3) the passive transport system.
The primary active transport system includes H+-ATPase (proton pump), which transports H+ out of the cytoplasm into the apoplast, and Ca2+-ATPase, which transports Ca2+ out of the cytoplasm. These processes require metabolic energy that is obtained by the hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP).
The secondary active transport system is driven by the electrochemical potential gradient for H+ across the plasmalemma, called the proton motive force (PMF). The PMF consists of electric and concentration gradients generated by the proton pump in the process of extrusion of H+ from the cell. The PMF-induced transport of H+ across the membrane is coupled with an accompanying ion, which moves against its gradient of electrochemical potential. When the two ions move in the same direction, it is called symport, and the protein mediating that movement is termed a symporter carrier. When the two ions move in the opposite direction, it is called antiport or exchange, and the protein involved is termed an antiporter carrier. For example, K+ is transported across the plasma membrane by a specific K+-H+ symporter when external K+ concentrations are low, and specific antiporters mediate absorption of Cl-, NO3-, and H2PO4-.
The passive transport system involves ion movement across the plasmalemma or tonoplast via ion channels down an electrochemical gradient. Calcium is believed to enter the cell in this way. The electric gradient generated by the proton pump across the plasma membrane (-120 to -180 millivolts) is believed to be large enough to permit most microelements to enter passively into the cell via specific ion channels.
Mineral nutrients are transported upward in the xylem and downward or upward in the phloem. Prior to xylem loading and translocation to the aboveground parts, NO3-N is reduced by nitrate reduc-tase and incorporated into amino acids and amides. The process requires energy and C skeletons, which are generated by photosynthesis. All mineral nutrients are highly mobile in the xylem, but N, K, Mg, P, S, Cl, and Na also move easily in the phloem. Microelements are partially phloem mobile, whereas Ca is considered to be immobile.
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