Tomato (Solanum lycopersicum) grafting has been widely used because of its major benefits in conferring plant resistance to soilborne pathogens, alleviating the deleterious effects of abiotic stresses, and enhancing water and nutrient use efficiency (Singh et al., 2017). In the United States, many small-scale organic vegetable growers graft plants on their own (Kubota et al., 2008) for using particular rootstock–scion combinations to deal with site-specific issues; however, they may not be able to produce high-quality grafted tomato plants (Meyer et al., 2017). Grafted tomato plants are usually kept in a healing chamber for 7 to 10 DAG (Bausher, 2013; Hu et al., 2015). For the first several days, grafts are often placed in a high-humidity environment and ventilation is gradually increased (Fan et al., 2015). Grafted tomato typically requires 7 to 14 d to connect vascular bundles between the scion and rootstock plants (Fan et al., 2015), although the major hydraulic connections begin to function from 6 to 7 DAG (Turquois and Malone, 1996). It has also been suggested that vascular connection usually takes 5 to 8 d to form, and it takes 14 d for the graft union to fully heal (Johnson et al., 2016). Fernández-García et al. (2004) monitored root hydraulic conductance of grafted tomato plants at 4, 8, 12, and 15 DAG and observed a linear increase from 4 to 12 DAG starting from 0, but an evident increase was not observed after 12 DAG.
Many of the previous reports have indicated the importance of humidity level during the graft healing period on the survival and quality of grafted seedlings. According to Oda (2007), even a momentary drying of the cut surface can be lethal. Wei et al. (2018) tested three relative humidity (RH) regimens, including 70% to 80%, 80% to 90%, and 90% to 100%, to demonstrate that 97% to 98% was the optimal RH for successful healing of grafted tomato seedlings, whereas 70% to 80% and 80% to 90% resulted in apparent wilting of grafted plants. Guan and Hallett (2016) recommended putting newly grafted tomato plants in an environment with 85% to 95% RH for the first 48 to 72 h. Vu et al. (2014) compared combinations of four temperatures and three RH levels on the survival rate of grafted tomato plants and found that at 23 °C and 80% RH plants had a higher survival rate than healing at 70% or 90% RH at 23 °C. Buajaila et al. (2018) investigated 100% and 50% RH and 0%, 25%, and 50% light levels on eggplant (Solanum melongena), pepper (Capsicum annuum), and tomato graft survival rate. They found that at 21 DAG, when averaged over all crops tested, graft healing under 100% RH had a higher survival rate than healing under 50% RH regardless of the light level. Under 50% RH, the survival rate was highest with 25% light followed by 0% light, and 50% light treatment had only a 32% survival rate. However, Johnson and Miles (2011) suggested that high RH may not be essential for healing tomato grafts as they exhibited similarly high survival rates at average RH levels of 98%, 81%, and 52%. Masterson et al. (2016) also reported that high humidity (>85% RH) may not be required for grafted tomato healing. Both studies used self-grafted tomato scion plants instead of grafted scion plants with commercial rootstocks. Moreover, they only documented the survival rate of grafts without an examination of grafted plant growth and flowering. Hence, more research is needed to further validate the feasibility of healing tomato grafts using lower RH levels and determine how low RH healing conditions impact the growth and development of grafted plants compared with grafts healed under standard healing conditions.
An extended period under high humidity also promotes the formation of adventitious roots from the tomato scion during graft healing (Guan and Hallett, 2016), which could lead to rootstock growth decline in grafted tomato seedlings or even grafting failure and rootstock death if adventitious roots grow into soil/medium (Meyer et al., 2017). Adventitious rooting from the scion could also allow soilborne pathogens to bypass the resistance of the rootstock and invade the scion, and removing adventitious roots requires extra labor and time. Moreover, keeping tomato plants in a high humidity environment for a long time may be conducive to disease development (Vu et al., 2014) and cause physiological disorders such as edema, which is irreversible (Johnson et al., 2016). Modifying humidity levels and duration during tomato graft healing could potentially help reduce scion adventitious root formation and increase grafted plant quality.
In this study, we tested a novel tomato graft healing method using a chamberless approach in a regular indoor ambient environment. The objectives were to compare the survival rate, adventitious root formation, and posthealing growth of grafted tomato seedlings between chamberless healing and standard healing chamber methods to explore the feasibility of chamberless healing to assist small-scale production of grafted tomato transplants.
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