Heat and hypoxia stress and their effects on stored fruits

Purpose of review: This review gives an overview of the current knowledge of metabolic and molecular responses of fruit commodities to pre-storage heat treatment and to low oxygen conditions (hypoxia) during storage. Main findings: Both hot water and hot air treatments have been used on citrus fruit and peaches to control fungal pathogens and prevent chilling injury during storage. Proteomic and metabolomic studies of these two commodities found that the heat treatment led to the accumulation of stress and defense proteins, and caused an increase in sugars and a decrease in organic acids. Some studies also found increases in polyphenols and polyamines, which have antioxidative activity and protective effects against stress. The transcriptomic studies corroborated these findings and found that heat treatment led to changes in transcripts that were similar to those that occurred in cold storage. Hypoxia produced by decreasing oxygen concentration (generally together with increase in the carbon dioxide levels) is the basis for controlled and modified atmosphere (CA and MA) storage. As observed in vegetative tissues and model species, as well as in fruits, the responses to modified O2 levels include changes in primary and secondary metabolism as result of modifications at the levels of gene expression and mRNA translation. Recently, some molecular elements of low oxygen sensing and signaling in Arabidopsis have been elucidated, and similar sensing mechanisms are postulated to be present in fruit tissues to modulate the metabolic responses in relation to the different oxygen levels present in the storage rooms. Directions for future research: Integrated 'omics' approaches will result in a more precise understanding of postharvest stress physiology and adaptive mechanisms present in fruit crops, and elucidate the major pathways involved in the responses. This may lead to strategies that will allow commodities to resist stresses through targeting specific genes in a pathway, either by conventional breeding or other methods.