Annona is a large genus of approximately 170 species of trees and shrubs native to tropical regions of the Americas and Africa. Many species are edible, whilst several pantropically-cultivated species have become invasive. Perhaps the most well-known species grown for their fruit are Annona cherimola (cherimoya), Annona muricata (soursop), Annona reticulata (bullock’s heart) and Annona squamosa (custard apple). Common names in the genus are complicated as the same name can be used interchangeably among species in different countries.
The most widely grown species is Annona squamosa as it tolerates lower temperatures than many other edible species. The species with the most-revered taste is Annona cherimola, whose fruit flavour has been described as a blend of banana, pineapple, papaya, peach and strawberry. In many cultivated species the fruit is sweet, but soursop has a tart flesh. The fruits are oval, conical or heart-shaped with green skins that appear to have overlapping scales. Each fruit weighs between 150 grammes and 500 grammes, with some large fruits weighing up to 2.7 kilogrammes. The aromatic flesh is white with many, large, dark, glossy seeds.
Annona flowers are small and hermaphrodite with green-yellow, thick, fleshy petals arranged in two whorls – the outer one is larger than the inner. Female and male parts of the flower do not mature at the same time. Female parts mature first with pollinators attracted by scent; some Annona species even produce heat to aid scent dispersal. At the early female stage of flowering, petals are partially closed, providing shelter, food and mating sites for pollinators. When female parts are no longer receptive, the flower enters the male stage. As pollen is produced, the outer petals open and pollen-covered pollinators leave the flower. Pollen can then be taken to other flowers at the female stage. Small beetles from the family Nitidulidae are important natural pollinators.
One factor that limits the commercial cultivation of Annona species, outside of their native ranges, is poor fruit set. This is due to the absence of natural pollinators and the occurrence of mechanisms within the plant that minimise self-pollination. However, even in native areas yields can be low; poor pollination can lead to fruits with irregular shapes. Outside of their native range, and to increase yields, hand pollination is often necessary. This is done by collecting partially opened flowers, storing them overnight and then the following morning transferring the pollen, using a brush, to female-stage flowers.
Kundan Kishore AK et al. 2012. Pollination biology of Annona squamosa L. (Annonaceae): evidence for pollination syndrome. Scientia Horticulturae 144: 212-217.
Morton JF 1987. Fruits of warm climates. Julia F Morton.
Van Wyk, B-E 2005. Food plants of the world. An illustrated guide. Timber Press.