Globally, watermelon is the largest produced fruit crop (Tlili et al., 2011), with18.1 MT produced in 2018 (USDA, 2019). In the United States, seedless (triploid) watermelon comprises 90% to 95% of the market (USDA, 2019). One of the United States Department of Agriculture (USDA) grade defects in triploid watermelon is the appearance of hollow heart (HH), which is also known as internal cracking (USDA, 2006). In watermelon, HH occurs as an internal split or void that usually starts in the placental (heart) tissue and can extend through the carpels into the epidermal (rind) layers (Johnson, 2014, 2015). Depending on the season and demand for seedless watermelon, fruit rated with moderate to severe HH will be rejected for marketability (USDA, 2006), ultimately resulting in significant monetary losses for both growers and the industry (Fig. 1).
It is thought that HH develops as a result of unequal expansion of placental and rind tissues (Kano, 1993), and it most often occurs in fruit from the crown set (or first harvested fruit) (Diezma-Iglesias et al., 2004). The watermelon rind continues to expand and differentiate during fruit growth and maturation. Placental tissue cells stop dividing at 7 d after anthesis, and cells begin to enlarge with the accumulation of water, sugars, proteins, and nutrients (Elmstrom and Davis, 1981; Kano, 1993). A rapid change in water potential between placental cells and rind cells can lead to a separation of the carpels, ovule tissue, and placental tissue, causing hollowing or a cavity to develop in the flesh (Johnson, 2014, 2015; Kano, 1993).
Inadequate pollination is thought to be one of the leading causes of HH in watermelon and is generally worse in triploid (seedless) watermelons because they require a diploid (seeded) pollenizer with viable pollen (Diezma-Iglesias et al., 2004). Inadequate pollination can come from an improper diploid pollenizer–triploid combination (McGregor and Waters, 2014), reduced bee visits, or unfavorable weather conditions (e.g., decrease in pollen viability and/or pollinator activity) (Pisanty et al., 2016). Additionally, cytokinins known to promote cell division can affect watermelon flesh firmness and cell density (Soteriou et al., 2017) and may contribute to inadequate pollination.
Previous studies have reported that watermelon fruit from plantings with lower diploid-to-triploid ratios have a higher incidence of HH (Fiacchino and Walters, 2003; Freeman et al., 2007). Generally, any diploid-to-triploid ratio less than 20% of a field (e.g., four triploids to one interplanted diploid pollenizer) increases HH formation (Freeman et al., 2007). To achieve optimal fruit yields and high watermelon fruit quality, 25% to 33% of a field should be planted with diploid plants (Fiacchino and Walters, 2003; Freeman and Olson, 2007a). This is achieved by interplanting a seeded watermelon cultigen or diploid pollenizer in the same field either in the same row as triploid plants or as a dedicated row only for diploid pollenizers (Freeman et al., 2007).
In watermelon cultigen evaluations, the HH incidence is highly variable, even when planted at the same geographic location (Seminis Seeds, 2019), which increases the difficulty screening germplasms for the disorder. The relationship between watermelon fruit firmness and the incidence of HH has not been studied, although researchers have suggested fruit tissue firmness may provide some indication of HH disorder (Guan, 2018). Cultigen evaluations for watermelon productivity and fruit traits are routinely conducted across the United States (Coolong, 2015; Johnson, 2017; Schultheis and Thompson, 2014). Fruit firmness and HH incidence are collected as part of these evaluations in North Carolina, resulting in datasets that might be useful for detecting relationships. Therefore, in this study, 3 years of seedless watermelon cultigen evaluation data were used to determine possible germplasm trends for high and low HH incidences and tissue firmness.
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Watermelon cultigen names of the 13 common cultigens used in 2012, 2013, and 2014, and their relative fruit characteristics.
Fungicide, insecticide, and miticide programs used for watermelon in 2012, 2013 and 2014.
Logistic regression showing the incidence of hollow heart (HH) among 2012, 2013, and 2014 seedless watermelon evaluations.