RA 00110.pdf - OAR@ICRISAT

millet, to study penetration and the effect of systemic
fungicides. Plantlets have been successfully regenerated
in the laboratory from dual cultures (Prabhu
1985). Plants from such cultures were transferred to
soil and tested for their reaction to the downy mildew
pathogen, and the test indicated that the plants
were resistant to downy mildew. Further research is
in progress. In addition, the behavior of susceptible
and resistant pearl millet callus was studied on
media mixed with different concentrations of downy
mildew diseased leaf extract containing Sg-toxin.
The callus on the medium with Sg-toxin turned
black, whereas the medium with healthy extract
supported callus growth. This work is continuing.
Cytology
Nuclear behavior of the inductive fungus and formative
phases was studied cytologically by fixing the
material in Farmer's solution and staining with acetocarmine
and iron haematoxylin. When the knoblike
sporangiophore structures began to emerge
from stomata, nuclei migrated into them from
hyphae located in the leaf tissue. There was a great
rush of nuclei to gain entry into the branches of
sporangiophores. As a result, the nuclei often appeared
thread-like. Once nuclei entered into the sporangiophores,
the nuclei regained their normal shape.
Subsequently, the nuclei from the sporangiophores
migrated into the sporangia as soon as they were
formed. Three to five nuclei entered each sporangium.
Sometimes up to 13 nuclei were observed in
each sporangium. A l l the nuclei were functional and
a zoospore was formed around each nucleus. Zoospores
were liberated from the sporangium through
an opening in the region of the papilla and the liberation
process was over within 5-10 min. There was no
nuclear division in the zoospores, sporangia, and
sporangiophores. When released from the sporangium
the uninoculated zoospores moved forward
and rotated on their axes. They were of different
shapes and sizes. The zoospore wall was more distinct
after the contents were emptied into the germ
tube. There is a 30-45 min time lapse between the
formation of germ tubes and migration of nuclei into
them. Some of the germ tubes were without nuclei.
When appressoria were developed, nuclei occupied
the apical region of the germ tube. Nuclear divisions
are common in the germ tubes of zoospores (Safeeulla
1976b).
Differences in nuclear contents of sporangia of 5.
graminicola were observed in the two different
pathogenic races reported by Shetty and Ahmed
(1981). The number of nuclei in the sporangia of the
pathogenic race occurring on NHB 3 varied from
2-5, with 2 most frequent. In contrast, the number of
nuclei in the sporangia of the pathogenic race occurring
on Mysore pearl millet cultivar Kalukombu
varied from 3-13. Six was most frequent, but in no
case was the number less than three. The nuclei were
smaller and more or less round. This study suggests
that the nuclear cytology may be race dependent in
S. graminicola.
References
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