The cucumber (Cucumis sativus) is a widely cultivated Cucurbitaceae crop with a global trading volume of 60 million metric tons (FAOSTAT, 2017), which reflects its dietary importance. Cucumber is used as either a fresh or processed vegetable. Multiple cucumber varieties are grown throughout the world, distinguished by usage, length, diameter, color, color uniformity, skin thickness, and surface protrusions (Shetty and Wehner, 2002). Cucumber varieties in Taiwan are mostly of the oriental trellis type, which was introduced by a Japanese seed company. Wen Nong 220, a green slicing cucumber, is currently the most popular variety in summer.
The most suitable temperature for cucumber growth is 20 to 30 °C, because temperatures higher than 35 °C can cause physiological disorders and seriously affect the fruit shape and quality (Yang et al., 2007).
Southern Taiwan has a tropical climate with high temperatures (26 to 28 °C) and humidity (70% to 80% in summer). However, global warming has caused a temperature rise in Taiwan, especially during the summer. According to news reports, the average temperature has risen ≈1.4 °C, which is twice the global average (Lu et al., 2012). As a result, summers in Southern Taiwan are not conducive to growing cucumbers. In Southern Taiwan, cucumbers are mainly cultivated in net houses to prevent pest infestations; however, if the indoor temperatures exceed 35 °C and moisture is increased because of restricted ventilation, then it results in the occurrence of downy mildew.
High temperatures and downy mildew not only interfere with the normal growth of cucumbers but also yield a bitter tasting fruit with a lower quality. Therefore, the Kaohsiung District Agricultural Research and Extension Station (KDARES) has initiated the breeding of a new cucumber variety with tolerance to abnormally high temperatures and moisture. The new Kaohsiung No. 3 variety has the potential to flower early with vigorous growth and strong adaptation to the summer temperatures of Southern Taiwan. It is suitable for fresh slicing, with a crisp texture and no bitter taste.
F1 hybrid is commonly practiced in breeding of the gourd family. The inbred lines are obtained from the monoclonal selection of several F2 generations (Brim, 1966). The parents of Kaohsuing No. 3 were selected by adopting the pedigree method at early stages (F2–F4), the bulk-population method at F5–F6, and then selfed for five generations as inbreds (S5). The crosses between inbred lines were conducted and followed by observation and line trials, as well as regional trials. A new, high-quality F1 hybrid with heat tolerance was identified.
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