Timing of Ethylene Inhibition Affects Internal Browning and Quality of 'Gala' Apples in Long-Term Low Oxygen Storage.

The objective of this study was to evaluate the timing of ethylene inhibition with preharvest and postharvest 1-methylcyclopropene (1-MCP) treatments on internal browning and quality of 'Gala' apples in long-term low O2 storage. 'Gala' apples were obtained from the same commercial orchard during their harvesting period for 2 years of study. Preharvest 1-MCP orchard spray (3.8% a.i) was applied at the label rate of 60 g 1-MCP per acre in the first year. Postharvest 1-MCP (1 µl L-1) treatments were made for 24 h at 0.5°C either at harvest time (1 day after harvest) or after storage in controlled atmosphere (CA) in both years. Apples were stored in 1.5 kPa O2 + 0.5 kPa CO2 or 0.6 kPa O2 + <0.5 kPa CO2 for 9 months in the first year and 1.5, 1.0, or 0.5 kPa O2 + 0.5 kPa CO2 for 8 months in the second year. Storage regimes with O2 concentrations less than 1 kPa were based on fruit respiration using SafePod™ technology. After removal from storage, all apples were then evaluated for internal browning and other quality attributes after 1, 7, and 14 days at room temperature (RT, 21-22°C). Internal browning developed in 'Gala' apples during both years of study, with up to 16% incidence across treatments in the first year and up to 84% in the second year. Apples stored in 0.5-0.6 kPa O2 had significantly less internal browning during both years of study, compared to apples stored in higher O2. The effect of 1-MCP on internal browning was negligible in 0.5-0.6 kPa O2 storage. 'Gala' stored in 1.5 kPa O2 and treated with postharvest 1-MCP after storage had significantly less internal browning with preharvest 1-MCP than those without preharvest treatment. Apples treated with postharvest 1-MCP at harvest time, instead of after storage, did not exhibit this same effect. Preharvest 1-MCP-treated fruit maintained greater firmness retention than those without preharvest 1-MCP, and this effect was further enhanced when 1-MCP was applied after storage. Postharvest 1-MCP had no effect on firmness retention in fruit without preharvest 1-MCP, but lower O2 maintained greater firmness in those apples. Preharvest 1-MCP had no significant effect on internal ethylene concentration, whereas it was reduced by postharvest 1-MCP at harvest time in the first year of study, regardless of storage regimes. However, internal ethylene was only affected by storage regime in the second year, with lower concentration in fruit from 0.5 kPa O2 than in those from higher O2. Greasiness developed only in the second year and postharvest 1-MCP consistently reduced it, regardless of treatment timing and storage regime. There was no greasiness in apples treated with postharvest 1-MCP at harvest and then held in 0.5 kPa O2 for 8 months plus 14 days at room temperature. Soluble solids concentration and malic acid content were slightly higher in 'Gala' apples with preharvest 1-MCP compared to those without, whereas there was little and inconsistent effect of postharvest 1-MCP on these attributes. Overall, storage regimes with less than 1 kPa O2 provided the least amount of internal browning and best quality attributes. Ethylene inhibition provided further benefits, but this was dependent on the timing of 1-MCP treatment.

Metabolic Alterations in Postharvest Pear Fruit As Influenced by 1-Methylcyclopropene and Controlled Atmosphere Storage.

This study assessed the impact of 1-methylcyclopropene (1-MCP) and controlled atmosphere (CA) on the metabolism of targeted amino acids, organic acids, and antioxidants in stored 'AC Harrow Crisp' pears and their relationships to storage disorders. Pears were treated with 0 or 300 nL L-1 1-MCP and stored at 0 °C under ambient air or CA. Spectrophotometric assays demonstrated that glutathione levels fluctuated with storage and were most preserved by 1-MCP under ambient air. HPLC analysis revealed that ascorbate concentrations declined with storage and were little affected by 1-MCP and CA. Citrate, lactate, and fumarate accumulated with storage but were differentially affected by 1-MCP. Aspartate and glutamate concentrations were greater with 1-MCP; γ-aminobutyrate accumulated in disordered fruit. Principal component analysis demonstrated that alterations in citrate and fumarate were, respectively, correlated with internal breakdown and senescent scald. γ-Aminobutyrate and alanine were associated with internal cavities. All disorders were associated with antioxidant depletion.

Oxidative metabolism is associated with physiological disorders in fruits stored under multiple environmental stresses.

In combination with low temperature, controlled atmosphere storage and 1-methylcyclopropene (ethylene antagonist) application are used to delay senescence of many fruits and vegetables. Controlled atmosphere consists of low O2 and elevated CO2. When sub-optimal partial pressures are used, these practices represent multiple abiotic stresses that can promote the development of physiological disorders in pome fruit, including flesh browning and cavities, although there is some evidence for genetic differences in susceptibility. In the absence of surface disorders, fruit with flesh injuries are not easily distinguished from asymptomatic fruit until these are consumed. Oxidative stress metabolites tend to accumulate (e.g., γ-aminobutyrate) or rapidly decline (e.g., ascorbate and glutathione) in vegetative tissues exposed to hypoxic and/or elevated CO2 environments. Moreover, these phenomena can be associated with altered energy and redox status. Biochemical investigations of Arabidopsis and tomato plants with genetically-altered levels of enzymes associated with the γ-aminobutyrate shunt and the ascorbate-glutathione pathway indicate that these metabolic processes are functionally related and critical for dampening the oxidative burst in vegetative and fruit tissues, respectively. Here, we hypothesize that γ-aminobutyrate accumulation, as well energy and antioxidant depletion are associated with the development of physiological injury in pome fruit under multiple environmental stresses. An improved understanding of this relationship could assist in maintaining the quality of stored fruit.