Pitaya is increasing in popularity as a crop partial as a result of its tolerance to arid environments and resistance to pathogens (Mizrahi et al., 1997, 2002; Mizrahi and Nerd, 1999; Nobel and De La Barrera, 2004). Both Hylocereus undatus (Haworth) Britt. & Rose and red pitaya (H. sp.) have been introduced to Taiwan in recent decades. The most commonly grown red pitaya varieties were selected from crosses between H. undatus and H. spp., and they produce fruit in summer and fall (Hsu, 2004). Breeding for winter-bearing cultivars as well as developing methods to regulate flowering to produce winter crops would benefit the red pitaya industry (Chang, 2003; Jiang, 2005; Jiang et al., 2011; Yen and Chang, 1997).
Factors affecting flowering of pitaya include shoot age, temperature, light, and application of growth regulators. For yellow pitaya (Selenicereus megalanthus), shoots that sprout in spring normally flower in winter, whereas shoots that sprout in winter flower in the next fall (Jiang et al., 2011). Regardless of daylength, extreme temperatures in summer may restrict the flowering of pitaya (Nerd et al., 2002). Light intensity may affect the nutritional status of shoots and thus influence the number of flowers produced, but it does not promote off-season flowering in H. undatus (Khaimov and Mizrahi, 2006; Raveh et al., 1998). Flower thinning and application of growth regulators such as [N-(2-chloro-4-pyridinyl)-N-phenylurea] (CPPU) and gibberellic acid (GA3) could accelerate flowering or extend the flowering period and therefore result in earlier or later harvest, but they did not reverse the seasonal flowering phenology in H. undatus (Khaimov and Mizrahi, 2006). A number of other chemicals related to floral initiation, including sodium naphthalene acetic acid, ethephon (EthrelTM), ethephon with urea, a commercial mixture of gibberellins (GA4 and GA7) with benzyl adenine known as PerlanTM, paclobutrazol, hydrogen cyanamide, 2-ethylene chlorohydrin, KNO3, and CaC2, have been tested but did not promote winter flowering of pitaya (Chang, 2003; Khaimov and Mizrahi, 2006; Yen and Chang, 1997).
Pitaya is generally known as a long-day plant, which flowers in several flushes between May and October in the northern hemisphere according to Luders and McMahon (2006). To verify the long-day plant status of pitaya, other researchers have conducted experiments involving daylength manipulation. Yen and Chang (1997) prolonged the light period to 2200 hr between November and March to encourage H. undatus plants to flower inside a plastic house but only had limited success. Khaimov and Mizrahi (2006) obtained a contradictory result from their experiments extending daylength between March and July. To date, the photoperiodism of pitaya has not been clearly demonstrated by conclusive data, and the application of night-breaking or daylength extension for winter crop production remains unstable in subtropical Taiwan. This study verified that red pitaya is a long-day plant and that night-breaking treatment can promote its winter crop production.
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