Insight into the physiological and molecular mechanisms of hot air treatment which reduce internal browning in winter-harvested pineapples

Internal browning (IB) in pineapple is a serious physiological disorder that occurs during storage which results in serious quality deterioration and losses. This study investigated the inhibitory effects of postharvest hot air (HA) treatment on pineapple IB occurrence during storage. Winter-grown 'Comte de Paris' pineapples were treated with HA (38 °C and 80–90% RH for 3 d) before storage at 25 °C. The results showed that HA treatment reduced the incidence of IB where HA treatment also resulted in lower malondialdehyde (MDA) content and superoxide anion (O2-.) production rates, suggesting that oxidative stresses are involved in the development of IB in pineapple. Further metabolomic and related enzyme activity results showed that the HA treatment inhibited the accumulation of p-coumaric acid which resulted from the increase of tyrosine ammonia-lyase, cinnamate-4-hydroxylase and p-coumarate: coenzyme A ligase enzyme activities. Lower levels of 4-methoxy-4-vinylphenol, L-tyrosine, and ascorbic acid and higher levels of polyphenol oxidase were also observed in control fruit with IB symptoms. Transcriptome analysis revealed that the pathways of alpha-linolenic acid metabolism, phenylpropanoid biosynthesis, MAPK signal pathway and the protein processing in endoplasmic reticulum were significantly affected by HA treatment. Identification of the IB-related genes family in these pathways showed that HA treatment inhibited the expressions of phospholipase D (PLD), lipoxygenase (LOX), dioxygenase (DOX), resiratory burst oxidase homologue (RBOH), phenylalanine/tyrosine ammonia-lyase (PAL/TAL), cinnamate-4-hydroxylase (C4H), p-coumarate: coenzyme A ligase (4CL) and polyphenol oxidase (PPO) and up-regulated the expressions of heat shock factors (HSF), heat shock proteins (HSPs), phospholipase A2 (PLA2) and ascorbate peroxidase (APX), therefore alleviating IB in winter-harvested pineapples. A conceptual model elucidating the alleviation of IB by HA in pineapples was proposed.