also utilize these latter pathways. Most of the Ca^2 that enters the symplast via Ca^2 channels is se-
questered in the vacuoles and thus prevented from moving to the xylem. The relative amount of water by-
passing the Casparian strips varies between a few percent [192] and more than 10% [186]. It seems to in-
crease under conditions of stress damage [192]. For metabolically transported solutes at low external
concentration, the amount transported in the bypass flow is an insignificant fraction of the total transport
to the xylem. It may, however, constitute the whole amount of a xenobiotic solute that is transported to
the tops [186].
C. Resorption and Exchange Binding
Solute transport in xylem vessels is driven strictly by mass flow. Nevertheless, the composition of xylem
sap varies along its path. Two processes, absorption from the xylem sap by adjacent living cells and bind-
ing by xylem walls, are responsible for these variations: The walls of xylem vessels, like other cell walls,
contain immobile negative charges. These charges constitute a Donnan phase that retains cations moving
in the vessels. In particular, polyvalent cations such as free Ca^2 [193], Zn^2 [194], and Fe^2 and Fe^3
[195] are retained. Polyvalent cation retention can be diminished, or prevented, by chelating agents
[195,196] and by displacement with similar or other cations [196].
Stout and Hoagland [178] demonstrated absorption of solutes from the xylem by surrounding tissues.
Such absorption of Nais rather pronounced and selective in some plants [197] and has been shown to
depend on energy metabolism [198,199]. Xylem-parenchyma transfer cells are apparently involved in
Naabsorption from the xylem sap and its transfer to the phloem [200,201].
VIII. SUMMARY
Mineral nutrients in the root cortex-apoplast equilibrate with those in the root medium. These mineral ions
are then transported into the root symplast, moving across the plasma membranes of epidermis, cortex, or
endodermis cells. Depending on the specific ion, this transport is facilitated by passive uniport through
channels, by carrier-mediated cotransport with protons, or by primary active transport.
Once in the root symplast, the ions may be transported into the vacuole, across the tonoplast, or ex-
creted to the xylem, across the plasma membranes of xylem parenchyma cells. Some mineral ions, such
as Naand Ca^2 , may also be reexcreted from the symplast to the apoplast. In the xylem, mineral ions
and other solutes move by mass flow, primarily to the leaf apoplast. There, the ions are again absorbed
into the symplast and vacuoles of leaf cells. Secondary transport from the leaves to sinks occurs in the
phloem, together with the products of photosynthesis. Metabolic transport across the roots, to the xylem,
regulates the amount of mineral ions conveyed to the tops. Normally, this amount is very little affected
by the velocity of xylem sap flow.
All the membrane transport processes mentioned depend on energy metabolism. A part of the Ca^2
transport is driven by a primary active Ca^2 -ATPase. Most of the mineral ion-transport processes are
driven by the proton motive force, which is composed of an electrical potential gradient and a pH gradi-
ent across the membrane. At the plasma membrane, a P-type ATPase generates the proton motive force.
At the tonoplast, two pumps that function in parallel, a V-type ATPase and a pyrophosphatase, generate
it. The latter may also pump potassium into the vacuole. Membrane-bound channels or carriers utilize the
proton motive force and facilitate the membrane transport of mineral ions. Genes encoding some of the
transport proteins have now been sequenced.
ACKNOWLEDGMENT
During the composition of this chapter, BJ’s laboratory was supported by the Endowment Fund for Ba-
sic Sciences: Charles H. Revson Foundation, and NM’s laboratory was supported by grant 108/97 from
the Israel Science Fundation, both administered by the Israel Academy of Sciences and Humanities.
REFERENCES
- DG Nicholls. Bioenergetics. London: Academic Press, 1982, p 189.
- PS Nobel. Physicochemical and Environmental Plant Physiology. San Diego: Academic Press, 1991.
354 JACOBY AND MORAN