Annona Species Monograph.pdf - Crops for the Future

Chapter 9. Genetic Improvement
the Philippines. However, he faced a lot of limiting factors and because he
evaluated only a small number of progenies, no new cultivars were selected
at that time.
9.4.1 Limiting factors and major constraints
Climate and soil are the factors with greatest influence on the variation in
growth, fruit set, fruit size and quality of commercial annonas. They
represent the main constraints in the establishment of an Annona breeding
programme, since they directly influence response via the genotypeenvironment
interaction.
Rainfall and high humidity during the peak flowering season greatly enhance
fruit production of most annonas by preventing desiccation of stigmas,
prolonging their receptive period and increasing fruit set and early fruit
growth (Nakasone and Paull, 1998). The sugar apple is the contrast, as it is
probably the most drought-tolerant species, and it grows, but produces
poorly, where rains are frequent. This is shown by the fact that sugar apple
does much better in northern Malaysia, where dry periods occur, than in the
southern part, which has year-round high moisture (Nakasone and Paull,
1998). This climatic adaptation of sugar apple to semi-arid conditions is
confirmed by Coronel (1994). Sugar apple's deciduous growth habit
contributes to its drought resistance, as it does not have any leaves during
most months of the dry season. In contrast, soursop grows and produces very
well under high rainfall conditions in the Amazon region. However, both
sugar apple and soursop grow and produce very well in the semi-arid
conditions of north-eastern Brazil, with very low rainfall, but they both
require irrigation. Given these good responses to environmental control,
breeding of sugar apple and soursop have a greater likelihood of success
under semi-arid conditions.
Temperature is also a limiting factor, mainly for the tropical annonas,
soursop, custard apple, sugar apple and wild soursop, since low temperatures
(< 14°C) may damage or even kill young trees, although adult plants may
show some tolerance. Poor pollination is frequent in all species when high
temperatures (> 30°C) and low RH (< 30%) occur, even with handpollination
(Nakasone and Paull, 1998). These authors also comment that
cherimoya is more tolerant to low temperature (7-18°C) and soursop is the
least tolerant (15-25°C). Therefore, improvement of cherimoya would be
better in the northern hemisphere (temperate and subtropical regions) and
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