Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 81

such growth in plants, the respiration of food produced by
plants, is dependent on temperature-controlled processes,
such as gas exchange, water evaporation, and transpiration.
The rapid growth under high summer temperature under
non-limiting nutrient conditions can result in cells rapidly
stretching until they break.
The two factors that are the largest regulators of transpi-
ration are water availability and stomatal closure. When
sunlight is available, leaf stomatal resistance is low. When
sunlight is not available, leaf stomatal resistance increases.
For example, the optimum rates of photosynthesis occurred
in Tamarix when the air temperature was 23-28 C, which
occurred in the morning when ET demand remained low
(Anderson 1982). Photosynthesis decreased by 35% as the
air temperature increased.
The factors that affect transpiration include those related to
the pathway that water takes from the soil to the atmosphere
through the plant. This includes the soil in which the roots
grow, the availability of water in the capillary fringe or satura-
ted zone, the structure of the root and its hydraulic conductiv-
ity, the type of xylem in the trunk and branches, stomatal
resistance, and ultimately the moisture content of the air.
Many factors affect the removal of water vapor from
leaves. These include (1) climatic factors, such as the
amount of sunlight energy, the driving force in initiating
evaporation, the relative humidity of the air near the leaves,
the vapor pressure deficit, and wind speed; (2) water factors,
such as its availability in soil, and the hydraulic conductivity
of the soil to move the water to the roots, and (3) the
resistance of water flow through plants. Evaporation doubles
for every temperature rise of about 10 C, as we saw in
Chap. 2. If the air is drier than the gas spaces in the spongy
parenchyma, water loss from the leaf is greater than when
the air is more humid. Wind speed affects the humidity near
plants; higher wind speeds tend to remove humidity near the
leaf more quickly than calm winds and results in increased
transpiration.
If all these factors are held constant, the predominant
factor controlling transpiration is the bioavailability of
water in the subsurface. If water is not limiting, then the
maximum amount of water transpired will be constrained by
the solar energy input and VPD. Too little water for plant
uptake, however, can result in xylem cavitation in certain
trees, as has occurred in poplars exposed to drought
conditions. Essentially, this blocks water flow through the
xylem and results in shoot death (Tyree et al. 1994).
In a gross sense, the more biomass a particular plant has,
such as number and size of leaves, the higher its transpira-
tion is if water is not limiting. This was demonstrated in a
study by de Wit (1958) in which the amount of water
transpired by a crop during the growing season was linearly
related to the production of harvested biomass. Moreover,
the slope of the line was specific for different plants.

Tsao (2003) presented a range of transpiration rates for
grasses and woody plants that often are chosen for the
phytoremediation of contaminated groundwater. Grasses,
such as buffalo grass, winter rye, and alfalfa, ranged from
0.02 to 0.55 in./day (0.5 to 14.1 mm/day), and for woody
plants transpiration ranged from 0.3 to 100 gal/day/tree (1.1
to 378 L/day/tree). Nagler et al. (2003) and Wilcox et al.
(2006) report that saltcedar, cottonwood, and willow used
about 13.2 gal/day/tree (50 L/day/tree). These values do not
discriminate whether the transpired water was precipitation,
soil water, or groundwater, however.
Contrary to popular belief, transpiration by plants can
occur during the night and when plants are dormant. There
may be no leaves and, therefore, no stomata, but the stems
contain lenticels that are open to the atmosphere from at
least as deep into the tree as the phloem. Transpiration at
night is possible if certain climatic and water conditions
occur. Decker et al. (1962) reported for a stand of woody
phreatophytes, that evapotranspiration measured during the
night (8:30 p.m to 5:30 a.m.), was not zero but approached
11% of the daily evapotranspiration rate.
As stated in Chap. 2, transpiration can be estimated by a
number of different methods. Ferro et al. (2003) suggested

ET y f LA T ¼ ; (3.9)

where T is the water use (volume per tree), ET is a reference
evapotranspiration, y f is the water-use multiplier, and LA is

the leaf area of the tree. This equation is for an individual
tree but can be assessed at a forest scale by multiplying the
value for an individual tree by the area planted. The use of
this equation assumes no limitations on transpiration, such as
stomatal closure and disease.
As we saw above, transpiration is affected by factors that
determine the resistance of the diffusion of water from the
leaf to the air. The water potential of the air, C air ,is

C air ¼ RT = V w ln RH ð ; (3.10)

where R is the gas constant, T is temperature ( K), V w is the
partial molar volume of liquid water, and RH is the relative
humidity of the air, the fraction of current saturation relative
to total saturation at a given air temperature:

RH ¼ C wv = C wv at saturation ; (3.11)

such that relative humidity ranges between 0 and 1 and, if
multiplied by 100, provides the percentage of relative
humidity.
For leaves to perform gas exchange, the relative humidity
must be near 100%. As the air is often less than this value, a
gradient exists for water vapor to exit the leaf. This
continues as the wind removes saturated air, and drier air