Environmental Engineering Reference
In-Depth Information
roots, the diffusion of water into the root hairs at least will
equal the removal of water by evaporation from the leaves.
These physical processes of diffusion and osmosis are
passive processes, because no energy is spent by the plant
to facilitate the entry of water; the solute concentration in the
cell increases, the water concentration decreases, and water
enters the cell by osmosis. Increased pressure in the cell
results from the water entry—during osmosis, plant cells
can obtain pressures up to 15 atms (atmospheres), which is
great enough to explain why sidewalks and driveways near
trees often are lifted.
When an individual plant cell encounters waters and it
passes through the cell wall and cell membrane, water and its
solutes, which will be discussed later, can then enter the
storage compartments of the cell vacuoles. The vacuoles
themselves are contained within a selectively permeable
membrane called the tonoplast. Vacuoles contain a dilute
concentration of sugars and salts, also known as sap, which
is about 98% water. Because of its solute concentration and
lower water concentration, or high osmotic pressure and low
osmotic potential, dilute water can enter the vacuole. In
general, other plant cells contain a more concentrated solu-
tion inside separated by a semipermeable cell membrane
from the external, more dilute solution of the cytoplasm.
Water entering through the plasmalemma creates an
internal pressure, called turgor, in the vacuole, that is exerted
on all parts of the cell structure. This makes the plant rigid;
plants that are in a water deficit appear limp and wilted
because they lack turgor. Turgor is analogous to tire inner
tubes that hold air; upon inflation of the inner tube with air,
or the plasmalemma with water, pressure is exerted outward
on the inside surface of the inner tube, or cell wall, which
make both rigid.
Wilting also can occur, however, when cells are exposed
to a higher concentration of solution external to the plant
relative to the internal solution. The water potential is higher
in the plant and lower outside, and water exits the plant cells,
a process called plasmolysis. Moreover, the removal of
water by transpiration at faster rates than water uptake in
the roots causes the same wilting phenomenon, because it
reduces the hydraulic pressure. Water may be bioavailable,
but it is being removed at a faster rate than it can be taken up.
Conversely, the water may be held onto soil grains at too
high tension levels for removal by plants.
Once sufficient osmotic pressure to produce turgor has
taken place and the cell membrane presses against the cell
wall, excess water is pushed into the cortex at the same rate
that it is taken in by osmosis. This is the same cortex
penetrated by the hyphae of ectotrophic mycorrhizae.
Because the cortex has a greater concentration of solutes, it
also has a lower, more negative water potential than the
epidermis or area outside the root. Thus, water flows to the
endodermis, and from there to the xylem where the flow
Fig. 3.11
The loss of liquid water, rather than water vapor, by plants is
called guttation and is a result of root pressure caused by osmosis when
soil water is not limiting and relative humidity is high (Photograph by
author).
creates a slight pressure, called root pressure. Although this
pressure can move water to low heights, this process does
not supply water to the tops of most plants. This pressure is
responsible, however, for the occurrence of water droplets
on the tips of leaves, a process called guttation, when suffi-
cient water supplies are available to meet and exceed the
transpiration demands of a plant (Fig.
3.11
). Typically, water
exits plants as vapor from the leaves, but here water exits as
a liquid.
3.4.2 Solute Entry
The mechanism that drives the passive uptake of water by
osmosis in the root zone is primed by the initial intake of
solutes by root cells and the resultant increase in intra-cell
solute concentration. This is important especially in terms
of water limitation, because a cell that has a higher solute
concentration will be able to take up water at greater
negative water potentials than a cell with a lower solute
concentration.
As stated earlier, the semipermeable structure of the plant
cell membrane is the key component to the survival of
individual plant cells and, hence, the total plant. Plants
have the disadvantage of not being able to move, at least
great distances at appreciable rates, to reach new water
sources, and most water sources are characterized by an
extremely dilute solution of solutes. Hence, the concentra-
tion of these substances, such as salts, is much lower outside
of the plant cell relative to inside the plant cell. Even in areas
where the salts are more concentrated in solution, such as
near the ocean, the cells of plants in such areas, for example
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