Red pitaya (Hylocereus sp.), a climbing cactus, was introduced into Taiwan as a fruit crop during 1980s and has increased in popularity (Chang, 2003; Hsu, 2004; Jiang, 2005; Jiang et al., 2011; Yen and Chang, 1997). In Taiwan, the most commonly grown red pitaya varieties are selected from crosses between Hylocereus undatus and Hylocereus sp. Pitaya is a long-day plant, which flowers in several flushes between May and October in the northern hemisphere (Luders and McMahon, 2006). The critical daylength of red pitaya was reported to be ≈12 h, and the two switches between inductive and noninductive periods for red pitaya occur at the vernal and autumnal equinoxes, respectively (Jiang et al., 2012).
Southern Taiwan is located in the tropical region (≈23.5 °–21.5 °N) and its temperature and light intensity in the winter are adequate for pitaya growth. Off-season production can be induced by interrupting the length of the night with about 4 h of artificial lighting (NB) (Jiang et al., 2012). Consequently, in orchard management, shoots developed during winter flower in the inductive period (spring–summer), whereas shoots that developed in early spring flower during the noninductive period (i.e., as daylength shortens) (Jiang, 2005). However, a promising year-round production schedule remains lacking because floral buds fail to emerge after December. Thus, investigating that the factors hinder winter flowering in Taiwan is a priority if year-round production is to be achieved.
At the early developmental stages of cactus bud transformation, several bracteoles are differentiated before the formation of floral bud (Almeida et al., 2010), which result in progressive bud swelling before floral bud emergence. In southern Taiwan, 8 weeks of 4 h of NB treatment after the vernal equinox is sufficient for the floral bud emergence of red pitaya (Jiang et al., 2012). Because of apical dominance, numerous areoles, except those that flowered, remain dormant despite having been exposed to adequate long daylength. Toward the end of September, the rested buds are of different sizes, with the larger ones located at the distal end of shoots. In general, these buds tend to respond more effectively to the NB treatment before December (personal observation). Therefore, we hypothesized that bud swelling could be associated with the number of bracteoles that progressively differentiated, and could be used as an indicator for bud development. Furthermore, the effectiveness of NB treatment in the off-season production of pitaya might be associated with the bud development stages. Thus, the effects of bud development with or without NB treatment between January and March were investigated.
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