‘Empire’ is a major commercial apple cultivar produced in the northeastern United States and Canada, as it appeals to consumers for fresh quality attributes, texture, and flavor profile. Despite its popularity, ‘Empire’ apples are sensitive to elevated CO2 and chilling conditions, making them susceptible to certain physiological disorders in storage, including external CO2 injury and flesh browning (DeEll and Ehsani-Moghaddam, 2012; Fawbush et al., 2008; Watkins and Nock, 2012).
External CO2 injury is characterized by rough bronze lesions that are often partially sunken on the peel with well-defined edges (Fawbush et al., 2008; Meheriuk et al., 1994; Watkins et al., 1997). Development of external CO2 injury usually occurs within the early stages of CA storage and it is more prevalent with rapid establishment of elevated CO2 levels, especially if apples are not sufficiently cooled before storage (Burmeister and Dilley, 1995; DeEll et al., 2016; Meheriuk et al., 1994; Watkins and Liu, 2010; Watkins et al., 1997). In most cases, symptoms of external CO2 injury progress with minimal internal flesh browning or damage.
Flesh browning, also known as internal browning, is characterized by diffuse browning of the flesh tissue, and it is typically not visible from the external surface (DeEll and Ehsani-Moghaddam, 2012; DeEll et al., 2007; Meheriuk et al., 1994; Watkins and Liu, 2010). The onset of flesh browning in apples is postulated to be associated with low temperatures during storage (Jung and Watkins, 2011; Watkins and Liu, 2010).
Postharvest treatment with 1-MCP, an inhibitor of ethylene action, has been shown to improve quality characteristics of apples, including reduced ethylene production and respiration, as well as improved firmness and acidity retention (DeEll et al., 2007; Watkins, 2007). Unfortunately, 1-MCP can also exacerbate the susceptibility of ‘Empire’ apples to external CO2 injury and flesh browning disorders (DeEll and Ehsani-Moghaddam, 2012; DeEll et al., 2003, 2005; Fawbush et al., 2008).
Previous studies have reported the practice of delaying CA storage to be beneficial for reducing disorders in apples (DeEll and Ehsani-Moghaddam, 2012; Watkins and Nock, 2012). Delayed CA storage involves delaying the establishment of CA by holding fruit in ambient air at low temperatures before exposure to low O2 and elevated CO2 (Argenta et al., 2000; de Castro et al., 2007). Delaying CA for 1 or 2 months at 1 °C reduced external CO2 injury and flesh browning disorders in ‘Empire’ apples compared to fruit placed immediately into CA (DeEll and Ehsani-Moghaddam, 2012). Similarly, delaying CA for 2 or 4 weeks at 0.5 °C reduced CO2-induced flesh browning in ‘Pink Lady’ apples (de Castro et al., 2007). However, inconsistencies and year-to-year variation in the effects of delayed CA establishment for alleviating certain storage disorders in apples have also been documented (Argenta et al., 2000; DeEll and Ehsani-Moghaddam, 2012; DeEll et al., 2016; Watkins and Nock, 2012).
In recent years, there has been a renewed interest in low O2 storage of apples due to the advent of dynamic CA in which fruit response to stress is monitored. The application of low O2 (<2 kPa) during storage has shown advantages for maintaining apple quality characteristics, including reduced ethylene production and respiration, maintained fruit firmness, sugars and acidity levels, and delayed fruit senescence (Both et al., 2016; Thewes et al., 2015). Exposure to low O2 can also alleviate superficial scald in certain susceptible apple cultivars (Lumpkin et al., 2014; Pesis et al., 2007; Wang and Dilley, 2000; Zanella, 2003). When apples are held in standard CA (2–3 kPa O2), fruit are generally held in atmospheric conditions greater than their anaerobic compensation point, the point where fruit O2 consumption and CO2 production are at minimal rates (Thewes et al., 2015; Yearsley et al., 1996). Consequently, fruit respiration and associated metabolic processes are not at minimum, leading to potential fruit quality loss in standard CA storage. However, further reduction of O2 levels during storage also raises concerns for increased low O2-related stress and injury (Wright et al., 2015).
Fruit RQ, defined as the ratio of CO2 production to O2 consumption, can be used to assess low O2-related stress in fruit and as a signal to adjust atmospheric composition within storage systems (Gran and Beaudry, 1993; Yearsley et al., 1996). By monitoring O2-related stress in real-time with adjustments of O2 partial pressures throughout the storage period, the respiratory health of apples under low O2 can be observed autonomously. This is considered dynamic CA storage, which involves the reduction of O2 partial pressures to the lowest possible tolerance level without inducing excess anaerobic metabolism affecting fruit quality and off-flavours associated with fermentation products (Bessemans et al., 2016; Thewes et al., 2015).
Although the benefits of low O2 storage on apple quality are well-documented in the literature (Bessemans et al., 2016; Köpcke, 2015; Lumpkin et al., 2014; Rebeaud and Gasser, 2015; Veltman et al., 2003; Zanella, 2003), the response of a CO2 and chilling sensitive apple like ‘Empire’ to low O2 levels (<2 kPa) in combination with 1-MCP is not understood. The objective of this study was to investigate the effects of low O2 and 1-MCP on storage disorders and fruit quality of ‘Empire’ apples. RQ-based dynamic CA storage with 0.6 kPa O2 was also evaluated in the second year of this study.
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