Agriculture Reference
In-Depth Information
XYLEM
CORTEX
EPIDERMIS
ENDODERMIS
PAREN-
CHYMA
TRACHEARY
EXTERNAL
SOLUTION
Apoplasm
Cytoplasm
Vacuole
Nucleus
Plasma
membrane
Tonoplast
Plasmo-
desmata
Casparian
strip
Figure 6.9
Idealized structure of a root showing apoplastic and symplastic pathways for
solute transport to the xylem
the plasma membrane of a cortical cell, and thence continue to the xylem. But
it cannot reach the xylem entirely through the apoplasm because its passage
is blocked by a suberized layer of endodermal cells called the Casparian strip.
These effectively block entry of water and mineral ions into the stele via the
apoplasm. They also prevent back diffusion of solutes from the xylem apoplast
to the cortical apoplast.
Finally the ion must 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 parenchy-
mal cells.
The immediate means of regulating ion transport and hence absorption into the
root is through the control of active uptake across plasma membranes. Changes
in root anatomy in response to changes in nutritional or other external conditions
are necessarily slower.
Membrane Transport Processes
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.
