RA 00110.pdf - OAR@ICRISAT

They are not strong competitors in the soil since they
die back to original population levels within a few
weeks following inoculation. They do not enjoy a
biologically unique or highly specific association
with their plant hosts. Root invasion is superficial at
best and appears to be limited to intercellular penetration
of the outer cortex and colonization of dead
cells. Hence, it must be characterized as primarily a
rhizoplane/rhizosphere phenomenon. It is clearly
established that nitrogen is fixed in this situation.
However, the activity is low, and the final product is
not released and transferred directly to the plant.
Most or all is immobilized in microbial tissue and
must be mineralized prior to plant utilization. The
small but consistently observed beneficial effects on
plant growth resulting from inoculation with Azospirillum
can be attributed largely to production of
PGPS by the microbes, analogous to the Russian
work discussed earlier. The microbes can apparently
compete successfully for carbohydrates or other
energy substrate sufficient to maintain a low population
level, but not enough to support a significant
level of nitrogen fixation. Their inability to establish
a unique, highly specific, and selective niche within
the root tissue denies them access to a constant high
supply of energy, optimum (microaerophilic) conditions
for fixation, and a mechanism for release and
direct transfer of the nutrient to the plant.
Utilization of Beneficial R - M
Associations
In considering the possibility of utilizing or agronomically
exploiting beneficial R - M associations
via inoculation, certain difficult questions must be
asked. Is inoculation biologically feasible In most
cases it is not. Successful inoculation is dependent
on the fortunate but rare coincidence of numerous
required circumstances or conditions.
Implicit in the information given above are compelling
reasons for the ability or inability to utilize
these three representative R - M associations on a
field scale. Rhizobium can be used because the unique
features of the plant-microbe asssociation make
it biologically possible. In the case of V A M fungi,
wide utilization on a field scale in future is being
established as a strong theoretical possibility, the full
realization of which may await only the development
of a medium for independent growth of pure
cultures of the V A M fungi in large quantities.
There is a sharp distinction between theoretical
(laboratory) and practical (field) success of inoculation.
The former demonstrates the existence of a
phenomenon and the potential for manipulation of
genetically based characteristics of the organism(s),
while the latter demonstrates the feasibility of adopting
inoculation as a standard management practice.
Utilization traditionally implies field inoculation.
Because in the majority of cases inoculation fails, an
early conclusion is that associative R - M systems
cannot be utilized directly, via conventional methods
of seed or soil inoculation, to enhance plant growth.
The best available scientists and facilities make such
inoculation systems work unpredictably, to a limited
extent, or not at all in carefully controlled and managed
laboratory, greenhouse and field conditions.
This cannot be expected from farmers on any scale.
These are strong negative statements but they have
one important and perhaps all-redeeming qualification.
The redeeming qualification is this: studies to date
indicate that various microbes, loosely associated
with plant roots, have the ability to enhance plant
growth by different means, including nitrogen-fixation
(a long term effect); PGPS production, leading to
enhanced root production and increased nutrient
uptake (an early short-term effect); and possible
inhibition of root pathogens. These are demonstrated
effects and, moreover, there is increasing
evidence that they are genotype-specific. This means
that they are controlled by plant genetic factors
(Neal et al. 1973). As suggested earlier, there is a very
real potential to utilize these systems, not by technological
manipulation of the microbes (inoculation)
but by manipulation of the plant genetic information.
Establishing and successfully maintaining a beneficial
R - M association by inoculation is the exception
rather than the rule. Success depends on meeting
certain unique requirements which rarely are met
naturally and even more rarely provided by man.
The organisms are there in the required diversity in a
dynamic soil environment which maintains the
diversity through the process of constant change.
The rhizosphere of a particular plant genotype provides
a relatively stable microbial niche or environment
created by unique biochemical properties of
the root. Such environments can be designed to
select specifically or predictably for specific microbes
or groups of microbes which will preferentially
thrive in that environment and, as a consequence of
their activities, enhance plant growth. This might
include for example, microbes functioning as nitrogen-fixers,
producers of PGPS, or microbes antagonistic
to root pathogens indigenous to the soil.
In order to accomplish this expeditiously, bota-
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