Environmental Engineering Reference
In-Depth Information
potential liabilities, which will be enumerated in Chap. 7—
briefly, the use of too much auxin can actually inhibit root
growth, much as the presence of auxin in the terminal meri-
stem enhances its growth but inhibits that of adjacent lateral
buds.
Another synthetic auxin was an impurity in the 2,4-D, or
Agent Orange, used to defoliate tropical plants in Vietnam.
In commercial applications, 2,4-D is a synthetic auxin that is
widely used as an herbicide to kill dicots but not monocots.
These hormones stimulate plants to essentially grow them-
selves to death through overstimulation of respiration, which
rapidly depletes carbohydrate stores at rates faster than the
production of new carbohydrate by photosynthesis. This
occurs because synthetic auxins can influence RNA tran-
scription and the production of proteins used for many
growth purposes in the cells.
In addition to the effect of these hormones on cell elon-
gation and respiration, there are other hormones that affect
cell division. One group of such hormones is the cytokinins.
In most plants, cytokinins are synthesized in the roots and
reach the shoots after transport in the xylem. Their effect on
cell division occurs between DNA replication and mitosis
and is relatively unknown.
with respect to seed dormancy because water is the solvent
in which cellular respiration takes place. Where there is
growth, there is respiration; even non-growing cells respire
slowly during dormancy. This is why trees retain water even
during dormancy; the water was taken up during periods of
previous transpiration, and loss of water to the atmosphere
during the dormant period is limited by the thick, waterproof
bark. The lenticels in the bark permit gas exchange to occur,
but this exchange is limited to the cortex and cambium just
beneath the bark and does not extend to the deeper xylem
except possibly by diffusion when water flow rates are at a
minimum.
Another example of dormancy and water availability is
the common mistletoe (
Viscum album
) plant. The word
mistel is the Anglo-Saxon word for dung, and the word toe
or tan means twig, so the plant typically used during the
Christmas holiday to promote amorous activity literally
means dung on a twig. This made sense to early plant
observers who could only fathom that the mistletoe in the
high branches of trees were there on account of bird
droppings that contained mistletoe seeds. In ancient times,
the fact that a green plant grew during the bleak winter gave
rise to its reverence by the Druids, who used it to ward off
evil spirits at the start of a new year.
From a plant-water relation viewpoint, the presence of
mistletoe in trees has important implications. Mistletoe
makes it own food but with water taken by the roots from
the host plant's xylem. It also uses the host plant for support,
sun exposure, and protection from predators. Mistletoe can
be found in most parts of a dormant tree, but its presence in
treetops is unequivocal evidence that dormant trees not only
contain water, but that water flow can support the mistletoe.
Another feature of these plants is that they contain toxic,
organic compounds called alkaloids. A few mistletoes do not
harm an otherwise healthy tree, but massive infestations can
lead to the tree's death.
Plants go dormant in colder weather but even in freezing
weather most trees do not die. This is because cold
temperatures induce genes to produce antifreeze compounds
in the cells to decrease the freezing point of water. This
process takes advantage of the colligative property of
water and can protect cells from death down to
3.1.7 Dormancy
Dormancy in plants is a period of slow or no growth and is
primarily a function of ambient air temperatures. If
temperatures are too cold for a particular species, the
enzymes that regulate the life of individual cells and operate
within a range of temperature will be affected and growth
will slow or cease.
As might be expected for multicellular organisms such as
plants that grow exposed above ground as well as below
ground, dormancy in different parts of a plant is achieved
under different temperatures. In general, dormancy for
above-ground buds occurs at temperatures around 45
F
(7.2
C) for a time period between 4 and 8 weeks. Roots
will still grow under these conditions, however, because
changes in soil temperature generally lag behind changes
in air temperature due to the insulating properties of soil and
the high specific heat capacity of soil moisture. Roots con-
tinue to grow after leaf loss because of respiration of stored
carbohydrate; growth will occur as long as oxygen in the
subsurface is available. As we will see later in Chap. 7, in
most parts of the United States, early fall is when many
woody plants are typically installed, because the root
systems are not dormant and continue to grow even while
the shoots remain dormant.
Dormancy also is controlled by water availability. This
fact is exemplified by dried seeds that can survive long
periods of time and still remain viable. Water is important
50
F
45
C). Unfortunately, the production of antifreeze
compounds by human cells does not occur, and such cold
temperatures cause ice to form in cells, or frostbite.
(
3.2
Roots
The height of a plant and its crown of leaves are usually the
most impressive features that catch our attention. The depth
and distribution of roots below ground, however, are equally
impressive but often less well recognized. Leaves can be
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