RA 00110.pdf - OAR@ICRISAT

located through the root phloem. In exchange,
nitrogen fixed in the bacteroid tissue is released
to the root xylem which then translocates it
throughout the plant. Required substrates and
products of the process are thus exchanged with
high efficiency, in the absence of factors which
would diminish or eliminate the beneficial effect,
such as the presence of other organisms competing
for the final product.
These, then, are the main characteristics of the
Rhizobium-legume association which make it an
R - M system beneficial to the host plant. The same
characteristics make it successful as an agronomically
exploitable system. A comprehensive treatise
on the associated technology of its utilization is
available (Somasegaran and Hoben 1985). Briefly,
this is accomplished by inoculation of seed or soil at
planting time with selected rhizobia strains of established
quality. This greatly enhances the chances for
early onset of infection, nodulation, and nitrogen
fixation. Successful attempts to manipulate this system
are directly proportional to the knowledge that
has accumulated through prolonged basic and applied
study of the nature of the system (Vincent 1982,
Broughton 1982, Alexander 1984). Successful inoculation
of legumes with rhizobia is by no means inevitable,
but its high success rate is more than sufficient
to justify its use in standard agronomic practice.
This system remains the best understood example
of a biologically intimate and beneficial R - M association.
Its closest competitor for this honor is the
ubiquitous root-fungus associations referred to generically
as "mycorrhizae". These R - M associations
are critically important in enhancing root uptake of
phosphorus when available soil phosphorus is limited.
This second example of a biologically intimate
and beneficial R - M association has also been long
recognized. A subgroup, ectomycorrhizae, alter the
morphology of infected roots in a visually distinctive
way. This subgroup is found predominantly on the
roots of woody perennial plants and is therefore of
lesser interest agronomically.
The other major subgroup, the endomycorrhizae
or vesicular-arbuscular mycorrhizae ( V A M ) , occur
on the roots of most herbaceous plants and virtually
all of the agronomically important crop plants. For
this reason alone they are worthy of exhaustive longterm
investigation. They are no longer studied
merely as a biological oddity, but rather as a symbiotic
R - M association which plays a significant and
often critical role in the survival, growth, and productivity
of agronomic crops. We currently have
enough information to appreciate their overwhelming
potential value in this respect.
Progress in the study of V A M has been rapid and
substantial. However, since the V A M fungi are obligate
symbionts on plant roots, our current inability
to grow the fungi alone in massive amounts severely
restricts the quality and quantity of both basic and
applied experimentation. Virtually all "pure culture"
studies must be conducted with spores, laboriously
sieved from the soil, or with a mixed inoculum
of spores, hyphae, and root fragments derived from
the roots of axenically maintained potted plants.
Until this limitation is removed, progress in basic
understanding and agronomic exploitation of the V A M
will be held largely in abeyance. This current limitation
will eventually be removed by such systematic
studies as those of Siqueira et al. (1985). The current
dilemma is probably because researchers do not
know enough about the nutritional requirements of
the fungal symbiont. The numerous and complex
roles of these fungi have been discussed recently and
comprehensively by Bagyaraj (1984), Tinker (1984),
Subba Rao and Krishna (In Press), Krishna (1985)
and many others. Applied aspects of V A M have
been recently reported (Ferguson 1984). A current
and thorough treatise on the methodology associated
with mycorrhizal research is available (Schenck
1982).
Aside from the beneficial effect of enhancing plant
growth, this R - M association bears scant resemblance
to the previously described Rhizobiumlegume
system. The V A M fungi are ubiquitous and
normally infect their respective host plants under
most conditions permitting plant growth. However,
they produce no macroscopically visible sign of
infection (i.e., altered root morphology), and their
presence is detected only by microscopic observation
of root tissue.
Although the Rhizobium-legume and V A M associations
are functionally similar (providing a critical
mineral nutrient which often limits plant growth),
there are important dissimilarities between them.
The rhizobia have an inexhaustible supply of the
nutrient (atmospheric nitrogen), a supply of energy
(photosynthate generated by sunlight) required to
reduce the nitrogen to a utilizable form, and a closed
system which permits a direct and efficient transfer
of the nitrogen to the plant host.
The V A M fungi, on the other hand, do not generate
utilizable phosphorus as a plant nutrient. They
merely increase plant uptake of available phosphorus
via hyphal exploration of a much larger soil
volume, followed by increased uptake of available
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