VII. TRANSPORT TO THE SHOOT
Since the classical experiments of Stout and Hoagland [178], it has been accepted that mineral ions are
transported from roots to shoots in the xylem. This pathway also applies to organic nitrogen compounds
that are products of ammonium fixation and of symbiotic N 2 fixation [179]. The mechanism of this trans-
port is mass flow in the aqueous xylem solution. The control of mineral ion transport to the shoots occurs
during their passage across the roots to the xylem.
A. Transport Across the Root
Water moves across the root in the symplast, in the apoplast, or in both. The root symplast [180] com-
prises the plasmodesmata-connected cytoplasmic continuum of the root cells; it does not include the vac-
uoles. Symplastic mass flow across the root traverses two membranes: one upon entering the symplast
and another upon exiting to the xylem. Biological membranes do not constitute a serious barrier to the dif-
fusion of water, but they impede the free passage of the mineral ions dissolved in the water. Thus, sym-
plastic movement of solutes across roots can be regulated by metabolically controlled transport across
these membranes [181].
Apoplastic water flow across the root occurs in the root cell-wall continuum. The apoplast [180] is
the free space continuum throughout the plant. This continuum is interrupted by the Casparian bands,
which are hydrophobic incrustations in the radial and transverse walls of the endodermis [182]. Water by-
passes the Casparian bands by movement across the membrane into the endodermal symplast and out
again. Mineral ions that are dissolved in the water also bypass the Casparian bands and migrate across the
membranes of endodermal cells. However, membrane transport of the mineral ions is involved. Whatever
course mineral ions choose, they traverse two membranes, where control may occur. Solutes moving to
the xylem must be absorbed into the symplast, from the medium at the root surface, or from the apoplast,
by either cortical cells or the cortical face of the endodermis. The solutes move out again into the stelar
apoplast or the xylem, at the stelar face of the endodermis or from the stelar parenchyma, respectively.
These membrane transport processes are qualitatively and quantitatively controlled.
Vacuoles of root cells are another site for selective regulation of solute transport across the root. Vac-
uoles accumulate various solutes, thus removing them from the symplastic stream. In particular, most of
the cellular Ca^2 is sequestered in the vacuoles, while the cytosolic Ca^2 concentration is maintained very
low [104]. Under saline conditions, much of the Clis also sequestered in the vacuoles, where it serves
for turgor regulation. The same applies to Nain plants that have conserved the Na/Hantiporter that
is needed for Natransport to the vacuole [122]. Other solutes are stored in the vacuoles when available
in excess of requirement. They may then be released again upon demand [183].
B. Effects of Transpiration
Haberlandt [184] concluded more than a century ago that transpiration is not of major importance for
plant mineral nutrition. This has since been reconfirmed repeatedly (see, e.g., references in Refs. 185 and
90). Indeed, the amount of solutes transported to the tops of plants should not be affected by the rate of
water flow if membrane transport-dependent delivery of solutes to the xylem is the rate-limiting process.
A low solute concentration would be expected in the xylem sap at high transpiration rates and a high con-
centration when transpiration is low. This seems essentially to be the situation under conditions of low
external salt concentration and low salt status of the roots [185] and for solutes that are recognized by the
transport proteins (not xenobiotics).
Broyer and Hoagland [185] stated in 1943 that delivery of ions to the xylem is metabolically con-
trolled, whereas upward movement in the xylem is passive mass flow. They found that in plants of high
salt status the delivery of the salt to the xylem is rapid and the velocity of mass flow in the xylem may be
the rate-limiting process. Consequently, variation in the rate of transpiration may affect salt transport to
the shoots under saline conditions. A good correlation between transpiration and transport to the shoot
was also found for some nonessential elements such as cadmium [186] and silicon [187] and for xenobi-
otic organic compounds [188]. The transport proteins of the root may not recognize these solutes. They
are apparently transported in a fraction of the apoplastic mass flow that bypasses the Casparian strips
[189,190]. It was suggested that such bypass occurs when solutes enter at sites of secondary-root emer-
gence [189,190] or through the apical region of the root [191]. Calcium ions that reach the xylem sap may
MINERAL NUTRIENT TRANSPORT IN PLANTS 353