Grafting is an old but common horticultural technique in which plant tissues from separate plants are joined together to form a new composite plant (Hartmann et al., 2011; Lee and Oda, 2010). The most well-known function of grafting is improving the soil-borne disease resistance of the new composite plants (Davis et al., 2008a, 2008b; Lee, 1994; Sakata et al., 2007). The prohibition of methyl bromide has further increased the popularity of grafting as an alternative approach to disease control (Davis et al., 2008a, 2008b; Oda, 2007; Passam et al., 2005). Furthermore, grafting can provide other advantages to the new composite plants, such as improved tolerance to abiotic stresses (e.g., water stress, thermal stress, salt stress, heavy metal toxicity), increased water and nutrient uptake, greater plant vigor, increased yield, and the induction or promotion of flowering (Davis et al., 2008b; Lee, 1994; Lee and Oda, 2010; Lee et al., 2010; Oda, 2007; Rivero et al., 2003; Schwarz et al., 2010). Vegetable grafting is already widely used for commercial production of cucurbitaceous vegetables [e.g., watermelon (Citrullus lanatus), cucumber (Cucumis sativus), melon (Cucumis melo)] and solanaceous crops [e.g., eggplant (Solanum melongena), pepper (Capsicum annuum), tomato (Solanum lycopersicum)] (Gaion et al., 2017; Lee et al., 2010; Schwarz et al., 2010). The variety of vegetable species for which grafting techniques have been developed has increased (Lee, 1994), and an increasing number of countries are involved in vegetable grafting (Davis et al., 2008a, 2008b; Kubota et al., 2008; Lee et al., 2010; Mudge et al., 2009).
Although grafting culture is commonly used in several vegetables, its application has been generally limited to cucurbits and solanaceous vegetables. The application of grafting techniques to more vegetables is an important issue. Cabbage is an important crop that belongs to the Crucifer family (Brassicaceae) and has never been seedling-grafted for commercial production. Based on a literature survey, we found that cabbage can be grafted onto kale (B. oleracea Acephala group) or kohlrabi (B. oleracea Gongylodes group) rootstock successfully by cleft grafting, but the survival ratios are very low (5% to 20%) (Oda et al., 1992). These low survival ratios may limit the commercial production of grafted cabbage seedlings. Chinese cabbage (Brassica rapa Pekinensis group) was reportedly grafted onto turnip (B. rapa Rapifera group) rootstock by cleft or horizontal methods, with survival ratios of ≈50% (Oda and Nakajima, 1992). The greater survival ratios of chinese cabbage than of cabbage suggests the potential to improve cabbage grafting.
The survival ratios of grafted seedlings depend on the grafting compatibility between scion and rootstock as well as on seedling age and quality, grafting technique, and management after grafting (Andrews and Serrano Marquez, 1993; Davis et al., 2008b; Oda, 2007). Diverse grafting methods have been developed for different vegetables, such as cleft grafting, tongue approach grafting, hole insertion grafting, and splice grafting (Lee, 1994; Lee and Oda, 2010; Lee et al., 2010; Oda, 2007). Among these techniques, tube grafting, also known as slant-cut grafting or splice grafting, can be applied in small plants grown in plugs (Lee and Oda, 2010; Oda, 2007). Tube grafting is a common method for grafted commercial tomato seedling production in Taiwan, and is also popular in Japan, Korea, and the United States for plug-cultured tomato seedlings (Lee et al., 2010; Oda, 2007). The tube-grafting procedure is simple and rapid (Lee and Oda, 2010). Importantly, this technique can be performed by hand or by using a grafting robot (Lee and Oda, 2010). Furthermore, the seedlings produced via the grafting method are healthy and strong (Lee and Oda, 2010), and there is no need to remove the tube after successful grafting.
Proper acclimatization, which includes healing and hardening, is a very important factor for success after grafting (Jang et al., 2011; Lee and Oda, 2010; Lee et al., 2010; Oda, 2007). Environmental factors related to healing include temperature, RH, and light intensity (Jang et al., 2011; Mudge et al., 2009). Grafted tomato seedlings allowed to heal at 23 °C had greater survival rates than seedlings that healed at 17, 20, and 26 °C (Vu et al., 2013). At 14 d postgrafting, watermelon seedlings that healed at 27.5 ± 1.5 °C had greater shoot fresh weight, root fresh weight, and leaf area than those of seedlings healed at 30.5 ± 1.5 °C and 33.5 ± 1.5 °C (Chang et al., 2003). To prevent scion wilting, low light intensity and high humidity conditions are preferable for healing (Nobuoka et al., 1996). However, greater light intensity during acclimatization improves the quality of grafted tomato and cucumber seedlings (Jang et al., 2011; Nobuoka et al., 2005). Furthermore, grafted tomato seedlings healing at 80% RH had greater final survival rates than seedlings healing at 90% RH because a lower percentage of diseased plants were found under 80% RH conditions (Vu et al., 2013). These examples suggest that different plant materials used for grafting require different healing conditions, and the best healing conditions can increase survival rates and improve seedling quality.
In this research, we attempted to develop a cabbage grafting method using tube grafting for potential diverse purposes in the future. First, we evaluated whether the tube-grafting method could be applied to cabbage. Second, we identified the best healing conditions for the grafted cabbage for future commercial production. Last, we determined the effects of grafting on cabbage head traits.
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