Agriculture Reference
In-Depth Information
and the occurrence of high chiasma frequency and the absence of multivalents
during meiosis suggest that it is of amphidiploid origin.
Cross-pollination between species is conducted primarily to determine
compatibility for increasing fruit set. Atemoya is the only hybrid that has
gained importance and it has inherited the glabrate leaf character of A.
squamosa and a leaf size almost as large as that of A. cherimola . Flowering and
fruiting seasons are similar to those of sweetsop. Skin, pulp and seed characters
of both parents are inherited in varying degrees by each plant. A desirable
hybrid would be between the cherimoya and soursop that combines the larger
fruit size and acidity of the soursop and the cherimoya's sweetness, fl avour and
texture. Attempts to cross the soursop with cherimoya, ilama, bullock's heart
or sweetsop have not been successful and may refl ect a considerable genetic
distance of soursop from the other species (Samuel et al. , 1991).
Problems in breeding
The long reproductive cycles, higher levels of heterozygosity and the costs
associated with evaluating large populations of crosses limits breeding
programmes. Existing commercial cultivars show considerable variation in
growth, fruit set, fruit size and quality. No single variety has all the desirable
characteristics. The length of the juvenile period varies, with earliest
production occurring in 2 years and full production in 5-6 years. This
juvenile period is extremely variable with scions on seedling rootstocks. The
seedling rootstocks are derived from extremely heterogeneous, open-pollinated
seeds; hence it is dii cult to fi x specifi c characters in a short period. Breeding
programmes have focused on selections from seedling populations. Early
maturity, better fruit appearance and, in the subtropics, greater cold tolerance
are the most frequent objectives.
Cultivar development
Except for cherimoya and atemoya, very few named clonal cultivars have been
developed among the Annonas (Table 6.1). Most of the plantings have been of
seedlings. In California, some old cultivars of cherimoya include 'McPherson',
'Deliciosa' and 'Bays'. Considerable work has been done in Peru on the
development of cultivars, but they are not widely known outside Peru; one of
them, 'Cumbe', is considered exceptional, because of size, surface smoothness,
fl avour and very few seeds. Chile, Spain and New Zealand grow the cherimoya,
as it is more tolerant of cold temperatures, with more successful self-pollination
than the atemoya. New Zealand's principal cultivars are 'Reretai', 'Burton's
Wonder' and 'Burton's Favourite'. Chilean cultivars 'Bronceada' and 'Concha
Lisa' have also performed well in Australia, the former having been reported to
 
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