Study On The Relationship Between Abnormal Chilling Injury Behavior And Glycometabolism In Postharvest Papaya Fruit

Papaya（Carica papaya L.）is popular among consumers because of its rich nutritional value.However,postharvest papaya fruit is easy to soften and rot.Low temperature storage is one of the most effective methods to maintain postharvest quality and prolong storage period.Whereas,chilling injury（CI）is very easy to occur in cold sensitive fruits during low temperature storage.Generally speaking,within the temperature range of inducing CI,the lower the temperature or the longer the duration of low temperature,the more severe the CI will be.In our previous study,our team found that papaya fruits stored at 1℃appeared CI symptoms later and maintained better quality than those stored at 6℃.In this study,Hainan"Daqing"papaya was used as material to study the mechanism of CI from the perspective of sugar metabolism.Based on the changes of tissue structure of papaya fruit at different refrigerating temperatures,the dynamic changes of sugar and its metabolism,cell membrane and reactive oxygen species in the abnormal CI behavior of papaya were studied,so as to clarify the relationship between abnormal CI behavior and sugar metabolism.The results are as follows:1.During low temperature storage,the CI symptoms of papaya fruits stored at 6℃were earlier than those stored at 1℃,and the CI symptoms were more serious.Papaya stored at 1℃maintained high fruit firmness and inhibited the accumulation of malondialdehyde（MDA）and the increase of membrane permeability.Moreover,through microscopic and ultrastructural observation,the fruit stored at 1℃can better maintain the integrity of cell structure during refrigeration.Thus,the degree of cell damage is lighter during cold storage.2.During low temperature storage,the content of hydrogen peroxide（H₂O₂）and the production rate of superoxide anion（·O₂^-）in papaya fruits stored at 6℃were higher than those at 1℃.At the same time,the study on the sugar content of papaya fruits at different storage temperatures showed that the papaya fruits stored at 1℃maintained higher sucrose content than those stored at 6℃,and alleviated the decrease of glucose,fructose and starch.The results indicated that sugar content may affect the occurrence of CI in the abnormal chilling injury of papaya.Correlation analysis showed that H₂O₂ and·O₂^-were significantly negatively correlated with starch(r=-0.887^**,r=-0.911^**),glucose(r=-0.878^**,r=-0.898^**)and fructose(r=-0.952^**,r=-0.952^**)contents in papaya fruit after harvest under low temperature storage.The results showed that starch,glucose and fructose played a positive role in antioxidant activities of Postharvest papaya fruits.3.The activities of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）and ascorbate-glutathione related enzymes（ascorbate peroxidase（APX）,glutathione reductase（GR）,monodehydroascorbate reductase（MDHAR）and dehydroascorbate reductase（DHAR））and the contents of ascorbate（As A）and reduced glutathione（GSH）in papaya fruits stored at 1℃were higher than those stored at 6℃.The results showed that the activities of SOD,CAT and POD,the ascorbate-glutathione cycle（As A-GSH）,and the contents of As A and GSH were increased in papaya fruits stored at 1℃.Therefore,the scavenging ability of reactive oxygen species in papaya fruit was significantly improved.4.Fruits stored at 1℃alleviated the decrease of glucose and fructose due to their lower amylase,acid invertase（AI）and higher sucrose synthase（SS-c）activity.The fruits stored at 1℃had higher activities of neutral invertase（NI）,sucrose synthase（SS-S）and sucrose phosphate synthase（SPS）,which resulted in the maintenance of higher sucrose content.In addition,the gene expression levels of sugar metabolism-related enzymes（Cpα-AM、Cpβ-AM、Cp SS、Cp SPS、Cp AI and Cp NI）in papaya fruits were analyzed.It was found that the expression levels of Cpα-AM,Cpβ-AM and Cp SS in papaya fruits were down-regulated,and the expression levels of Cp SPS,Cp AI and Cp NI were up-regulated when the fruits were stored at 1℃and 6℃respectively.Among them,the relative expression level of Cp SPS in papaya fruits stored at 1℃was higher,while the relative expression level of Cp AI was lower.These results suggested that papaya stored at 1℃might affect the activities of related enzymes by regulating the expression levels of Cp SPS and Cp AI,and then maintain higher sucrose,fructose and glucose contents,so as to enhance the chilling resistance of fruits and reduce the occurrence of CI.