Environmental Engineering Reference
In-Depth Information
in that they can penetrate smaller pore spaces between soil
grains that may contain large volumes of water, such as
when substantial amounts of clay are present as either
aggregates or layers in the soil.
The density of root hairs varies with the type of plant and
prevailing environmental conditions, such as water avail-
ability and oxygen content. For example, trees can have
from 20 to 500 root hairs/cm
2
, and grasses as much as
2,500 hairs/cm
2
(Kramer 1983). When exposed above
ground, root hairs are covered by cutin, and the root cap is
covered by mucigel; this polysaccharide-rich material acts
as a lubricant for root-hair penetration into tight soils. As the
root increases in length, the zone of root hairs also moves,
with new hairs forming near the growing tip and old hairs
dying at the end of the root-hair zone.
Because individual root hair cells contain water and
solutes, the water potential in the root hairs is lower than
outside the root hairs; consequently, water enters the root
hair by osmosis as previously discussed (Fig.
3.5
). After this
initial entry of water, the root hair cell is now at higher water
potential than adjacent cells, and water moves from the root
hair to these cells and ultimately into the cellular vacuole
and xylem. The root hair becomes depleted in water after
this transfer and is, therefore, receptive to more water entry
from outside the root hair as long as water is bioavailable. If
this occurs along a series of interconnected cells, such as the
xylem, a mass flow of water will occur.
In general, water that enters the root hairs can enter the
internal xylem tissue of the plant through two mechanisms:
(1) through the cell walls and membranes of the individual
cells that compose the outer layer of epidermal cells in the
roots, or (2) between the cell walls of individual epidermal
cells (Fig.
3.12
). As previously discussed, the first mecha-
nism requires water to go through the living cytoplasm of
cells and is called the symplastic pathway. The second
method has water going between intracellular spaces and is
called the apoplastic pathway. The apoplastic pathway of
water is similar to finding one's way through a maze. For the
apoplastic pathway, soil water enters the root between adja-
cent cell walls of epidermal cells (Fig.
3.12
). Water passes
from cell wall to cell wall of the epidermis and cortex cells
by diffusion. Upon reaching the endodermis, however, water
must pass through the Casparian strip similar to the
symplastic pathway. The Casparian strip cells are not per-
meable to water. As such, the membranes of these cells
regulate the entry of water, solutes, and gases into the plant
as a whole. Within the Casparian strip and outside the stele is
the pericycle cells, which give rise to branch roots that grow
through the cortex into the sediment.
Various analytical and numerical models have been
developed in attempts to simulate the relation between
plant roots and water and solute uptake. Somma et al.
(1998) developed a three-dimensional model of root water
and solute uptake. Water uptake is simulated as being
affected by water potential and osmosis and solute uptake
is simulated by passive and active uptake. In essence, plant
transpiration and solute assimilation are coupled through
water-use efficiency data.
3.4.4 Effect of Rhizosphere Surfactant Release
on Water Uptake
The production of organic exudates along root sheaths and
tips increases the solute concentration in the soil water and,
therefore, can affect water surface tensions. Water is under
considerable tension in the unsaturated zone because of the
presence of air in the pore spaces. As the concentration of
organics increases in the soil water from plant roots, such as
sugars and phosphatidylcholines, the surface tension of
water becomes lowered and may facilitate the entry of
water into plants (Passioura 1988; Read and Gregory
1997). Compared to pure water, a surfactant and water
solution will decrease surface tension between 10% and
50% (Read et al. 2003).
For plants, the surfactant property of root exudates is espe-
cially beneficial as the soil profile dries, for the exudates will
have a greater ability to wet the remaining water under higher
tensions and thereby lower the tension to facilitate plant
uptake. Surfactant addition to the unsaturated and saturated
zones of the subsurface has been shown, however, to decrease
the diffusivity and hydraulic conductivity of the sediments.
3.4.5 Hydraulic Lift and Water Redistribution
The vertical movement of water from roots deep in the
unsaturated zone, capillary fringe, or saturated zone and
redistribution of this water to shallower roots is called
hydraulic lift (Richards and Caldwell 1987). The physical
basis behind hydraulic lift is best explained in terms of the
various components of water potential in the root zone. In
areas where transpiration and evaporation remove soil mois-
ture from shallow surface soils, the total water potentials
decrease or become more negative. Deeper soil with higher,
less negative water potential then moves upward toward the
surface to replenish the lost water. Much of this occurs at
night when transpiration and evaporation cease. This upward
movement of water from deeper wetter zones to a drier
unsaturated zone is a principal mechanism of unsaturated
flow in some arid regions (Andraski et al. 2005). Of course,
this process only continues as long as there are deeper
sources of water in the capillary fringe with less negative
water potential or groundwater at atmospheric pressures. If
this source of water is beyond root growth or dries up, the
plants wilt and will not recover the following day.
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