Agriculture Reference
In-Depth Information
enforced heterozygotes, while the female genotype (mm) is a double recessive.
Seeds derived from these cross-combinations will have twice the number of
hermaphrodites compared with females (Storey, 1938, 1941a). It is possible to
self and cross male trees when there is reversal of sex from a staminate fl ower
to a form that has a functional ovary, and these are used to develop inbred lines
of dioecious papayas. Sex is determined at fl owering time, usually 4-6 months
after planting. Molecular probes can be used to predict the sex of papaya at
seedling stage.
Fruit size shows wide variation, ranging from about 5 cm in diameter
and 50 g in weight to over 50 cm in length and 10 kg or more in weight.
Fruit weight is a quantitative character determined by multiple alleles (Chan,
2001). Fruit shape in papaya is a sex-linked character. The female fl ower
has a globose ovary, which develops into round or ovoid fruits. In contrast,
the elongata or hermaphrodite fl ower has a slender, tapering ovary and this
subsequently develops into a fruit that is elongated and cylindrical or pyriform
in shape, depending on the variety of papaya.
Precocity or earliness to fruiting is a factor of the number of nodes
produced to fi rst fl owering node, while the number of nodes to fl owering
and the internode length determine height to fi rst fruit. These characters are
governed by additive gene ef ects, with the hybrid having the arithmetric mean
between the two parents (Nakasone and Storey, 1955). Subhadrabandhu and
Nontaswatsri (1997) reported that height of fi rst fl ower and number of nodes
to fruiting were controlled by both additive and non-additive genes.
Carpellody of the stamen is the development of misshapen or 'cat-faced'
fruits due to fusion of the stamens to the ovary tissues in hermaphrodite
fl owers (Fig. 11.6). Carpellody and female sterility may involve a number of
gene pairs involving three loci, two for carpellody (c/c, c/c) and one for sterility
(s/s). Their normal alleles (+) are partially or completely dominant. The s/+
combination is normal. In c/+, the + is partially dominant over 'c', so there
will be some carpellody. The 's' factor, whether homozygous or not (s/-), is
epistatic over the 'c' allele when the carpellody factors are heterozygous. The
phenotype would be normal but unstable, depending upon environmental
conditions. If either of the 'c' factors is homozygous (c/c), the combined
strength of the two alleles at one locus can overcome epistasis, thus exhibiting
carpellody. High heritability (h
2
= 82%) is obtained for carpellody, but ef ective
phenotypic selection may be interfered with by the 'change-in-rate' type
of interaction between genotype and plant age (Chan, 1984). Increased
expression of carpellody may also be brought about by cool temperature, high
soil moisture and high nitrogen (Awada, 1961).
Flavour and odour can range from pleasantly aromatic, as in the 'Solo'
and 'Eksotika' varieties, to musky, as in the 'Maradol' variety. Muskiness is
due to the homozygous recessive allele of a single gene (Storey, 1969). Total
soluble solids content is usually associated with sweetness and may range from
5% to about 19% total soluble solids (
0
Brix) in the 'Solo' varieties. This trait is
