North Carolina is the seventh leading state in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] production and value in the United States (Arney et al., 2006). Watermelon is second to cucumber (Cucumis sativus L.) in cucurbit area planted in North Carolina (North Carolina Department of Agriculture and Consumer Services, 2004). The land devoted to watermelon production in North Carolina from 1994 to 2004 ranged from 3238 to 4616 ha (Arney et al., 2006).
Temperatures below 10 °C may injure tropical and subtropical crops such as species of the Cucurbitaceae (Raison, 1974). The damage is often referred to as CI and was reviewed by Lyons (1973). There have been several reports on chilling tolerance in cucumber seedlings (Chung et al., 2003; Kozik et al., 2007; Kozik and Wehner, 2008; Smeets and Wehner, 1997), but not watermelon seedlings.
Chilling damage in cucumber, like in other thermophylic plants, depends mainly on chilling temperature, duration of chilling, and on light intensity during chilling (Minchin and Simon, 1973; Rietze, 1988; Rietze and Wiebe, 1987, 1989; Van Hasselt, 1972; Wang, 1986; Wright and Wilson, 1973). The environment before and after chilling also is important. Chilling damage is affected by the temperature, light conditions, and water status of the plants before chilling (Lafuente et al., 1991; Pomeroy and Mudd, 1987; Rietze and Wiebe, 1989; Rikin et al., 1976; Saltveit, 1991; Wilson and Crawford, 1974) and the light conditions after chilling (Lasley et al., 1979; Rietze and Wiebe, 1989). Watermelon is susceptible to CI but is more resistant than cucumber (Wehner and Mirdad, 1994).
Low-temperature effects have been studied on germination, seedling damage, and fruit damage in cucurbits. For example, there is genetic variation among cucumber cultigens for germination at low temperature (Lower, 1974; Nienhuis et al., 1983; Wehner, 1981). Cold germination has a heritability of 0.15 to 0.61, depending on test temperature and population used (Wehner, 1982, 1984), and can be improved without correlated changes in other important horticultural traits (Staub et al., 1988). Genetic variation for chilling tolerance in cucumber exists (Aoki et al., 1988, 1989; Cabrera et al., 1992; Liu et al., 1984; Saczynska et al., 1993), although not in all populations (Rietze, 1988; Staub, 1988).
Smeets and Wehner (1997) developed a method for screening seedlings of cucumber using specific environmental conditions and cultigens that were tolerant (AR75-79, ‘Chipper’, ‘Pixie’, and ‘Wisconsin SMR 18’) or susceptible (Gy14, ‘Marketmore 76’, NCSU M28, NCSU M29, and ‘Poinsett 76’). They concluded that genetic variation for chilling damage was greater at the first true leaf than at the cotyledon stage. Using this method, comparisons of cultigens for their tolerance to low temperatures during the seedling stage of development in cucumber have been reported. Chung et al. (2003) investigated inheritance of CI in progenies of both tolerant ‘Chipper’ and AR75-79 crossed with susceptible Gy14. Their data suggested that chilling tolerance was maternally inherited. Wehner and Kozik (2007) also demonstrated that there was low heritability for chilling tolerance in two cucumber populations that were developed from elite cultivars not chosen for chilling tolerance. A later screening of the cucumber germplasm collection resulted in the identification of a high level of tolerance of PI 246930. Genetic studies in cucumber inbred NC-76 (developed from PI 246930) showed that chilling tolerance was the result of a single, dominant, nuclear gene Ch (Kozik and Wehner, 2008).
Sensitivity of watermelon fruit to CI has been studied by Risse et al. (1990), but chilling tolerance of watermelon plants has not been reported. Because efficient testing methods have been developed for screening cucumber cultigens for tolerance to chilling, it may be possible to adapt such methods for investigation of resistant watermelon cultigens and the inheritance of chilling tolerance for plant improvement. Therefore, a study was designed to develop an efficient testing method for chilling tolerance in watermelon and to use that method to identify chilling-resistant cultigens.
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