The Technical Evolution of CA Storage Protocols and the Advancements in Elucidating the Fruit Responses to Low Oxygen Stress

Abstract Innovations in postharvest technology also deal with the modulation of gas composition in storage rooms and/or packaging, in particular concerning oxygen levels, which, for some storage protocols and fruit crops, is set at extremely low concentrations (less than 1 kPa). Since the establishment of the first commercial CA rooms, the oxygen concentration showed a constant decrease throughout decades, reaching the lowest levels used in ULO (Ultra Low Oxygen), ILOS (Initial Low Oxygen Stress), and DCA (Dynamic Controlled Atmosphere) applications. This decreasing trend in oxygen concentrations used in storage rooms resulted in a general improvement of the quality parameters and the marketable life of the commodities (apples, in particular). The optimization of these techniques and the reduction of the risks (development of physiological disorders, off-odors, off-flavors) associated with keeping the fruit at such extreme conditions need to be based on a better knowledge of the metabolic responses to hypoxia. The metabolic responses of fruit tissues to low-oxygen stress are, as observed in model plants, mainly related to dramatic changes in mitochondrial respiration and the activation of the fermentative metabolism that appear to be differently affected (resulting in different levels of ethanol accumulation) by different low oxygen levels and in relation to the genetic background. Among others, ethanol, alanine, asparagine and aspartate concentrations in cortex tissue markedly change in relation to hypoxic conditions, thus representing possible metabolic markers of this kind of stress. Specific volatile compounds of apples are also differentially affected, whereas other volatile classes appear more stable. Using large-scale transcriptomic approaches coupled with metabolic profiling analyses it is now possible to better describe the global fruit responses to low oxygen conditions. In apples, in addition to the expression of genes involved in primary metabolism (major CHO, fermentation), hypoxia also affects specific secondary metabolic pathways that appear to be selectively modulated by different low oxygen treatments. Members of the ERF-VII transcription factors (TFs) gene family displayed differential expression suggesting their involvement in the modulation or controlling mechanisms of hypoxic responses, as observed in model species.