Seedless cucumber, also known as greenhouse cucumber, is a popular and high-value crop found in many local food markets. Like tomato, cucumber is sold for premium prices in early-season markets (Curtis et al., 2012). Seedless cucumber plants are parthenocarpic with the capacity to set fruit without pollination. There are different types of seedless cucumbers. The cucumbers with extended length often are referred to as european, english, or japanese cucumbers. They have thin skin with longitudinal ridges. European or english cultivars often are individually wrapped to prevent quality loss (Reid, 2015). Beit Alpha cucumbers are smaller, with a range of sizes (Shaw et al., 2000). Large Beit Alpha cucumbers have similar sizes to typical slicing cucumbers. Pickling cucumbers with the parthenocarpic character are also available in the market.
Seedless cucumbers have a climbing growth habit, which allows crop trellising to maximize the use of vertical spaces. Worldwide, seedless cucumber is the third most important high tunnel crop after tomato and pepper (Lamont, 2009). However, the primary use of high tunnels has been to extend the growing season for tomato, with lesser popularity of cucumber in Indiana and the other states in the midwest (Knewtson et al., 2010). One challenge limiting seedless cucumber production in high tunnels is the relatively low soil temperature in early season that suppresses plant growth even when the air temperature would be adequate for cucumber production. Soil temperature lower than 63 °F greatly inhibits absorption of water and mineral nutrients of the crop (Welbaum, 2015). In USDA plant hardiness zone 6, soil temperature inside high tunnels in March and April is frequently less than 63 °F, which may prevent crop establishment or delay early cucumber production. As a result, it is difficult to achieve the early-season advantages for growing cucumber in high tunnels. To overcome this obstacle, heating the soil within the high tunnels is essential for early production of seedless cucumbers in Europe (den Nijs, 1980). However, this approach requires additional energy input and is less sustainable for high tunnel farmers. Presently, most cucumbers are planted in May in unheated high tunnels or open field in Indiana, and few locally produced cucumbers are available in markets before June.
Vegetable grafting is a cultural practice used for controlling biotic and abiotic stresses. A plant with specific disease resistance or stress tolerance is used as a rootstock to be grafted with a typical cultivar (called a scion) that has desirable fruit characteristics. Grafted plants combine the beneficial characteristics of both the rootstock and scion plants (Lee et al., 2010). In China, almost all the cucumbers grown in fall and winter seasons in Chinese solar greenhouses are grafted (Davis et al., 2008). The adoption rate of cucumber grafting in Japan and Korea is as high as 75% (Lee et al., 2010). The primary reason for cucumber grafting in Asia is to enhance plants’ tolerance to suboptimal temperatures. In addition, grafting is known to increase cucumber yield, improve fruit quality, extend harvest period, and enhance salt tolerance (Davis et al., 2008). Squash (Cucurbita maxima, C. moschata), interspecific squash hybrid (C. maxima × C. moschata), and figleaf gourd (Cucurbita ficifola) are commonly used as rootstocks for cucumber grafting (King et al., 2010).
In addition to low-temperature tolerance, interspecific squash hybrid rootstocks are tolerant to high temperatures; thus, they are preferred rootstocks for cucumber production in the summer. Cucumbers grafted onto some genotypes of winter squash rootstock produce fruit with shiny skin. These cucumbers have distinct appearance and longer shelf life. They are well received in the Japanese market (King et al., 2010). Figleaf gourd rootstocks have superior cold tolerance, but its rapid seedling growth and lack of uniform seed germination make grafting difficult (Davis et al., 2008).
Although grafting technique has been widely used in Asian and European countries, little information is available in the United States about seasonal extension potential of growing grafted cucumbers in high tunnels. To help high tunnel growers increase profitability of growing seedless cucumbers, this study was designed to explore the potential of extending the growing of seedless cucumbers in early spring by grafting with two commercial cucumber rootstocks in high tunnels.
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