Sweetpotato [Ipomoea batatas (L.) Lam.] is an important crop in developing countries and has many uses, ranging from consumption of fresh roots or leaves to processing into animal feed, starch, flour, candy, and alcohol. In the United States, it is an important vegetable crop with more than 59,000 ha planted in 2012 (U.S. Department of Agriculture–National Agricultural Statistics Service, 2013). ‘Beauregard’ is the most popular sweetpotato variety grown for the fresh market in Mississippi and the Gulf South (Boudreaux, 2012; Meyers et al., 2013) but is prone to skinning (surface abrasion) during harvest and postharvest handling. Skinned areas become susceptible to pathogen infections and moisture loss, which results in an unattractive appearance for the fresh market (Clark et al., 2013). Therefore, skinning is associated with losses resulting from decay as well as unmarketable produce.
The periderm in sweetpotato is the first line of defense and is critical to reduce or avoid skinning and wounding at harvest (Clark et al., 2013). It is composed of three layers: phellem, phellogen, and phelloderm. The phellogen (meristematic cell layer) generates the phelloderm toward the inside of the root and the phellem (skin) toward the outside. Skinning occurs when an abrasion forces the periderm to fracture across the phellem (tensile fracture) and along the phellogen (shear fracture) resulting in the separation of the phellem from the phelloderm (Hammerle, 1970; Lulai, 2002; Sabba and Lulai, 2002; Webster et al., 1973). The wound healing process is characterized by suberization/lignification of the exposed cells and developing of the new periderm beneath (Lulai and Suttle, 2004; St. Amand and Randle, 1989). In addition, ethylene has been reported to have a role in wound healing (St. Amand and Randle, 1989).
Lignin and suberin contribute to both cell wall strength and resistance to water loss during plant growth and development (Bernards and Lewis, 1998; Boudet, 2000). Although lignin is composed of phenolic (aromatic) polymers, suberin contains both a polyaliphatic domain and polyphenolics domain, which are often linked to lignin. Biosynthesis of lignin/suberin shares common intermediates (hydroxycinnamoyl-CoA derivatives) from which the lignification pathway diverges from the suberization pathway. Lignification/suberization has been extensively studied in other plants during wound healing (Bernards and Lewis, 1998; Boudet, 2000); however, very little information is available about the relationship between lignification/suberization of the native periderm in sweetpotato and skinning resistance. A trend, although non-significant, between skin adhesion and lignin content was suggested in sweetpotato exposed to different temperatures (Villavicencio et al., 2007). Furthermore, setting the skin to reduce skinning in sweetpotato at harvest has been achieved to some degree by preharvest foliar application of ethephon or devining (LaBonte and Wright, 1993; Schultheis et al., 2000), but skin lignin/suberin content was not determined. Therefore, we hypothesize that foliar application of ethephon enhances skin lignification and/or suberization in storage roots promoting skinning resistance. The objectives of this study are 1) to compare the effectiveness of preharvest foliar applications of ethephon and defoliation/devining in reducing skinning incidence and severity at harvest; 2) to determine the changes in skin lignification/suberization produced by these preharvest practices; and 3) to associate skin lignification/suberization with skinning resistance in sweetpotato storage roots.
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