Impact of CO2 on the Energy Status of 'Honeycrisp' Apple in Controlled-atmosphere Storage

‘Honeycrisp’ fruit is very sensitive to the low O₂ and elevated CO₂ levels found in controlled atmosphere (CA) storage. CA-related injury causes jagged brown lesions and lens-shaped voids in the apple cortex within the first six weeks of storage. We have previously shown that fermentative metabolism was influenced by CA conditions, but the volatiles do not appear to be indicative of CA-related injury per se. The presence of fermentation suggested an inhibition of the apple fruit's ability to efficiently form energy-related metabolites. We investigated whether adenylate energy charge (AEC) might contribute to cell death in ‘Honeycrisp’.  Energy compounds including adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) were determined in apple tissues by HPLC/UV at 259.1nm. Samples were lyophilized apple fruit cortex from fruits subjected to CO₂ levels of 0, 1.5, 3, 10 and 20 kPa in combination with 3 kPa O₂ in addition to the fruits treated with diphenylamine (DPA) drench (1000 µL·L^-1, 30 s) held in 10 kPa CO₂ + 3 kPa O₂ during storage at 3°C. The fruits were sampled at day 0, 3, 7, when reaching half maximal injury and at maximal injury, depending on the treatments. ATP levels in apple tissues decreased during CA storage, especially at 10 kPa and 20 kPa CO₂. ADP contents in apple tissues were very low at harvest, increased at day 3 and then steadily decreased. AMP contents did not change much except at 20 kPa CO₂ which increased dramatically during storage time. Adenylate energy charge (AEC) decreased during CA storage except for apple treated with DPA which maintained above 0.75. We propose that CA conditions might suppress the AEC, leading to cell death and cortex browning in ‘Honeycrisp’ as a symptom of CA injury and that as long as ‘Honeycrisp’ apple is maintained with an AEC above 0.75, it likely remains healthy.