Grafting onto disease-resistant rootstocks is an important technology for overcoming soil-borne disease (Buller et al., 2013; Guan et al., 2012; Louws et al., 2010) and is used for watermelon production primarily in Asia, Europe, and the Middle East (Cohen et al., 2007; Davis et al., 2008). However, the high cost of grafted transplants has prevented the use of grafted transplants in U.S. production. As methyl bromide, an effective and inexpensive soil fumigant, has been phased out (U.S. Department of Agriculture, 2012) and disease-free land has become scarce, there has begun to be a need for grafted watermelon transplants in the United States. With current disease conditions in U.S. soils, it is estimated that at least 6.69 million grafted watermelon transplants per year would be required to combat soil-borne diseases previously controlled with fumigants (D. Liere, personal communication).
Much of the cost of grafted transplants is associated with the labor involved in their production and maintenance (Davis et al., 2008). The majority of these labor costs are a result of rootstock regrowth (Choi et al., 2002; Memmott and Hassell, 2010), which competes with the watermelon scion for nutrients and sunlight. In countries currently using grafted transplants commercially, the meristem is manually removed at the grafting stage (Hassell et al., 2008). Meristematic regrowth is manually removed during graft healing, at the transplanting stages, and thereafter as needed (Lee and Oda, 2003). Rootstock regrowth removal is an expensive, labor-intensive process and has been prohibitive to the adoption of grafted transplants in U.S. watermelon production.
Rootstock treatment with fatty alcohol can control meristematic regrowth by burning the meristem tissue and allow for successful grafting without the risk of regrowth. Commercial watermelon grafting methods require at least one rootstock cotyledon to remain intact to ensure graft success, and grafting must be done soon after the cotyledons unfold to limit regrowth (Hassell et al., 2008; Lee and Oda, 2003). Rootstock cotyledons provide energy to maintain the plant and heal the graft. Cucurbit seedlings are dependent upon seed reserves only during preemergent growth (Bisognin et al., 2005) and depend completely on the leaf-like, highly photosynthetic cotyledons for further development (Penny et al., 1976). At least one rootstock cotyledon is needed for early growth and establishment (Bisognin et al., 2005).
Fatty alcohol was originally used to control growth of axillary meristems on topped tobacco (Nicotiana tabacum). Because the fatty alcohols target only the actively dividing cells in meristems, the tobacco leaves remain intact, and energy that would have been sent to new growth is now stored in the leaves (Steffens et al., 1967). An application of fatty alcohol to the apical meristem of cucurbit rootstock seedlings before grafting has been shown to be an effective means of eliminating meristem tissue and controlling regrowth (Daley and Hassell, 2014). We observed that over a 21-d period following fatty alcohol treatment, both bottle gourd and interspecific hybrid squash rootstock seedling cotyledons seemed to expand, becoming long and rigid. The hypocotyl also increased in length and diameter over the observed time, yet the seedlings remained viable for grafting. We hypothesize that the increase in seedling size was due to an accumulation of carbohydrates within the rootstock. Thus, this experiment was designed to determine changes in rootstock seedling development over 21 DAT with fatty alcohol, as well as the effect on carbohydrate concentrations within the hypocotyls and cotyledons of the rootstock seedlings.
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