RA 00110.pdf - OAR@ICRISAT

phosphorus by the roots via direct hyphal transfer.
In soils where total phosphorus is low and the element
is largely removed rather than recycled, this
inevitably leads to a "mining effect". Eventually,
phosphorus must be added, whether the V A M are
present or not. However, this may not be the case if
phosphorus is present in large quantity in a "fixed"
or insoluble form.
Having identified the role or importance of these
two classic examples of symbiosis, it now becomes
important to consider why these divergent R - M
associations are biologically successful. By understanding
why these systems work, study of other
potentially beneficial R - M associations becomes
more rational. There is great predictive value in
understanding the basic concepts of the functioning
of the mycorrhizae and Rhizobium-legume systems.
Briefly stated, these two symbiotic R - M associations
are biologically successful because of the high
degree of intimacy which has evolved between
the organisms which has evolved. In both cases,
morphological, physiological, and biochemical characteristics
have evolved in such a specialized way
that external factors mitigating against or limiting
the beneficial effects on plant growth are minimized.
Within limits, the biological mechanisms for nitrogen
fixation in nodulated legumes, and phosphorus
uptake in mycorrhizal plants are resistant (not
immune) to adverse or process-limiting influences.
Cryptic R - M Associations
It was pointed out above that plants growing in
natural soil have their roots exposed to diverse, indigenous
soil microorganisms. These R - M associations
are either overtly detrimental to the plant
(pathogenic), or beneficial, as in the two examples of
symbiosis documented above. Considering the number
of different plants and microbes involved, there
must be an infinite number of R - M associations, the
nature and significance or role of which must be
considered. It has been suggested that there are
many potentially beneficial R - M associations which
lie in the gray area of possible, equivocal, or unverified
existence. These have been referred to as "cryptic"
associations since their presence and/or effects
are not visually obvious (Hubbell and Gaskins
1980). Suggested possibilities were the effects on
plant growth of rhizosphere microbes which produce
plant growth promoting substances (PGPS) or
fix nitrogen as free-living organisms, or both.
Such possibilities are not hypothetical but their
recognition was not easy. Early work in Russia
attempted to demonstrate enhanced plant growth
(crop yield) by soil inoculation with free-living,
nitrogen-fixing and/or phosphate-solubilizing microbes.
Their results largely failed statistical tests
and independent confirmation (Mishustin and
Naumova 1962). However, certain g r o w t h -
enhancing effects, although small, have been confirmed.
These effects are now generally attributed to
the production of PGPS in the seedling rhizosphere
by the inoculated microbe(s) (Brown 1974). Subsequently,
a number of papers dealt with plant growth
effects produced by free-living, nitrogen-fixing microbes.
These reports were suggestive but inconclusive.
However, a landmark paper (Dobereiner and
Day 1976) bestowed scientific credibility on the concept
of 'associative' nitrogen fixation applied to the
Azospirillum-Digitaria system. The potential agronomic
significance of this research generated an
immediate international response among investigators
which exists, at abated levels, even to this day.
Numerous publications have covered this topic in
recent years (Vose and Ruschel 1981, Gaskins et al.
1983, Knowles 1983, Hubbell and Gaskins 1984,
Elmerich 1984, Wani 1985, Wani and Lee 1986).
Most of the research to date has not been done on
pearl millet, but it does not diminish the value or
validity of the generalizations which can be drawn
from studies of "associative" nitrogen-fixation in
grasses other than Pennisetum spp.
Early enthusiasm for the agronomic future of
associative nitrogen fixation has dwindled. Initial
estimates of nitrogen fixed and crop yield increases
attributed to these associations were either unrepeatable
or were found to have been badly overestimated
when attempted in other laboratories (van
Berkum and Bohlool 1980, Lethbridge et al. 1982).
Belatedly, much of this can be attributed to the
absence or inappropriate use of technology and a
truly exasperating level of biological variability
inherent in the system. Much of the difficulty may
have been that initial field studies were designed on
the assumption that the associative system was amenable
to the same inoculation concepts and methodology
applied so successfully with Rhizobium. It
was not. The reasons are based on the biological
nature of the association.
The associative microbes, such as Azospirillum
(and numerous others) are ubiquitous in agricultural
soils, but occur in low numbers, and are most frequently
found in the rhizosphere of many tropical
grasses and other plants exhibiting a C 4 metabolism.
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