Environmental Engineering Reference
In-Depth Information
conductivity,
K
, also describes the flow of a fluid, ground-
water, through porous sediments.
As water passively enters the root hairs by diffusion, the
water can only reach the xylem after first passing through
multiple cell walls, the symplastic pathway, or simply
through cell walls and spaces, the apoplastic pathway.
Water encounters these cell membranes, which provide a
large resistance to flow, along its path. Once in the xylem,
however, this resistance is no longer present, as the xylem
contains no cytoplasm or cell membranes. In roots, this
resistance is quantified as root hydraulic conductivity,
L
p
,
and can be estimated using
L
p
¼
W
v
=Dc
(3.4)
where
W
v
is the velocity of water transport from one cell to
another and
is the difference in water potential between
cells; the concept of water potential will be more fully
described later in this chapter.
Quantification of root hydraulic conductivity provides a
meaningful way to compare the magnitude of the degree of
resistance to diffusional water flow between cells, in volume
of water per unit area of membrane per unit time per unit
driving force (m
3
/m
2
/s/MPa). The velocity of water trans-
port,
W
v
, from one cell to another can be described by
Dc
W
v
¼
L
p
Dð :
(3.5)
Fig. 3.8
The generalized relation among depth, root density, and root
hydraulic conductivity common to many phreatophytes. The lower
root density near the water table is compensated for by increased root
hydraulic conductivity.
Over time, intracellular water movement will decrease as
the difference in water potential,
, between the cells
decreases and additional water movement ceases.
Tap roots often are characterized by much higher root
hydraulic conductivities relative to shallower roots. The tap
root of most plants may grow to greater depths but are fewer
in number relative to the more abundant roots in the shallow
parts of the soil horizon, especially in temperate, well-
drained nutrient-poor soils. This is the case for longleaf
pines (
Pinus palustris
) in the Coastal Plain geophysical
provinces of the United States, where tap roots average
about 60 ft (180 m) in length (Heyward 1933; Pessin
1939). The higher root hydraulic conductivity may result
from a difference in physiology of these deeper roots com-
pared to shallow roots, where the tap roots have long and
continuous xylem (Le Maitre et al. 1999). The higher root
hydraulic conductivity also may reflect the increasing poten-
tial to encounter the water table, and subsequent increase in
water potential. Therefore, the fact that fewer roots are at
depths nearer the water table is compensated for the roots
having higher root hydraulic conductivities (Fig.
3.8
).
There also is a relation between root hydraulic conduc-
tivity, root age, and hydraulic conductivity of the soil. For a
given plant, older tap roots tend to have lower root hydraulic
conductivities than younger tap roots, as the former contain
Dc
more suberin. Moreover, vigorous tap-root growth actually
can increase the hydraulic conductivity of the soil, both
aerially and vertically. For example, in Australia,
researchers measured the hydraulic conductivity of silty-
clay soil in the unsaturated zone beneath and just outside
of a tree plantation and found that the hydraulic conductivity
of the soil was lower outside than inside the plantation
(Rural Industries Research and Development Corporation
2000). The researchers also measured the hydraulic conduc-
tivity at various depths beneath the plantation. Although
there was a gradual decrease in hydraulic conductivity with
depth, the highest hydraulic conductivity values were
associated with the root zone (Rural Industries Research
and Development Corporation 2000).
3.2.5 Effect of Redox Condition on Roots
Plants are autotrophic, aerobic organisms; plants require
oxygen during respiration to release the energy stored in
the food made during photosynthesis. In most terrestrial
environments with little sedimentary organic matter, oxygen
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