Environmental Engineering Reference
In-Depth Information
Not all invasive plants that use copious amounts of
groundwater are related to the western United States, how-
ever. Another non-native woody plant found in the United
States that is characterized by fast growth rates and is,
therefore, considered to be invasive is the melaleuca
(
Melaleuca quinquenervia
). It also hails from Australia and
grows about 6 ft (1.8 m) per year. Introduced into south
Florida in 1906, it spread slowly but was greatly enhanced
after planes were used to disperse seed throughout the
Everglades. In 1993, it was estimated that melaleuca had
spread to cover nearly 400,000 to 1.5 million acres
(1.61
previously had provided the plant with food. This process
is called photoperiodism and is controlled by a light-
sensitive protein called phytochrome. Ethylene, essentially
a plant hormone in gas form, promotes leaf drop by increas-
ing the production of an enzyme that breaks down the
cellulose in plant-cell walls.
Other environmental changes, or stresses, can induce the
production of hormones in order to aid the survival of the
plant. Abscisic acid (ABA) is a hormone that promotes plant
dormancy and leaf abscission, and it is produced by leaf cells
under conditions of water stress. The production of ABA, in
turn, causes the stomata guard cells to close, which reduces
transpiration and photosynthesis.
Another common example of the effect of hormones on
plant growth is contained in a growing shoot on any land-
scape plant. In the spring, an increase in length occurs in the
terminal bud, formed the previous year. Other buds, how-
ever, also are present along the stem behind the main termi-
nal bud. The question, then, is what keeps these buds from
growing as rapidly as the terminal bud or not at all in some
cases? The terminal bud releases a hormone that suppresses
the growth in other buds, a process called apical dominance.
If the terminal bud is removed, the source of the hormone
that has inhibited the growth of the other lateral buds is
removed, and the lateral buds start to grow. This hormone
is formed in the cell cytoplasm, but transported throughout
the plant.
Another hormone that relates to cell elongation is gibber-
ellin. It encourages cell elongation especially in stems. One
of the possible reasons that it is advised to put aspirin in a
vase of cut flowers is because the aspirin, or acetyl salicylic
acid, acts as a surrogate hormone to induce the cut-end stems
of flowers to root.
The hormone responsible for terminal bud elongation and
root tip elongation is called auxin, or indoleacetic acid
(IAA). Auxin is synthesized by the plant by conversion of
an amino acid called tryptophan. Apparently, the purpose of
the IAA hormone in a particular cell is to make the cell wall
more pliable and to promote cell elongation. When exposed
to light from one direction only, auxin production is induced
in the shaded side of the terminal bud, and these cells
elongate at a faster rate than the cells exposed to light with
the net effect being that the bud bends toward the light
source; this process is called phototropism. Other plant
auxins include the synthetically derived naphthalenacetic
acid.
Auxin also can be transported from the shoots through the
phloem to the roots to stimulate root meristem growth. This
process can be forced by gardeners or horticulturalists by
addition of synthetic auxin, orindole-3 butyric acid at a
concentration of 0.1%, to plant cuttings prior to installation
in soil media. Such an approach also can be applied to
cuttings used in phytoremediation projects, but it has some
10
9
m
2
) of the Everglades, spreading
at a rate of 50 acres (202,000 m
2
) per day. As would be
expected, water losses also are higher in areas where mela-
leuca has invaded. They are hard to kill, and any threat to
their survival leads the tree to release millions of seeds. It is
perhaps ironic that the one person responsible for encourag-
ing the use of melaleuca, as well as kudzu, in the United
States, was the famous plant scientist David Fairchild, who
spent his career introducing plants to the United States (see
Fairchild (1938) for more information on kudzu and mela-
leuca introduction, as well as for compelling reading).
There are other reasons why some plants grow faster than
others, and it has to do with survival, competition, and
reproduction. To ensure survival over generations, however,
seed production and dispersal are the main driving forces for
accelerated growth and height. But the main one for some
plants that use wind for seed dispersal, such as cottonwoods,
oaks, and poplars, is height—they need to be tall to take
advantage of the wind for a wide range of seed distribution
and survival. Willow (
Salix spp.
) seedlings were found to
have greater survival rates in fine-textured sediments high on
point bars relative to coarse-grained sediments closer to
surface-water fluctuations (Gage and Cooper 2004). The
roots of seedlings need to reach the water table, which
requires adequate water and nutrient availability, and may
take several years.
If growth occurs in the tips of roots and shoots and the
cambial layer where the meristem cells differentiate, what
controls do the plants have, if any, on growth? In general,
plant growth is controlled by hormones, which are organic
molecules synthesized by the plant but not used directly for
energy. Hormones can accomplish their work at very low
concentrations. They can either increase or decrease plant
growth, often in separate tissues of the plant at the same
time. Hormones can be produced in one tissue and exert an
effect in another tissue, sometimes a great distance away.
An excellent example of the influence of hormones on
plant growth is the control of the color and lifespan of leaves
of deciduous trees in temperate forests. As the growing
season ends, the plant responds to lower levels of solar-
radiation intensity and shorter days by stopping the produc-
tion of chlorophyll and then disposing of the leaf that
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9
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