Environmental Engineering Reference
In-Depth Information
cortex. Mycorrhizae that exist on the outside of roots are
called ectotrophic mycorrhizae (Bowen 1984). Endotrophic
mycorrhizae are more common than ectotrophic
mycorrhizae, and they are found in more than 90% of the
world's herbaceous plants (Dhillion and Zak 1993). Endo-
trophic bacteria rarely are associated, however, with woody
plants, which tend to be dominated by ectotrophic
mycorrhizae. In fact, some tree species, such as oaks,
maples, and hickories, have few to no root hairs and, there-
fore, rely solely on ectotrophic mycorrhizae for water
uptake.
Mycorrhizae are believed to be useful to plants in that the
mycorrhizae's mycelium, or hyphae, increase the total sur-
face area of the roots for increased uptake of water and
minerals (MacFall 1994). Ectomycorrhizae form a mantle
around the root or between cells in the cortex so that they
actually inhabit the space between cells of the plant root. In
this manner, plants use ectomycorrhizae to access previously
unavailable minerals in decaying leaf litter at the ground
surface. The fungi commonly seen on the forest floor and
called toadstools or mushrooms provide visual evidence of
the presence of mycorrhizae in the rhizosphere of a nearby
tree, as these mushrooms are the fruiting bodies of fungi
(Fig. 3.10 ). These fungi are carpophores, the reproductive
part of the underground mycellium.
The mutually beneficial interaction between plants and
fungi can probably find its earliest beginning in a humble
plant that many pass by today unaware—the lichen
discussed previously and shown in Fig. 3.10 . The part of
lichen visible to the eye is the fungal part; it is believed that
this acts to keep the sequestered algal cells inside from
drying out. The fungi provide the algae access to water and
minerals, which can be absorbed from sources other than the
ground, such as precipitation, rocks, and bark, in a manner
analogous to the mycorrhizae of tree roots.
Mycorrhizae benefit plants because of enhanced water
uptake as well as other processes necessary for plant sur-
vival. As we saw earlier in this chapter, dissolved iron is
essential for the production of chlorophyll. The iron encoun-
tered by most plant roots is in the oxidized form and, there-
fore, not bioavailable. To increase iron bioavailability and to
facilitate iron uptake, plants and mycorrhizae produce
organic acids that can chelate iron and increase iron solubil-
ity; this process is discussed in Chap. 11. Also, it is possible
that fungi degrade organic matter in the soil and release
stored nutrients for uptake by trees. In return, the fungi
essentially hitch a ride toward new sources of water and
soil organic matter as the roots grow.
Some rhizosphere bacteria can release substances to
decrease the germination of certain seeds relative to other
seeds for which they are better suited. Such allelopathic
relations and the implication for plant exposure to ground-
water contamination is discussed in Chap. 11. Fungi may
Fig. 3.10 The mushrooms often seen on the ground at the base of trees
are the fruiting bodies of mycorrhizae. A similar symbiotic association
is provided above ground by lichen that contains algae and fungi.
also protect the tree from bacterial root pathogens, as they
are rendered inactive by the antibiotic effect of root fungi.
Another geochemical consequence of plant and
mycorrhizae interaction is seen in flooded soils charac-
terized by little dissolved oxygen. Although oxygen from
the atmosphere can diffuse through the plant cortex to sup-
port root respiration, the production of toxic hydrogen sul-
fide (H 2 S) from sulfate reduction in anoxic soils can be a
detriment to plant growth. Sulfur-oxidizing bacteria such as
Beggiatoa , associated with plant roots in these environments
can oxidize the H 2 S to harmless levels and, therefore,
remove the threat to plant growth. This relation has impor-
tant implication for phytoremediation plantings where the
trees will interact with anoxic, contaminated groundwater.
3.4
Roots and Water Absorption
Plants, on average, are greater than 80% water by volume.
A germinating seed will send a single meristem upward to
capture light and CO 2 , but the seed would die if it didn't also
send downward a single root meristem to capture water. The
same seed could have remained dormant, sometimes for
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