Environmental Engineering Reference
In-Depth Information
u o )/ V , in Joules/m 3
Newtons/m 2
c ¼
Pa (pres-
sure). If pure water is placed in contact with a water-solute
solution of less chemical potential, it is possible to measure
the chemical potential of the water or the solute molecules.
The potential of water, however, typically is investigated.
Water potential,
( u w
¼
¼
, is the force of water in a system and
the ability of water to perform work, or in the case that we
are interested in, the potential for water to enter or exit a
plant cell. In other words, water potential is the potential
energy of the water per unit mass of water. Water moves
from higher
c
. Similar to the example of the
boulder moving freely from an area of high potential to
lower potential, the flow of water in a plant is from areas
of high water potential, or negative
c
to lower
c
c
, to areas of lower
c
water potential, or more negative
. The advantage offered
by using water potential as a measurement of the water status
of plants is that it is based on physical, rather than biological,
reference standards.
Water potential is composed of many other factors or
subcomponents that affect the movement of water in soils,
such as presence or absence of solutes, called the osmotic
potential,
Fig. 3.20 Typical water potential (
, in MPa) along the flow of water
from the subsurface to the atmosphere (Modified from Kozlowski and
Pallardy 1997). Plants that tap groundwater do not have to overcome
the stronger tensions in unsaturated soil.
c
c o ; the weight of the water present, or pressure
potential,
c p ; the affect of gravity on the water, or gravita-
tional potential,
c g ; and the extent of water adhesion to the
soil by surface tension forces and hydrogen bonding, called
the matric potential,
the unsaturated zone becomes more negative and approaches
field capacity, the hydraulic conductivity decreases as more
air is added to the system, which increases the soil tortuosity
encountered by the water. Conversely, the hydraulic conduc-
tivity of saturated sediments does not change, as long as the
sediments remain saturated. This fact provides an additional
advantage to facultative phreatophytes when soil moisture
decreases, as flow is not impeded under saturated conditions.
This status of water potential affects the rate of photosyn-
thesis and, therefore, transpiration. Too-negative water
potentials cause stomata to close in response to increased
resistance, which reduces transpiration and photosynthesis.
Under conditions of unlimited water, however, turgid cells
that have positive
c m :
c ¼ c pie þ c p þ c g
(3.17)
Figure 3.20 represents water potential and how it relates
to the potential for water to flow from soil to plant to the
atmosphere.
As shown in Fig. 3.20 , the flow of water is from the least
negative water potentials, or the wettest soils (but not
groundwater, because it is at atmospheric pressures rather
than under tension), to the area of most negative water
potential, the area of least water, such as the plant leaves.
Plants, in general, have negative water potentials because the
amount of free energy in the plants is less than free water
under the same conditions. When transpiration is not occur-
ring, the standing water potential as affected by gravity
should equate to about
c o can lead
to near-zero values for total water potential. This is because
the pressure potential,
c
and overcome the negative
c p , represents the hydrostatic pressure
in the system, which becomes negative as turgor is lost
during transpiration.
Relative humidity and water potential also are related and
have an effect on plant water status. The water potential of
the atmosphere is strongly negative throughout most ranges
of relative humidity, and water is lost from less-negative
water potentials in the leaves and to the air. When relative
humidity approaches 100%, however, the water potential of
the air decreases rapidly, becomes less negative, and
approaches zero. Therefore, little water loss by transpiration
occurs even though the stomata are open. This fact supports
what drives transpiration—the large difference in water
potential between the soil and the atmosphere.
0.01 MPa/m increase in height
(Woodruff et al. 2004). In other words, a 10-m tall tree
would have leaf water potential 0.1 MPa more negative
than the lower leaves. The water potential from the plant's
perspective is the measure of energy in each component of
water transport, from outside in the subsurface, to inside the
plant, and into the atmosphere. A decrease in water avail-
ability results in even more negative water potentials.
Because water potential can be used to examine water
flow, there is a relation between soil water potential,
, and
soil hydraulic conductivity, K . As the soil water potential in
c
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