High-value vegetable crops such as eggplant, tomato, and watermelon are grafted to increase vigor, yield, tolerance to salinity and temperature extremes, and disease resistance (Lee, 1994, 2003, 2007; Paroussi et al., 2007; Rivard and Louws, 2008). Production and demand for grafted vegetable plants continues to increase across Asia and Europe and has begun to expand to North America (Kubota et al., 2008; Lee, 2003). Grafted vegetable plants are not widely available in North America, and large-scale plant propagators as well as small-scale producers are looking for recommendations for grafted vegetable transplant production (Kubota et al., 2008).
The splice grafting technique, also known as top grafting or tube grafting, is widely used among vegetable grafting operations as it is efficient and easy to learn. By using splice grafting, producers can graft two to three times faster than with other grafting techniques, thereby optimizing labor costs and greenhouse space (Hartmann et al., 2002; Lee, 2007; Oda, 2007; Rivard and Louws, 2006). In splice grafting, rootstock and scion are cut at 45° angles, and cut surfaces are held together by a grafting clip (Hartmann et al., 2002; Oda, 2007). It takes an average of 5 to 8 d for the graft union of an herbaceous plant to develop vascular connectivity between rootstock and scion (Fernandez-Garcia et al., 2004; Turquois and Malone, 1996). During this time, the scion is not able to access water from the rootstock. Therefore, it is important to maintain proper environmental conditions to prevent water loss from the scion and promote rapid formation of the graft union (Davis et al., 2008). Survival of grafted plants will be higher if plants are kept in a controlled environment with high RH and optimal temperatures (Davis et al., 2008; De Ruiter Seeds, 2006; Hassell et al., 2008; Oda, 2007).
Research programs and industry guidelines recommend a wide range of temperature and RH levels for healing grafted vegetable seedlings. De Ruiter Seeds (2006) suggests that the optimal temperature range for healing grafted tomato is 21 to 22 °C with a maximum temperature range of 28 to 29 °C. Other research programs suggest healing cucurbit plants at 28 to 30 °C (Hassell et al., 2008; Lee, 2007; McAvoy, 2005; Oda, 2007). The healing environment should be maintained between 85% and 100% RH for both tomato and cucurbit plants (Davis et al., 2008; Rivard and Louws, 2006). Some authors specifically state that the environment should be maintained at greater than 95% RH for cucurbits (Hassell et al., 2008; Lee, 2007; McAvoy, 2005; Oda, 2007). Although some large-scale commercial grafting operations often use environmentally controlled growth chambers to hold plants during the healing process, these chambers are not cost effective for most operations (Hassell et al., 2008; Oda, 2007). Instead, many growers and grafting researchers construct healing chambers, which are small structures covered in plastic and placed within a greenhouse (Davis et al., 2008; Hassell et al., 2008; Rivard and Louws, 2006).
Healing chambers are an economical option for creating a humid environment. Maintaining temperatures within the optimal range and high RH is of primary concern when healing grafted vegetable seedlings. Temperature within the healing chamber is directly impacted by temperature within the greenhouse. RH can be increased by spraying the healing chamber with water, flooding the bottom of the chamber with water, or using a humidifier within the healing chamber (Hassell et al., 2008; McAvoy, 2005; Rivard and Louws, 2006). Little information is available regarding recommended design, dimensions, and management of healing chambers. The objective of this study was to compare the survival of grafted eggplant, tomato, and watermelon plants in three different healing chamber designs modeled after research, industry, and small-farm designs commonly used in North America.
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