Pitaya, or dragon fruit (Hylocereus spp.), is a climbing cactus native to South and Central America that grows in the shade of canopy trees (Mizrahi et al., 1997). It is an emerging commercial fruit crop grown worldwide, particularly in Vietnam, Taiwan, Malaysia, Indonesia, and Israel (Mizrahi et al., 1997; Ortiz-Hernández and Carrillo-Salazar, 2012). In Taiwan, the production of pitaya has increased substantially over recent decades (Council of Agriculture, 2019) due to its high price and increasing market demand. Normally, red-fleshed pitaya (H. polyrizus) is produced in Taiwan from May to December, and the production peak begins from July to September (Chu and Chang, 2020); however, a night-breaking technique has been used to extend off-season production (Jiang and Yang, 2015).
‘Da Hong’, also known as ‘Big Red’, is an elite red-fleshed cultivar preferred by growers in Taiwan and Southeast Asia because of its favorable traits including self-compatibility, abundant yield, large fruit (more than 400 g per fruit), and high total soluble solid content (TSSC, above 20 °Brix) (Jiang and Yang, 2015; Liu et al., 2015). However, unlike the red-fleshed cultivar Fu Gui Hong or VN White, a white-fleshed pitaya (H. undatus) cultivar, Da Hong does not produce high fruit yield all of the year in Taiwan (Chien and Chang, 2019). It has been found that although ‘Da Hong’ produces the most abundant flowers and fruits during the hottest period (August to September), the fruits are small (less than 150 g) and the color of the cladode becomes yellow under field conditions (Y.C. Chu and J.C. Chang, unpublished data; Chiu et al., 2015) and in net-house grown plants (Chien and Chang, 2018). Therefore, these conditions result in an erratic yield and variable fruit quality. It appears that ‘Da Hong’ may be more vulnerable to high temperature stress compared with ‘Fu Gui Hong’ and ‘VN White’; however, a more detailed clarification of the responses of ‘Da Hong’ to high temperature stress has yet been done.
Pitaya produces fruit that contains thousands of tiny seeds (Hsu, 2004). Moreover, Weiss et al. (1994) reported a positive relationship between seed weight and fruit/pulp weight in pitaya under favorable conditions; however, there is no information available on how heat stress affects fruit production, such as its effect on yield and quality, in terms of fruit weight and sweetness. It has yet to be reported whether flowering under high summer temperatures results in a decrease in seed setting (which is expressed as the estimated number of seeds), and if seed weight exacerbates fruit drop or reduces fruit size.
Although pitaya has a crassulacean acid metabolism (CAM) characteristic, it is prone to yellowing of the cladode, i.e., slight sunburn with no incidence of necrosis on the sunny (sun-exposed) side of the adaxial cladode that has three flat wavy ribs with one sunny side and two shaded sides in red-fleshed pitaya, during the hot season in Taiwan, followed by regreening during cooler seasons (Chien and Chang, 2019). Of the pitayas, the cladode of the white-flesh pitaya displays the highest total daily net uptake of CO2 at day/night air temperatures of 30/20 °C, but the total daily net uptake of CO2 falls remarkably at 40/30 °C (Nobel and De la Barrera, 2002), indicating that dry matter accumulation may be affected by temperature regimes. Cladode necrosis begins to occur after 6 weeks of exposure to 40/30 °C, especially when light intensity exceeds photoinhibition, and severe necrosis occurs with prolonged duration of high temperature (Nobel and De la Barrera, 2002). In Israel, it has been shown that extremely high summer temperatures (38/20 °C) inhibit flower bud formation, which directly leads to yield reduction (Nerd et al., 2002; Nobel and De la Barrera, 2002; Raveh et al., 1998). However, the resulting effects of heat stress on dry matter accumulation and how that change affects flowering, fruit set, and fruit development are still unknown.
The aim of this study was to ascertain whether HT during flowering affects floral bud development duration, flower opening behavior, fruit set, fruit weight, seed setting, which was expressed as the estimated number of seeds, and seed weight per fruit, and cladode discoloration in ‘Da Hong’ red-fleshed pitaya plants under controlled conditions.
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