RA 00110.pdf - OAR@ICRISAT

covariance analysis and to establish the energy requirement for N 2 fixation, and that the crop should be
studied as a community of plants. Subba Rao said that in general, leaf area need not be considered for
acetylene reduction assay.
Relationship between soil temperature and nitrogenase activity is important, but this may not determine the
time of inoculation. The possibility of 33° C as optimum for N 2 fixation, may be indirect because the same
temperature is also optimum for leaf expansion. It was suggested these factors should be studied further. It was
also clarified that nitrogenase activity is maximum around flowering, and is likely to be affected by drought at
this stage.
Quantification of nitrogen fixed by bacteria was discussed. Under field conditions, it could be 10 kg ha -1 N,
but some pot-culture studies using 15 N have indicated that 17% of total plant N at low N levels is derived from
the atmosphere by nitrogen-fixing bacteria. Subba Rao said that besides fixing nitrogen, these bacteria could
have other positive effects on plant growth.
Bagyaraj stressed the importance of mycorrhyza in agriculture. He said it is very important to develop
methods of culturing mycorrhiza in the laboratory, and until such time, mycorrhiza research may be only of
academic interest. Two areas that are not well understood are whether mycorrhiza needs recurring applications,
and the residual effects of inoculation. Bagyaraj said that mycorrhiza-infected plants are resistant to
parasitic fungi such as Sclerotium, possibly because mycorrhiza produce certain orthodihydroxy phenolic
compounds.
Krishna presented the mycorrhiza work in progress at ICRIS A T . During the discussions, it was pointed out
that currently in Africa, acid treatment of rock phosphate is being tried, and it was suggested that the
economics of acid treatment of rock phosphate, and the alternate method of rock phosphate and mycorrhizal
fungi need to be established. To a question on the mode of action of mycorrhiza, Krishna said that mycorrhiza
do not solubilize rock phosphate, but these fungi exploit more of the available P.
Questions were raised on the interaction of mycorrhiza and other plant pathogenic fungi. Krishna clarified
that mycorrhiza colonization offers resistance only to root pathogens. He also defined susceptibility, which
indicates infection by mycorrhiza, and responsiveness, which indicates plant response.
Doubts were raised about whether mycorrhiza produce phytoalexins, and thereby protect roots from root
diseases. Subba Rao wondered if there is a genotype x strain interaction, but Krishna said that he is looking for
horizontal susceptibility, as in the case of endemic diseases. He also explained that infected roots show greater
affinity for P, and mycorrhiza increase the root surface area. Wani said that since one hybrid without
mycorrhiza performed well, why not select such lines Krishna clarified that the plant is required to produce
less energy per unit surface area explored when inoculated with mycorrhiza, compared to the energy required
for root production and nutrient exploration.
Tilak summarized the interaction of mycorrhiza and Azospirillum, and said that while inoculation with a
mixture is advantageous, mycorrhizal inoculum production is a major constraint. The culture filtrate of
Azospirillum enhances the spore germination of mycorrhiza, and it was suggested that this information may
help to culture the V A M in the laboratory. Tilak clarified that he used 1500 kg ha -1 inoculum, and the
Azospirillum inoculation could contribute about 10 kg ha -1 N, and that the remaining 10 kg ha -1 N could be
applied. At higher N levels, Azospirillium has no effect.
Subba Rao summarized the session: V A M could be seen inside the root, but cannot be cultured, while in
contrast, Azospirillum can be cultured, but cannot be located inside the plant cell. Both microorganisms have
immense potential to increase millet production, and in both Indian and African agriculture, inoculation with
these microorganisms is very important, and research in this area should be strengthened. Methods to culture
mycorrhiza need to be developed immediately.
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