Geoscience Reference
In-Depth Information
Figure 6-6.
Willow (
Salix
sp.) trees behind cattails.
Willow converts the harmful byproducts of anaerobic
glycolysis into pyruvic and glycolic acids (Dugan 2005).
Photo by J.S. Aber; shore of Lake Kahola, Kansas,
United States.
Figure 6-5.
Looking downward into slow-l owing
water i lled with submerged water milfoil
(
Myriophyllum
sp.). Field of view
∼
2 m across. Photo
by J.S. Aber; Wascana Creek, Regina, Saskatchewan,
Canada.
growth (Fig. 6-6). Still, most hydrophytes can
survive only for short periods in this manner.
In saline, tidal, or marine environments, salt
is another factor that limits most hydrophytes.
Plants that tolerate high salinity are known as
halophytes. They have few competitors and may
l ourish in such environments. Certain man-
groves (
Rhizophora
sp.) have specialized root
cells that block sodium while allowing desirable
nutrients (potassium) to pass through (see Fig.
6-4). Other mangroves, such as
Avicennia
marina
, and cordgrass take up salt and secrete
excess salt from their leaves. Saltcedar (Fig. 6-7),
for example, may grow in soil with salinity up
to 50‰ (sea water is
chloroplasts are found near the surface of leaves
and stems. The amount of sunlight that pene-
trates determines how deeply plants could grow.
This depends on water turbidity (suspended
i ne sediment) and light wavelength (Lahring
2003). In clear water, blue-green light penetrates
deepest; for medium turbidity yellow light
reaches deepest, and in highly turbid water only
orange-red light is able to penetrate deeply.
The depth at which one percent of surface light
reaches is usually the lowest limit that may
support photosynthesis (Mitchell 1974).
∼
35‰). Excess salt is col-
lected in special glands in the leaves and then
excreted onto the leaf surface; when these
leaves fall to the ground, they contribute to soil
salinity (Zouhar 2003). Red samphire (
Salicor-
nia rubra
) is another hydrophyte that takes salt
into its body (Fig. 6-8).
The i rst step in photosynthesis for most
kinds of vegetation is the production of phos-
phoglyceric acid (C
3
H
7
O
7
P), which is a three-
carbon compound. Hence, these species are
known as C
3
plants. Many hydrophytes, however,
produce oxaloacetic acid (C
4
H
4
O
5
), a C
4
com-
pound. C
4
plants are more efi cient for both rate
of carbon i xation and water use. Increased
water efi ciency means that C
4
plants need less
water. This reduces the rate at which potentially
6.1.2 Biochemical adaptations
Even with all the structural features noted above,
some hydrophytes have insufi cient oxygen for
normal plant respiration. This is particularly
likely to happen during periods of prolonged
l ooding, saturation or submergence. Some
plants are able to switch to respiration without
oxygen. Anaerobic glycolysis converts food into
energy, but at a much lower rate than normal
respiration and with the added problem of toxic
byproducts such as alcohol and acetaldehyde
(Niering 1985; Welsch et al. 1995; Dugan 2005).
Some plants are able to excrete these com-
pounds through i nely divided roots, and others
are able to immobilize or convert the toxic
compounds into organic acids that are used for
