Grafting watermelon onto disease-resistant rootstocks can confer resistance to soil borne diseases such as fusarium wilt (Fusarium oxysporum f. sp. niveum) and monosporascus root rot (Monosporascus cannonballus) (Beltran et al., 2008; Guan et al., 2012; Louws et al., 2010). With the loss of methyl bromide as part of the Clean Air Act (U.S. Department of Agriculture, 2012), watermelon grafting is currently the most promising method of fusarium wilt control (Louws et al., 2010). While only 5% of watermelon acreage in the United States is currently reported to be affected by this disease, arable land for rotation is decreasing and the cost of traveling to disease-free soil is difficult for growers to overcome (D. Liere, personal communication). Although the demand for commercially produced grafted plants is apparent, high production cost remains a major impediment to grafted transplant adoption in U.S. production.
The two most common commercially used grafting methods (over 90%) are the hole-insertion and the one-cotyledon method (Hassell et al., 2008). These methods require at least one cotyledon to remain intact to ensure graft success (Hassell et al., 2008), and both require manual meristem removal with a blade during grafting and even before transplanting of grafted plants. Manual meristem removal often removes the meristem only partially, allowing meristem regeneration to occur. Previous studies have demonstrated the success of fatty alcohol rootstock treatments in controlling meristematic regrowth (Daley and Hassell, 2014). Fatty alcohol products are traditionally used in tobacco (Nicotiana tabacum) production to remove axillary meristems and promote growth of remaining leaves. When fatty alcohol products are applied to rootstocks used for grafting watermelon, the rootstock meristematic tissue is destroyed and the rootstocks remain viable for grafting (Daley and Hassell, 2014).
In addition to regrowth control, rootstocks treated with fatty alcohol continue to live and photosynthesize, as the cotyledons are also functional leaves (Bisognin et al., 2005). Rather than putting energy into new growth, carbohydrates are stored in the hypocotyl and cotyledons of the rootstocks. Previous experiments have revealed a starch increase of 100- and 200-fold in hypocotyls of bottle gourd and interspecific hybrid squash rootstocks, respectively, over 21 d after fatty alcohol treatment (Daley et al., 2014). We hypothesize that this increase of stored energy in the rootstock could be harnessed by the plant to improve current grafting methods by providing sufficient energy to increase graft survival, rootstock rooting, and overall grafted transplant quality. The first experiment outlined in this article was designed to determine the effect of increased rootstock carbohydrate content on graft survival and rootstock rerooting using the one-cotyledon grafting method as described by Hassell et al. (2008).
With current grafting methods, at least one cotyledon is left on the rootstock during grafting. Because the rootstock cotyledons are larger than the cotyledons of watermelon seedlings, the rootstocks require an increase in individual tray cell size over standard cell size for grafted watermelon transplant production. This larger cell size is needed to accommodate the rootstock cotyledon when grafting. In addition to requiring a greater cell size, the large rootstock cotyledons can also harbor foliar disease such as powdery mildew [Podosphaera xanthii (Kousik et al., 2008)] that can prevent successful graft healing or infect successfully grafted transplants.
Decreasing the tray cell size and preventing the spread of disease via the rootstock cotyledon is an important objective in improving the efficiency of grafted transplant production. The development of a successful grafting method that removes both cotyledons would be advantageous to commercial production; however, results of previous studies on this type of method proved to be rather inconsistent for commercial application (Memmott, 2010). Because the cotyledon has been shown to provide energy to the developing rootstock seedling (Bisognin et al., 2005), we hypothesize that the inconsistencies in previous attempts to graft without the cotyledons were a result of a lack of energy in the hypocotyl to support the graft healing and rerooting of the transplant. The increased starch reserves in rootstocks treated with fatty alcohol over time may provide the required energy to overcome the reliance on the cotyledon and make grafting to the rootstock hypocotyl without the cotyledons feasible (Daley et al., 2014). Thus, a second experiment was conducted to test this hypothesis and demonstrate the effect of rootstock age after fatty alcohol treatment on graft survival and rootstock rerooting using the hypocotyl-only grafting method.
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