Grafting is a horticultural technique that joins scion (top) and rootstock plants via a graft union to create desirable above-ground and below-ground characteristics in the grafted plant (Davis et al., 2008). Grafting has been used for decades in commercial vegetable production in Asia and Europe to manage plant diseases, increase plant vigor, and improve plant production, but is still not widely used in the United States (Kubota et al., 2008; Lee, 1994; Lee et al., 2010; Peregrine and Binahmad, 1982; Sakata et al., 2007). Adoption of grafted plants among small-scale growers in the United States is limited by a shortage of research-based information regarding the production of grafted plants. While large-scale growers in the United States likely purchase grafted transplants, many small-scale growers are grafting their own (Kubota et al., 2008). Solanaceous crops (Solanaceae) are generally considered easy to graft because the splice grafting method that is most commonly used is simple and quick with a high graft survival rate (≥95% survival for tomato under controlled healing conditions). While grafting generally takes less than a minute per plant to complete, the healing process takes a week. Healing is the process of construction and establishment of the vascular connection between the scion and the rootstock plants (Davis et al., 2008; Fernandez-Garcia et al., 2004). In general, an environment that includes high relative humidity (85% to 100%), low or no light, and temperature ranging from 75 to 80 °F is believed to be a requirement for obtaining high grafting success with solanaceous crops (Bausher, 2013; Jang et al., 2011; Lee, 1994).
The scion is incapable of absorbing water from the rootstock for several days following grafting, and a high humidity environment can reduce scion transpiration and prevent plant wilting (Fernandez-Garcia et al., 2004; Turquois and Malone, 1996). The light conditions during the healing process can also affect plant transpiration since reduced light levels minimize stomatal opening and inhibit the photosynthesis process (Oda, 1999; Rivard and Louws, 2011). Overall, grafting survival is improved due to the conservation of water in the plant (Jang et al., 2011). However, photosynthesis is required for cell division, which is needed for vascular regeneration and callus formation (Hunter et al., 2004; Taiz and Zeiger, 2002). Callus proliferation in both the rootstock and the scion are needed for vascular connection at the rootstock-scion interface (Ogata et al., 2005), and poor callus formation can lead to low rates of survival of grafted plants (Johkan et al., 2008, 2009; Oda et al., 2005). Carbohydrates are required for callus formation, and are produced in the plant through photosynthesis (Hunter et al., 2004; Milthorpe and Moorby, 1979; Taiz and Zeiger, 2002). When light is limited in the healing chamber, healing of grafted plants is dependent on stored carbohydrates (Daley et al., 2014). For example, the survival of watermelon (Citrullus lanatus) grafted using the splice grafting technique (both rootstock cotyledons were removed) was 89% when rootstocks received 2% sucrose solution before grafting and the rootstock stems had 52% starch accumulation. In comparison, rootstocks that were treated with water had 6% starch accumulation in the rootstock stems and grafting success was 58% (Dabirian and Miles, 2017). There are no reports regarding increasing the carbohydrate reserves of solanaceous crops before grafting, though previous research reports that when light is provided during the healing period, callus induction, growth and graft survival are improved (Afshari et al., 2011; Moon and Stomp, 1997; Nobuoka et al., 2005).
While large-scale producers of grafted vegetable transplants use controlled environments to heal newly grafted plants, small-scale growers are using simple healing chamber structures within a greenhouse, where it can be difficult to precisely control environmental factors. There are no standard guidelines for managing small-scale healing chambers, as climate conditions and greenhouse structures vary considerably across regions (O’Connell et al., 2009). An earlier experiment carried out in a greenhouse at Mount Vernon, WA measured grafting success following healing with three different healing chamber structures: a commercial propagator’s design that was covered in clear plastic and shadecloth, and was hand misted (used by a large-scale vegetable grafting company in British Columbia, Canada); a research design that was also covered in clear plastic and shadecloth but that used a humidifier (commonly used in the southeastern United States); and a simple structure that was only covered in shadecloth and was hand misted (used by small-scale tomato growers who graft their own transplants) (Johnson and Miles, 2011). The commercial propagator healing chamber structure had the highest temperature and RH (24.9 °C and 98%, respectively), while the research structure had a slightly lower temperature and moderately lower RH (23.4 °C and 81%, respectively), and the shadecloth structure also had slightly lower temperature but substantially lower RH (23.3 °C and 52%, respectively). The authors concluded that high RH during the 7-d healing period was necessary for eggplant and watermelon graft survival (82% and 7% grafting survival overall, respectively), but not necessary for tomato graft survival (98% grafting survival overall). However, the light levels were not measured in this study.
There is a need to provide more information for small-scale growers who are healing grafted plants using simple healing structures inside a greenhouse, with a range of fluctuating light and RH levels. Thus, the objective of this study was to investigate the effect of a range of light and RH levels on the grafting success of tomato, eggplant, and pepper in a small healing chamber within a greenhouse environment.
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