Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 80

reduce the surface area of the leaf by up to 50%. Although
this is a common adaptation to thin-bladed plants, such as
grasses, broad-leafed plants respond in a similar manner but
typically wilt the entire leaf surface at a steeper angle to
incoming radiation. Moreover, these processes are reversible
once water potentials are restored.
Even if water is unlimited, transpiration has been shown
to not be constant because of changes in the atmospheric
conditions that may cause the stomata to close.
Van Hylckama (1968) presented data from a study site
near Buckeye, Arizona, where if windspeed and tempe-
ratures increased each day, even if water was available, ET
decreased. He concluded from these observations that
widely held assumptions of the constant rate of water use
by riparian vegetation cannot be supported. These results
have implications for the assumption of constant water-
use rates by trees when simulated as part of some
phytoremediation models.
For tall trees, very negative leaf water potentials increase
stomatal resistance or decrease conductance, and this affects
the stable isotopic signature of the carbon being fixed as
sugars. Koch et al. (2004) analyzed the stable carbon
isotopes of leaf tissues collected from the upper part of
redwood trees and found that relative to the stable carbon
value of the lower leaves, the upper leaves were enriched in
the heavier carbon isotope ( 13 C). For most plants, there is
enrichment in the light carbon isotope ( 12 C), because the
lighter carbon isotope reacts faster in the carbon fixation
reaction during photosynthesis. At higher elevations, how-
ever, where stomatal resistances also are higher, less CO 2 is
available, and all is used regardless of whether it is isotopi-
cally light or heavy. The tissue in the upper leaves of the
redwood samples was

Fig. 3.15 The basic structure of a leaf that depicts the location of the
guard cells that surround the stomata on the underside of leaves. Some
phreatophytes, such as poplars, have stomata on both sides of the leaf.

Water transported to the leaf through the xylem enters the
leaf through vascular bundles, more generically known as
veins. The veins are primarily found in the spongy paren-
chyma, because that is where the spaces between cells facil-
itate gas exchange. Conversely, photosynthetic products are
removed from the leaf and transported to other parts of the
plant by the phloem.
Water can be translocated up the xylem to leaves and
within the various parts of a tree. This internal translocation
of water often is seen in trees with branches that are exposed
more strongly to the east. Water stored in the plant during the
night moves from the western part of the plant to meet the
earlier ET demand in the eastern part of the plant (Daum
1967).
But let us return to the movement of water from the soil to
the atmosphere through a plant. Because the palisade layer
of cells in the mesophyll is the location of photosynthesis,
these cells have a high concentration of sugars and, hence,
low water concentration or potential. The water potential
in the veins, attached to the mesophyl and xylem, is higher
than the mesophyll. Hence, water enters the mesophyll cells
and the water that is not used to support turgor or used in
photosynthesis is removed as water vapor. These vaporized
water molecules are replaced continually according to the
Cohesion-Tension Theory.
Changes in the water potential of a leaf not only changes
stomatal conductance but also the shape of the leaf itself.
Many plants respond to water deficits or high atmospheric
temperatures or low humidity by leaf rolling. This acts to

22.2 permil; by comparison, most
C 3 plants have tissue stable carbon isotope values near 27

permil. These differences in carbon isotopes and their use in
monitoring the phytoremediation of contaminated ground-
water systems is further explained in Chap. 15.
Stomatal regulation also is under some control of plant-
produced hormones. For example, the hormone ABA
accumulates in plant shoots deprived of water. This accumu-
lation, in turn, causes the stomata to close (Nilson and
Assmann 2007). These researchers go on to review some
of the insights gained from the recently sequenced genome
of Populus .

3.5.4 Factors Affecting Transpiration

There is a saying in Iowa that on a hot summer's night, you
can hear the corn grow. Although at first glance this seems to
be a mere exaggeration, there is some truth to this statement.
For example, growth is a biologically mediated reaction that
has higher rates at higher temperatures, and the energy for