THESIS

the investigations revealed that the failure of seed set in hybrids was due to the ovule
abortion. In order to overcome the sterility, the colchicine has been applied to the
interspecific hybrids.
The pollen fertility of S. americanum x S. torvum (59.34 %) (Table 21) was
lower than their parent but not too much different. This result could not confirm that
the hybrids from the crosses between S. americanum x S. torvum were true hybrids or
not; we have to confirm with poildy levels determination and also the morphological
characters.
The hybrids wihin S. americanum x S. americnum gave lower pollen fertility
than their parents. Similar results have been obtained for F1 hybrids from the same
species combination (Rajasekaran, 1970; Rangasamy and Kadambavanasundaram,
1974). The low pollen fertility of the F1 may be attributed to meiotic difficulty arising
from hybridity of the nucleus (Isshiki and Kawajiri, 2002).
Pollen of F1 hybrids between S. americanum x S. villosum was sterile (~
0.00 %) (Table 21). The variations in the level of low pollen fertility could be due to
either the differences in genetic constitution or cryptic structural differences between
the parent chromosomes, which might be too small to be detected cytologically or due
to combined effects of both factors (Rao, 1981; Anis et al., 1994). Moreover, the
hybrids of S00859 x S00860 and its reciprocal between S. americanum x S. villosum
had very high pollen fertility (Table 21), this could be due to both of accessions were
very closely related but the ploidy levels determination and the morphological
characters have yet to be confirmed.
In cantrast, pollen fertility of the interspecific hybrids between S. americanum
x S. nigrum and S. villosum x S. nigrum were insignificantly lower than their parents.
This also has to confirm the interspecific hybrids by polidy levels determination and
the morphological characters.
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