44 Papaya
of the ACC synthase and ACC oxidase genes (Neupane et al. 1998). Investigations
to characterise polyphenol oxidase and other enzymes obtained from fruits of her-
maphrodite and female plants have also been undertaken (Cano et al. 1996; Lin et
al. 1998) to examine alterations in fruit ripening and postharvest storability. Today
many transgenic papaya plants are under evaluation at various experimental sites.
2.9 Genetic Diversity
Genetic diversity has played a significant role in the development of cultivars suit-
able for growing under different agro-ecological regions for fruit as well as papain
extraction. The papaya germplasm ranges from very primitive types to major com-
mercial types. There are numerous local mixtures everywhere in all papaya grow-
ing regions. Collection, evaluation, documentation, conservation and utilisation of
papaya germplasm and thereby, contribution to development of varieties are pre-
requisite for future advancement (Ram 1992). Because of its complex genetic make-
up, there are few true cultivars of papaya, which are as uniform in horticultural
characters as the cultivars of other herbaceous crops. When seed results from open
pollination, it is impossible, in most cases, to obtain selections which are reasonably
uniform in flower type and fruit characteristics. Despite the lack of recognised culti-
vars, growers can maintain satisfactory strains by controlled pollination of selected
plants. Parent plants should be carefully selected for early and heavy fruit production
of desirable shape and size. A group of Hawaiian papayas referred to as Solo comes
closer to deserving cultivar rank than any other types due to its constancy in char-
acter expression to a high degree of natural self-pollination of its bisexual flowers
(Medina et al. 2003). This, in addition to continuous selection of pear-shaped fruits
produced by bi-sexual plants, has maintained Solo character relatively unchanged
over the years. Improved selections, such as Sunrise Solo, have resulted from rigor-
ous breeding work.
2.9.1 Germplasm
The papaya germplasm ranges from very primitive types to major commercial
varieties. Wide genetic variability is noticed in papaya growing regions due to seed
propagation and cross-pollinated nature of the crop. On account of high degree of
variability in economic characters, several selections from the existing populations
have been made in different papaya growing areas of the world. In India alone, over 10
such selections are available for commercial cultivation. To widen the genetic base, a
large number of superior cultivars have been introduced in India from other countries,
namely Solo, Sunrise Solo, Waimanalo, Peradeniya, Thailand, Pink-Flesh, etc. Some
hybrids are also available for use in further variety improvement programmes.
2.9.2 Description of Varieties
Despite the great variability in size, quality and other characteristics of papaya,
there were few prominent, selected and named cultivars before the introduction into
Hawaii of the dioecious, small-fruited papaya from Barbados in 1911. It was named