Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 65

soils. In the upper soil layers, such as the O and A
zones, bacteria can approach 10 5 to 10 8 cells/g of dry
soil because of the presence of roots. As a result,
concentrations of CO 2 in the rhizosphere are higher relative
to areas without roots. The root uptake of water physically
concentrates water near roots, and roots produce organic
compounds that act as surfactants to lower water surface
tension; this reduction in water tension provides an advan-
tage to rhizospheric microbes that seek to avoid strongly
negative water potentials, which would stop the diffusional
uptake of nutrients necessary to microbial life. Such a rela-
tion between plants and increased bacterial numbers in
response to water availability is particularly noted during
drought conditions, such that rooted areas act as oases of
water for microbes.

Fig. 3.9 The wider base than trunk of this baldcypress ( Taxodium
distichum L.) in Congaree National Park, near Columbia, South
Carolina, illustrates the adaptation of this particular species to the low
oxygen levels that characterize the saturated sediments in which these
trees grow. The increased surface area of the wider base permits oxygen
diffusion at rates to equal root respiration (Photograph by author).

3.3.1 Rhizosphere Bacteria and Nitrogen
Fixation

Probably the most investigated relation between plants and
the rhizosphere involve the soil bacterium Rhizobium . These
bacteria interact with plants after entrance through root hairs
during the seedling stage and extend into the cortex. The
Rhizobia are beneficial to plants because the bacteria reduce
gaseous nitrogen (N 2 ) in the soil air (although present at
concentrations near 80%, nitrogen is not bioavailable for
plant uptake) into nitrogen that is available to plants.
Rhizobia are associated with the roots of a class of plants
called the legumes, such as clover, beans, and alfalfa. Prior
to the manufacture of inorganic sources of nitrogen in
fertilizers, the addition of manure and interplanting of alfalfa
and clover among other crops were the principal methods of
delivering nitrogen to non-leguminous crops. Moreover,
today it is common practice to rotate fields of leguminous
crops with non-leguminous crops.
This interaction between plants, bacteria, and nitrogen
availability raises the question of how did this interaction
develop? After all, if plants are able to fix atmospheric CO 2
why didn't plants also develop the ability to fix atmospheric
N 2 ? The answer, in part, lies in the fundamentals of chemical
bonds. Atmospheric N 2 is held together by a triple bond
between the two nitrogen atoms—much energy is needed
to break a triple bond. Plant-associated bacteria provide the
source of energy needed to break the triple bond and make
the nitrogen available to plants. Moreover, the bacteria that
can break this triple bond can do so only in the absence of
oxygen; in the presence of oxygen the enzyme that the
bacteria use to beak the triple bond and fix N 2 is oxidized
and rendered unfit. This creates a dilemma: plants require
nitrogen to survive, but plants also require oxygen to support
respiration. Thus, a compromise evolved. The nitrogen-
fixing bacteria form large nodules on the roots of most

content of the soil and, therefore, create a microniche for
heterotrophic bacteria. The zone immediately surrounding
the roots that supports these bacteria was called the rhizo-
sphere in 1904 by Lorenz Hiltner (in Anderson et al. 1993).
It was later discovered that this rhizosphere exists first to
benefit the plant but also benefits the bacteria.
The rhizosphere can extend up to 0.397 in. (10 mm) from
individual roots. Another definition of the rhizosphere is that
it comprises the thickness of soil that remains attached to
plant roots if they are exposed and the plant is shaken. The
rhizosphere is occupied by a variety of bacteria that make
available atmospheric nitrogen to the roots for uptake by
reducing, or fixing, atmospheric nitrogen, a process that
most plants cannot do by themselves. The rhizosphere also
is occupied by a variety of fungi, or mycorrhizae, that aid the
plant in the uptake of minerals and water; this is in contrast
to other fungi and bacteria that often can infect plant roots.
The heterotrophic bacteria and fungi in the rhizosphere, in
turn, receive excess organic matter from the plant as a source
of carbon and energy. In many ways, the symbiotic relation
between plants and microorganisms was an evolutionary
negotiation between the heterotrophic bacteria intent on
damaging the plant and the plant intent on decreasing this
threat to survival. One consequence of the beneficial relation
between plants and root microbes is reflected in the habit of
many older gardeners that pour sugary soft drinks on the
ground near roots of some plants to facilitate this interaction.
(Probably the most famous organism that inhabits the rhizo-
sphere is the truffle.)
The number of microbes in planted soils can be at least an
order of magnitude greater than the number in unplanted