Hylocereus undatus (dragon fruit).
Authors: Rojas-Sandoval Julissa , Praciak Andrew 
Published: April 2021

Abstract

Genetics: Hylocereus undatus is a diploid species and the chromosome number reported for it is 2n = 22 (Lichtenzveig et al., 2000). Reproductive Biology: Hylocereus undatus has hermaphroditic flowers. They open at night and in Mexico nocturnal visitors include the nectar-feeding bats Leptonycteris curasoae and Choeronycteris mexicana which are the main pollinators of this species. Flowers remain open the next morning and are also visited and pollinated by bees. This cactus has a mixed breeding system in which selfing and outcrossing set fruits (Valiente-Banuet et al., 2007). Physiology and Phenology: In Mexico, flowering occurs from the end of May to the beginning of August; so production of the last flowers coincides with ripening of the first fruits. In China, the species has been recorded flowering from July to December (Flora of China, 2015). In Sri Lanka flower production is from April to November (Gunasena et al., 2007). Longevity: Hylocereus undatus is a perennial long-lived plant. In cultivation it starts fruit production from the second year onward (Gunasena et al., 2007). Environmental Requirements: Hylocereus undatus is a fast growing, vine-like, tropical cactus grown for its fleshy, succulent fruit. Originating in Central America, it is now cultivated in many countries with tropical and subtropical climates, including the USA (south Florida, California and Hawaii), Australia, Taiwan, Vietnam, Malaysia and Israel. It is frost- and chilling-sensitive and is largely produced in areas where temperatures do not exceed 38°C. Growing as a climbing cactus in shaded or semi-shaded positions under large canopies, it may be injured by extreme sunlight and can tolerate some shade; however, it is considered to be a full sunlight crop in Central and South American countries. When growing naturally, it attaches branched stems to trees or rocks via adventitious roots. Under cultivation the vine-like stems are supported by a post and trellis system. Optimum temperatures for growth are 18-25°C, with good relative humidity levels. Hylocereus undatus is tolerant of windy conditions and moderately saline soils. Like many cacti, it has a low water demand, which is related to its crassulacean acid metabolism (CAM) mode of photosynthesis - uptake of CO2 occurs during the night when the stomata are open, which restricts water loss via transpiration during the heat of the day. Horticultural Aspects: Flowers are pollinated by bats or moths; however, hand pollination is also used with self-incompatible varieties to ensure good fruit set and fruit size. This requires considerable labour input and many new commercial operations are utilising new cultivars which are self-fertile to avoid the cost of hand pollination. Many of the varieties bred in Asia are now self-compatible and will set fruit relatively easily without requiring hand pollination. The main disadvantage with many of the self-compatible varieties of dragon fruit is that the fruit is often smaller than if the flowers were cross-pollinated with pollen from a different clone or different species (Merten, 2002). This may be due to fruit weight, which is positively correlated with the number of viable seeds and dependent on pollination. Hand pollination is carried out by removing the anthers from one flower and brushing them against the stigma of another or by collecting the pollen and using a small brush to pollinate many flowers. Commercial growers have to determine if it is worthwhile hand pollinating flowers in order to obtain a greater fruit weight, given the cost of labour and returns received for larger fruit. Often the first wave of flowers will not set fruit in self-incompatible varieties; however, a process has been developed for long-term storage of pollen which allows hand pollination whenever it is required. Pollen collected from dragon fruit flowers can be stored after drying to a moisture content of 5-10% and stored at below-freezing temperatures. Pollen can be stored in this way for 9 months and used to pollinate the first blooms of the season, resulting in an earlier and larger crop. Commercial dragon fruit growers in Taiwan use supplemental night break lighting to increase the flowering period after the normal period of flowering has finished. Flowering is induced by breaking the dark period with lighting between 22:00 and 02:00 h, allowing off-season production from November to April. These fruits produced in the off-season often receive premium prices as they are larger and sweeter than those produced from summer crops. Dragon fruit, while being a type of cactus, perform poorly under extremes of temperature and cannot tolerate high light and temperature. Dragon fruit plantations must be sited in frost-free areas or incorporate some form of frost protection such as greenhouse production for cooler winter climates. Dragon fruit plants will show damage at temperatures below 0°C, and also above 40-45°C as they were originally adapted to shade canopy environments. In high radiation areas, overhead shading is often installed, which also helps reduce extremely high temperatures which can limit flowering and fruit set. High radiation levels cause the plants to become bleached in appearance (caused by the destruction of chlorophyll in the stems), growth will also be retarded and plants may eventually die. However, under heavy shade the plants may become etiolated with reduced flowering and production levels. Recommendations for shading are to apply the minimal amount of shade required to prevent bleaching of the stems and ensure the plants are not water stressed as this reduces the crop's resistance to high light damage.