Strawberry (Fragaria ×ananassa Duch.), the third most consumed fresh fruit in the U.S. market in 2017 (USDA, 2019), is attractive and highly valued, but it is delicate. Large cells with thin cell walls result in the fragile structure of strawberry fruit (Szczesniak and Smith, 1969), which become more susceptible to mechanical injury as the ripening process proceeds. Strawberries are harvested when mostly or fully ripe according to the commercial standard for U.S. No. 1 grade, which states that strawberry fruit must be at least 3/4 red (USDA, 2006). Bruising occurs mainly during harvesting, packing, and transportation for horticultural crops (Prussia and Shewfelt, 1993). When plant tissues are wounded, the physical and metabolic reactions change in the damaged tissues and trigger ethylene production, resulting in major postharvest losses, decay, and accelerated senescence, thus affecting strawberry quality and shelf life (Ferreira et al., 2009; Wills and Kim, 1995). The severity of wounding in strawberry depends on various factors, including the type of force applied on the fruit, cooling method, and rate of lowering pulp temperatures, along with the storage temperature (Ferreira et al., 2009). Internal fruit properties, such as texture, fruit maturity, water content, firmness, size, and shape (Hung and Prussia, 1989; Van Linden et al., 2006), and the nature of cultivars (Jiménez-Jiménez et al., 2013; Kunze et al., 1975) also contribute to the bruising response. Bruising severity and the tissues being affected can be determined by dissecting the fruit and measuring the bruise diameter, depth, and volume, which have strong positive correlations with impact energy (Schoorl and Holt, 1980).
Mechanical injury, such as cutting, abrasion, or bruising, has been shown to cause climacteric fruits such as banana and tomato to produce an increasing amount of ethylene (Moretti et al., 1998; Palmer and McGlasson, 1969). However, strawberry fruit is often regarded as a nonclimacteric and ethylene-insensitive crop. In previous research, some researchers have suggested that ethylene could not only stimulate the respiration rate of nonclimacteric fruit, but also accelerate fruit color development, softening, and ion leakage, with a deleterious logarithmic linear response to 10 to <0.005 μL·L−1 ethylene in terms of shelf life (Wills and Kim, 1995; Wills and Wong, 1996; Wills et al., 1999). In contrast, other researchers have presented evidence showing little or no effect of ethylene on strawberries (El-Kazzaz et al., 1983; Tian et al., 2000). The role of ethylene in nonclimacteric fruit wounding response is uncertain, and the differential response toward ethylene in previous studies was reported to be affected by maturity, storage time, and cultivar characteristics (El-Kazzaz et al., 1983; Picón et al., 1993; Tian et al., 2000).
In tests to determine the potential benefits of removing ethylene from the strawberry postharvest environment that were conducted before the present research (Brecht et al., 2016), we observed significant cultivar variation in response to ethylene scrubbing in terms of bruising severity and calyx yellowing and browning. We hypothesized that those differences in response to ethylene scrubbing might be related to differences in wound ethylene production or ethylene sensitivity among the cultivars that had been tested. Therefore, in this study, several major commercial strawberry cultivars, including Monterey, Sweet Sensation (Florida 127), Florida Radiance, and two proprietary cultivars (Cultivar A and Cultivar B) from a private breeding program, were investigated to determine their bruising susceptibility and wound response in terms of timing and the rate of wound ethylene production to inform further improvements in postharvest procedures.
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