40 Papaya
endosperm. By embryoculture two diploid plants were obtained which turned out to
be hermaphrodite.
2.7.6 Mutation Breeding......................................................
Bankapur and Habib (1979) attempted mutation in papaya through radiation and
observed that doses of gamma rays ranging from 5 to 15 kR were able to produce
significant changes in characters such as seed germination, survival of seedlings and
ratio of male to female sexes in the population.
Ram (1983) and Ram and Srivastava (1984) exposed dry papaya seeds to 10–70 kR
gamma rays and observed that the percentage germination and emergence decreased
significantly with increasing doses, and 50 kR and above doses were lethal. Ram
and Majumder (1981) evolved a dwarf mutant line by treating papaya seeds with
15 kR gamma rays. Initially, three dwarf plants were isolated from M 2 population.
Repeated sib-mating among the dwarf plants helped in establishing a homozygous
dwarf line which was later named ‘PusaNanha’. It is dwarf (106 cm) in height having
a thinner trunk girth (25 cm) and shorter leaf length (86 cm) as compared to 213 cm
tall parent plants having thicker trunk (36 cm) and larger (193 cm) leaves. The fruit-
ing in this strain starts at lower height (30 cm). Thus, it is more suitable for high den-
sity planting (Ram and Majumder 1988). Work on mutation breeding with gamma
rays at Pune, Maharashtra (India) did not produce any fruitful results.
2.8 Biotechnology
Papaya plants can be propagated clonally by tissue culture techniques (Litz 1978;
Litz and Conover 1978; Rajeevan and Pandey 1983; Reuveni et al. 1990). In addi-
tion to leaf, axillary buds and meristem or shoot-tip culture, seed-coat tissues,
embryos and anthers have also been used for raising in vitro plants (Yang and Ye
1992; Islam and Joarder 1996; Jordan and Velozo 1997). Good survival, more uni-
formity in sex and higher yield of tissue-cultured plants has been reported under
field conditions (Pandey and Singh 1988). Reports on cell suspension cultures of
papaya are few. The cultured conditions allowed initiation of a large number of
embryos directly from cell suspensions through adventive somatic embryogenesis
and indirectly from callus on axillary buds (Jordan and Velozo 1996). Somatic
embryos could be germinated on suitable culture medium until plantlets reach
a suitable size for transfer to soil. Somatic embryos have also been raised from
the callus of root explants after 3 months of culture (Chen et al. 1987). The ex-
plants were obtained from 4 to 6 weeks old seedlings of papaya varieties, Solo
and Sunrise. Castillo et al. (1997) could produce somatic embryos of papaya cv.,
Solo in a liquid production system and encapsulated the embryos in two different
encapsulation compounds. The encapsulated embryos showed 77.5% germination.
The embryogenic regeneration, though obtained efficiently from several kinds of
immature tissues in the presence of auxins, is nonetheless often difficult to achieve
on selective media employed in transformation procedures. However, improved
methods of preparing embryogenic cultures are likely to overcome this bottleneck
(Manshardt and Drew 1998).