Vegetable grafting is the process of combining two compatible seedlings (scion and rootstock) to comprise one new seedling. Grafting onto rootstocks resistant to soilborne diseases can reduce the disease incidence of scion and improve the yield. For this reason, vegetable grafting has been used to overcome issues associated with intensive cultivation in Asia and Europe (Lee et al., 2010), and recently evaluated in the United States (Pullano, 2016). Other advantages of grafting onto superior rootstocks include abiotic-stress tolerance, such as tolerance to low temperature (Davis et al., 2008; Ntatsi et al., 2014) or saline environments (Colla et al., 2010), overcoming nutrient deficiencies (Schwarz et al., 2013), and improving fruit quality (King et al., 2010).
Despite the benefits of vegetable grafting, this technology is currently underused by commercial vegetable growers in the United States. Current capacity of vegetable nurseries to supply grafted seedlings is insufficient to meet the large, narrow-window, seasonal demand required by growers at time of transplanting. Low-temperature storage of grafted seedlings is one strategy to augment labor-intensive grafted seedling production. Using low-temperature storage allows propagators to maintain a more stable labor input while allowing an earlier start to seedling production. Grafting new seedlings while others are stored allows propagators to amass larger quantities of grafted seedlings that will have emerged from storage at the same stage of growth. The technique of low-temperature storage has been successfully demonstrated using ornamental plugs (Heins et al., 1995) and vegetable seedlings including watermelon (Ding et al., 2011; Duan et al., 2014), muskmelon [Cucumis melo (Justus and Kubota, 2010)], eggplant [Solanum melongena (Kubota et al., 2002; Wang et al., 2009)], pepper [Capsicum annuum (Kwack and Chun, 2015)], and tomato [Solanum lycopersicum (Park and Fujiwara, 2009, 2015)]. However, some species are chilling sensitive, which results in deteriorating seedling quality during and after low-temperature storage. For chilling-sensitive plants, a higher storage temperature must be selected to avoid loss of seedlings. Crop species in the cucurbit family (Cucurbitaceae) are generally known to be chilling sensitive, which limits the storability. For example, Ding et al. (2011) and Duan et al. (2014) could store watermelon seedlings at 15 °C up to 6 d, whereas Park and Fujiwara (2015) stored tomato seedlings at 10 °C for 21 d. Obviously, a longer duration of storage is desired to provide more flexibility in managing production.
Within the same family, sensitivities to low temperature vary for different species. We examined seedling response of 22 nightshade (Solanaceae) and cucurbit scions and rootstocks treated in low-temperature storage (Spalholz, 2013). The study determined that watermelon was highly chilling sensitive when compared with other cucurbit species. In the cucurbits, interspecific hybrid squash was the most chilling tolerant, followed by bottle gourd. These two species are used widely as rootstock to graft watermelon seedlings.
Acquisition of chilling tolerance by grafting chilling-susceptible scion onto chilling-tolerant rootstock is one of the widely practiced objectives for grafting cucurbit crops. For example, productivity of cucumber (Cucumis sativus) was improved when grafted onto figleaf gourd (Cucurbita ficifolia) under cooler soil conditions (Ahn et al., 1999). Petropoulos et al. (2012) examined three different rootstocks, including gourd (two forms of L. siceraria) and ‘RS 841 F1’ interspecific hybrid squash, for two different watermelon cultivars Sugar Baby and Crimson Sweet, and showed that a specific combination of watermelon scion and rootstock performed better than other combinations under low postgrafting temperatures. Therefore, grafting onto chilling-tolerant rootstock can be expected to improve the storability of chilling-susceptible scions in low-temperature storage. In fact, Justus and Kubota (2010) showed that storability of chilling-sensitive ‘Olympic Gold’ muskmelon was enhanced when grafted onto chilling-tolerant ‘Tetsukabuto’ interspecific squash rootstock. Another study reported by Ding et al. (2011) showed that ‘Zaojia 84–24’ watermelon seedlings grafted onto ‘Zhuangshi’ squash (C. moschata) rootstock retained higher chlorophyll and soluble sugar during storage and exhibited higher net photosynthetic rate during the poststorage recovery than nongrafted watermelon seedlings.
In the present study, we grafted chilling-sensitive watermelon onto two selected rootstocks of interspecific hybrid squash and bottle gourd that were previously shown as chilling tolerant as well as the same watermelon cultivar, and stored them for 2 or 4 weeks to observe low-temperature storage response.
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