Regulatory Mechanism Of Elevated Oxygen Treatment On Quality And Physiological Characteristics Of Post-Harvest Winter Jujubes

Winter jujube（Ziziphus jujube Mill.cv.Dongzao）,a high-quality fresh jujube variety,is crisp and delicious with bright color and rich nutrition which is deeply loved by consumers.However,deterioration of jujube fruit characterized as darkening of peel color,ethanol fermentation,and softening will occur rapidly after postharvest,resulting in limited shelf life.In the present study,winter jujube was used to study the effects of elevated O₂treatment on storage quality,and the regulation effects of elevated O₂treatment on pigment metabolism,ethanol metabolism,cell wall metabolism and reactive oxygen species（ROS）metabolism of winter jujube were further discussed,in order to reveal the regulatory mechanism of elevated O₂treatment on quality and physiological characteristics of post-harvest winter jujubes.The obtained results were as follows:1.The effects of three concentrations of O₂（40%,60%,and 80%）treatment on quality of winter jujube were studied.The results showed that 60%O₂significantly maintained the color and cell structure of winter jujube,delayed the decline of firmness and total soluble solid（TSS）content,inhibited the increase of browning index（BI）,malondialdehyde（MDA）content and cell membrane permeability.At the end of storage,the BI of the 60%O₂treatment group was reduced by 11.1%and the firmness and TSS content were increased by 20.9%and 31.5%compared with the control group.2.Elevated O₂treatment slowed down the degradation of chlorophyll and the chlorophyll content was increased by 25.4%compared with the control group on 7 d.O₂treatment also maintained flavonoids and anthocyanins contents in the pericarp,which was 45.2%and 40.5%higher than that of the control group when storage ended,respectively.Meanwhile,O₂treatment induced increased phenylalanine ammonia-lyase,chalcone synthase,dihydroflavone reductase,anthocyanidin synthase,and flavonoid glycosyltransferase activities and gene expression.It indicated that elevated O₂treatment was conducive to delaying the appearance of the dim color.3.Elevated O₂treatment reduced the accumulation of pyruvate by inhibiting the conversion of hexose to pyruvate and maintaining the organic acid content.At the end of storage,the contents of sucrose,fructose,and glucose in the O₂treatment were increased by 10.2%,6.4%,and 5.4%compared with the control group,respectively.Meanwhile,the contents of tartaric acid and succinic acid were 13.6%and 18.4%higher than those in the control group.The low level of pyruvate content reduced the activities and gene expression of pyruvate decarboxylase and alcohol dehydrogenase,inhibiting the accumulation of acetaldehyde and ethanol in winter jujube.At the end of storage,the content of ethanol in O₂treatment group was 57%lower than that in the control group.4.Elevated O₂treatment induced the scavenging of ROS by the fruit antioxidant system and inhibited the activities of cell wall degrading enzymes,which was conducive to the maintenance of cell wall structure and component.O₂treatment induced the activities and gene expression of superoxide dismutase（SOD）,peroxidase（POD）,and catalase（CAT）,thus promoting the clearance of H₂O₂and O₂^-.At the end of storage,the activities of SOD,POD,and CAT in the O₂treatment group were 1.9times,1.3 times,and 1.1 times higher than those in the control group,respectively.And,O₂treatment also induced increased ascorbate peroxidase,glutathione peroxidase,glutathione reductase activities and gene expression,which inhibited the decrease of ascorbic acid and glutathione contents.O₂treatment also inhibited the activities and gene expression of polygalacturonase,pectin methylesterase,and cellulase,thus slowing the decrease of protopectin,cellulose,and hemicellulose contents and the increase of water-soluble pectin（WSP）content.The contents of cellulose and hemicellulose in the O₂treatment group were 71.4%and 25.5%higher than those in the control group when storage ended.However,long-term oxygen stress led to excessive accumulation of ROS and oxidative damage,which promoted the sharp decline of L*value and protopectin content,and the increase of MDA and WSP content at the later stage of storage.