Study On The Effects Of Near-freezing Temperature(NFT) Storage On Quality Control Of Postharvest Guava(Psidium Guajava L.cv.'Pearl')

Guava(Psidium guajava L.),is one of the most popular fruits widely grown in tropical and subtropical regions,which has received great attention for industry development in lingnan region of China.Guava is popular in fresh fruit consumption owing to its distinctive flavor,taste,rich nutritional compounds,high moisture content and low calorie characteristics.However,as a kind of climacteric fruits,the high respiration rate and fast ripening speed of harvested guava limited its postharvest shelf life at ambient temperature and led to its perishable characteristic during storage.Besides,guava fruits were susceptible to chilling injury under cold storage,presenting severe physiological disorders such as soaking,failure to ripen and flesh discoloration,which had detrimental impacts on the commercial value and edible quality of guava.Therefore,it is essential to explore an effective method for limiting the chilling injury occurrence of guava and retarding texture softening during postharvest preservation.Near-freezing temperature(NFT)storage has been proved to be a promising preservation technology to delay senescence and extend the shelf life of postharvest fruits.In this study,NFT,6?and 10?storage were applied in the storage of postharvest guava(Psidium guajava L.cv.'Pearl')to evaluate the effects of NFT on chilling injury and texture softening of guava during storage,which were considered as the core problems in postharvest guava.In order to elucidate the mechanism of NFT storage in regulating postharvest quality of guava,the effects of NFT on cell antioxidant metabolism,ethylene synthesis and cell wall polysaccharide metabolism were studied.The main results are as follows:(1)The NFT of'Pearl'guava was determined to be-1.3±0.1?.The results showed that,the occurrence of chilling injury and respiratory peak in guava were inhibited effectively by NFT after 16 days of cold storage and 2 days of shelf-life simulation at 25±0.5?.Scanning electron microscopy(SEM)and membrane permeability results also indicated that guava stored under NFT maintained higher cell integrity and lower membrane permeability.(2)Studies on the mechanism of NFT regulating the occurrence of guava chilling injury suggested that,compared with 6?and 10?groups,the reactive oxygen species metabolism of guava stored under NFT was significantly inhibited,characterized by the lower level of H₂O₂and O₂^-·.In addition,guava fruit stored at NFT possessed higher antioxidant capacity(DPPH,ABTS free radical scavenging ability).NFT storage was proved to be effective in activating the antioxidant metabolic system of guava fruit by improving the activities of catalase(CAT),superoxide dismutase(SOD),peroxidase(PDD),regulating the gene expression of key enzymes of ascorbate acid-glutathione(ASA-GSH)antioxidant metabolism cycle(Pgu APX?Pgu GR?Pgu MDHAR),and activating the activities of ascorbate peroxidase(APX),glutathione reductase(GR),monodehydroascorbate(MDHAR),dehydroascorbate reductase(DHAR).Besides,the increment in the contents of ascorbic acid(ASA),dehydroascorbate(DHA),glutathione(GSH),glutathione disulfide(GSSG)and other key antioxidant components in guava stored at NFT also contributed to the balance of redox status of guava fruit and inhibition in cell membrane damage during storage,and thus alleviating the occurrence of cold injury.Cluster heat map and Pearson correlation analysis showed that the occurrence of chilling injury in guava was significantly correlated with the change in antioxidant metabolism,so it was speculated that NFT might inhibit the occurrence of chilling injury in guava by acting on the antioxidant metabolism of guava cells,especially the ASA-GSH cycle metabolism.(3)The study on the mechanism of NFT in regulating the texture softening of guava showed that,the decline in guava firmness stored at NFT was significantly inhibited,and no deterioration phenomenon such as fruit core hydration was observed in NFT group during storage.The results indicated that NFT storage could reduce the synthesis rate of ethylene precursor ACC and regulate the key enzyme gene(Pgu ACS?Pgu ACO)of ethylene synthesis,thus inhibiting the production of ethylene in guava.Compared with 6?and 10?groups,the increments in the contents of water-soluble pectin(WSP)and chelated pectin(CSP)in guava fruit cell wall were remarkably inhibited,and the contents of salt-soluble pectin(NSP)and cellulose(Cel C)in guava fruit cell wall were maintained at NFT.It was hypothesized that NFT storage can regulate the expression of key enzyme genes(Pgu PG?Pgu PME?Pgu?-GAL)and inhibit the activity of PG,PME,?-GAL,CEL and other enzymes in guava.The crystallinity index(Cr I)of cell wall material(AIR)in NFT group measured by XRD was lower than the other groups,indicating that no obvious loosening of cell wall cellulose skeleton occurred during storage.The results of immunofluorescence labeling pectin with different degree of methylesterification and laser confocal microscopy combined with the results of degree of methylesterification of AIR material indicated that,NFT storage can effectively inhibit the desertification process in cell walls pectin,and then delay the degradation of cell wall polysaccharides such as pectin.(4)Statistical analysis of ethylene metabolism and cell wall polysaccharide degradation metabolism of guava showed that the changes in ethylene anabolism and cell wall polysaccharide degradation metabolism had synergistic effects on guava texture softening.Moreover,the ethylene synthesis metabolism and cell wall polysaccharide degradation metabolism of guava fruit could be regulated by NFT storage,and the maintenance of intact cell wall structure of guava during storage was also promoted,thus leading to the alleviation in texture softening.Overall,NFT storage can effectively improve the storage quality of postharvest“pearl”guava fruit,inhibit the chilling injury occurrence,improve fruit chilling tolerance and delay fruit softening by regulating gene expression and inducing activities of related enzymes.In this study,NFT storage was proved to have great development potential and application value in storage and preservation of postharvest guava.