Neoagaro-oligosaccharide delays the postharvest decay of passion fruit and its relationship with the respiratory metabolism

[Objective] The passion (Passiflora caerulea L.) fruit is a typical tropical and subtropical fruit native to Brazil. In China, passion fruit is widely cultivated in Guangxi, Fujian, Hainan, Guangdong and Yunnan Provinces. The passion fruit has unique flavor and contains a variety of nutrients with dietary therapy effect. However, as a climacteric fruit, the harvested passion fruit is highly sensitive to ethylene and thereby the respiration climacteric can be activated very soon, leading to rapid postharvest ripening, senescence, and serious decay, such as pericarp shrinking, disease outbreak, and mechanical injury-induced browning. These quality deteriorations will greatly reduce the shelf-life and commercial value of the fruits. Therefore, it is of great significance to develop effective postharvest technology to retard the postharvest ripening and senescence and enhance the storability of postharvest passion fruit. The utilization of marine biological resources is drawing lots of attention currently in the field of agri-food preservation. The oligosaccharides obtained from seaweed is a promising substance for maintaining fruit quality, but to date little information is known about neither its regulation mechanism underlying the quality-keeping effect, nor its effects on postharvest passion fruit. On the other hand, as the key factor in postharvest ripening and senescence, respiration is so crucial to fruit physiological change that should be given the first and foremost attention when considering quality variation. Also, respiration is an integrated reflection of a series of metabolic processes known as the hub of matter and energy metabolism. The respiratory metabolic pathways involving material metabolism of the harvested fruit include embden-meyerhof-parnas (EMP), tricarboxylic acid (TCA) cycle and pentose phosphate pathway (PPP), and the pathways involving energy production include electron transport chain (ETC). Besides, pyridine nucleotides act as cofactors in these pathways and thereby can be seen as the indicators of the balance in respiratory metabolic pathways. Therefore, in this study, the effects of neoagaro-oligosaccharide (NAOS) solution dipping treatment on the quality of postharvest passion fruit were investigated, with specific regards to its regulatory mechanism on the respiratory metabolism, aiming to provide technical reference and theoretical basis for retarding the postharvest decay and extending the shelf-life of the passion fruit. [Methods] The yellow passion fruits were harvested at commercial maturity and the fruits selected for test were divided into two groups. One group of fruits was soaked in 180 mg·L-1 NAOS solution for 15 min and the other one was soaked in distilled water as control. The fruits were then air dried and stored at (25±1) ℃ and 85% relative humidity for 18 days. During the storage, the changes of the fruit decay rate, respiration rate, firmness, weight loss rate, pericarp moisture content, and the contents of total soluble solids (TSS) and titrable acid (TA) in pulp were investigated to evaluate the effect of NAOS treatment on fruit quality. Furthermore, the enzyme activities of the respiratory metabolic pathways were assayed, such as phosphohexose isomerase (PGI) in EMP, succinate dehydrogenase (SDH) in TCA cycle, glucose-6-phosphate dehydrogenase (G-6-PDH) and 6-phosphogluconate dehydrogenase (6-PGDH) in PPP, cytochrome C oxidase (CCO) in ETC and other terminal oxidases including ascorbic acid oxidase (AAO) and polyphenol oxidase (PPO). Besides, the contents of pyridine nucleotides including nicotinamide adenine dinucleotide (NAD) and its reduced form (NADH), and nicotinamide adenine dinucleotide phosphate (NADP) and its reduced form (NADPH) and the activity of involving enzyme, NAD kinase (NADK), were determined. [Results] Compared with the control group, NAOS-treated fruits could maintain higher firmness, pericarp moisture content, and the contents of pulp TSS and TA during the storage, with lower weight loss rate and decay rate; moreover, NAOS treatment delayed the respiration climacteric of the postharvest passion fruits, lowered the activities of PGI, SDH, CCO, AAO and PPO, as well as the contents of NAD and NADH, but elevated the levels of G-6-PDH+6-PGDH, NADK, NADP and NADPH. [Conclusion] The NAOS treatment could regulate the respiratory metabolism of the harvested passion fruits via alleviating the EMP-TCA pathway and ETC, and enhancing the PPP, which jointly contributed to retarded respiration climacteric and reduced substrate consumption, leading to the slower postharvest ripeness and senescence, and thereby the decay occurrence was inhibited and the quality was maintained. Therefore, the NAOS treatment could be an effective handling to control the postharvest decay of the passion fruits via regulating its physiological activities.