Grafted tomato transplants (Solanum lycopersicum) have become an important production tool for vegetable growers within the United States (Grieneisen et al., 2018; Masterson et al., 2016a; Meyer, 2016; Rivard and Louws, 2008). Vegetable grafting of both solanaceous and cucurbit crops first emerged as an important method of overcoming soil-borne diseases (Lee et al., 2010), and the practice of grafting has been adopted on a larger scale in the United States within the past two decades (Albacete et al., 2015; Kubota et al., 2008). Grafting has been shown to be an effective tool for increasing farm profitability (Barrett et al., 2012; Rysin and Louws, 2015; Rysin et al., 2015). The benefits of vegetable grafting include increasing the marketable yield, improving resistance to soil-borne disease, and overcoming adverse environmental conditions (Albacete et al., 2015; Kubota et al., 2008).
Specifically, vegetable growers in the midwestern United States have expressed interest in learning more about using grafted transplants on their farms (Masterson et al., 2016b; Meyer et al., 2017). Researchers are working to break down the barriers to adopting grafting to provide growers with improved recommendations regarding how to perform grafting and how to manage plants immediately postgrafting (Bausher, 2013; Buajaila et al., 2018; Johnson and Miles, 2011; Masterson et al., 2016b; Meyer et al., 2017).
Tomato plants are approachable for growers to graft on-farm due to the simplicity of the splice grafting method and the potentially high survival rate of transplants postgrafting (Buajaila et al., 2018). When proper grafting techniques are used, grafted tomato transplant survival after healing has been shown to exceed 97% (Bausher, 2013; Johnson and Miles, 2011). A skilled worker can graft between 300 and 500 plants per hour using the splice grafting method (Kubota et al., 2008), making on-farm grafting for small-scale production a relatively expeditious process. However, reports indicate the postgrafting healing process requires ≈1 week inside a healing chamber and management of relative humidity (RH), light, and temperature (Bie et al., 2017; Lee et al., 2010; Rivard and Louws, 2011).
Healing chamber designs vary greatly, but the general structure for medium-scale to small-scale operations is a rigid frame fully enclosed by plastic sheeting (Lee et al., 2010; Masterson et al., 2016b; Rivard and Louws, 2011). This plastic may be semi-transparent (Lee et al., 2010) or opaque (Bie et al., 2017; Rivard and Louws, 2011).
Maintenance of humidity in the healing chamber is typically achieved through the use of commercial humidifiers (Masterson et al., 2016b; Wei et al., 2018), filling the bottom of the chamber with water (Masterson et al., 2016b; Rivard and Louws, 2011), or misting plants by hand using a spray bottle (Johnson and Miles, 2011). The recommendation is that grafted transplants should be kept at high RH between 80% to 100% for at least the first 4 d postgrafting (Kubota et al., 2008; Rivard and Louws, 2011). Recent work by Wei et al. (2018) found that 97% to 98% RH is optimal for the healing of grafted tomatoes. Buajaila et al. (2018) conducted a trial of small-scale healing chambers with a target rate of 100% RH that resulted in an actual average RH ranging from 96% to 98% and the highest plant survival rate.
Light levels within the healing chamber have an important role in the graft healing process (Buajaila et al., 2018). Standard recommendations for healing grafted vegetable transplants include blocking all light within the chamber for the first 2 to 4 d postgrafting (Hartmann et al., 2010; Rivard and Louws, 2011). Alternatively, Kubota et al. (2008) suggested the use of low-light conditions instead of complete light exclusion. Recent research has focused on varying methods of achieving and maintaining low-light conditions within healing chambers (Buajaila et al., 2018; Johnson and Miles, 2011; Masterson et al., 2016b; Meyer et al., 2017). After testing the effects of available light reduced to 0%, 25%, and 50% on grafted tomato transplants healed in small-scale chambers, Buajaila et al. (2018) suggested that the management of RH may be more important than the strict management of light level. Studies demonstrating high plant survival rates with only 50% light reduction (Buajaila et al., 2018; Masterson et al., 2016b) indicate to the need to measure the effects of no light exclusion on grafted tomato transplant healing.
The suggested optimum healing chamber air temperature varies; 27 to 28 °C has been recommended for general vegetable graft healing (Kubota et al., 2008) and 21 to 27 °C has been recommended for tomato graft healing (Rivard and Louws, 2011). Johnson and Miles (2011) suggested that tomato transplants may be more tolerant of wider fluctuations in RH and temperature compared with grafted watermelon (Citrullus lanatus) and eggplant (Solanum melongena) transplants. This flexibility in RH and temperature management is advantageous for small-scale growers who may not be able to tightly regulate the environmental conditions of a healing chamber.
Although the effects of air temperature have been examined, there has been no known work dedicated to examining the role of substrate temperature in grafted tomato transplant survival. Prior work involving tomato substrate and root-zone temperature modifications may be an indication of the potential role that managing substrate temperature can have in tomato graft healing. Hurewitz and Janes (1983) demonstrated that 30 °C was the optimal temperature for tomato transplant growth based on the response measured in plant fresh weight, dry weight, and leaf area. This optimum temperature was reaffirmed by McMichael and Burke (1998). However, Tindall et al. (1990) found that tomato shoot growth, plant height, and water use were optimized at a lower temperature of 25 °C. It is expected that changes in substrate temperatures for grafted tomato transplants would be observed in air temperatures in corresponding healing chambers.
Because of the lack of information regarding how substrate temperatures could affect tomato graft healing, and because of the various recommendations for the management of air temperatures and light levels, we aimed to explore the effects of altering tomato root-zone temperatures while simultaneously assessing various durations of light exclusion. The design of light treatments was based on evolving recommendations for healing chamber management that have been previously outlined. Specifically, we were interested in the response of grafted tomato transplants under no light reduction in small-scale healing chambers. Our experimental design combined current small-scale grower practices, prior work involving healing chamber modifications, and alternative approaches to temperature and light management. We took a novel approach to graft transplant temperature management by comparing treatments placed on propagation mats compared with those kept in ambient conditions. We hypothesized that 4 d of light exclusion and the use of propagation heat mats would improve grafted tomato transplant survival and growth during the healing period.
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