Effects Of Low Temperature Stress On Membrane Lipid Components And Key Enzymes Of Prunus Salicina Fruit And Methods To Reduce Cold Injury

Prunus salicina L belongs to the genus of Prunus in the family rosaceae,also known as hollow plum and Chinese plum.The low-temperature treatment is adopted in the process of postharvest storage,because fruits are easy to rot in the high-temperature season.However,the prunus salicina are sensitive to low temperature,and the low temperature can lead to cold injury of prunus salicina,resulting in the decline of fruit quality after harvest,reducing commodity value and shortening storage period,thus causing economic losses.The cold injury of fruits and vegetables starts from the phase change of cell membrane.Phospholipase D is involved in the response of cold resistance mechanism of fruits and vegetables,and plays the first role in a variety of phospholipases to change the membrane lipid components.It has been reported that inhibiting the activity of PLD can alleviate the cold injury symptoms of fruits and vegetables,but the research on exploring the cold injury mechanism of prunus salicina by combining the change of membrane lipid and the change of the activity of key enzyme PLD has not been reported.In this experiment,the prunus salicina is used as the test material,and the physiological and biochemical indexes of banana plum fruit under cold damage temperature（4°C）and control temperature（10°C）are analyzed.The response of cell membrane fatty acid components and phospholipid components to low temperature stress is studied.The phospholipase D activity and the relative expression of genes from different subfamilies are also investigated.The experiment identify the phospholipid component markers,cold sensitive genes,and coldresistant indicator fatty acids in banana plum.The 1-MCP treatment and ethylene absorbent treatment are carried out on banana plum fruits to study the differences of banana chilling damage at 4 °C.The effects of the two treatments on the changes of membrane lipid components and the expression of chilling-sensitive cold-sensitive gene 2 are analyzed.The experiment explore the regulation techniques to alleviate the chilling damage of bananas and plums.The main findings of this paper are as follows:1.Low temperature inhibites the normal post-ripening of banana plum fruit.The banana plum fruit which stored at 4 °C could not be softened normally,and the soluble solid content is abnormally reduced.At the same time,the shelf life of the fruit is shortened,the rot rate was increases,and the symptoms of cold damage appeared on the 20 d.The relative conductivity,free proline and malondialdehyde（MDA）content of banana plum fruits under low temperature stress are significantly increased,and lipoxygenase（LOX）enzyme activity is significantly higher than that of the control.The two antioxidant enzymes,peroxidase（POD）and catalase（CAT）,are stimulated by the cold response,and the activity increases in the early stage,inhibitesin the later stage.It indicates that the cell membrane lipid peroxidation is severe,and the low temperature stress causes great damage to the cell membrane.2.The fatty acid composition of postharvest prunus salicina is confirmed to be linoleic acid（36.99%）,palmitic acid（30.93%）,linolenic acid（24.50%）,stearic acid（5.65%）and oleic acid（1.93%）.Phosphatidylcholine（297.68 nmol/mg DW）,phosphatidylethanolamine（143.65 nmol/mg DW）,phosphatidylinositol（68.43 nmol/mg DW）,and monogalactosyldiglyceride（56.678 nmol/mg DW）)are the main components of phospholipids.As a hydrolysis product of phospholipids,the content of lysophospholipid is low,the content of phosphatidylcholine is 0.63 nmol/mg DW,the content of lysophosphatidylethanolamine is 0.09 nmol/mg DW,and the content of lysophosphatidylinositol is 0.06 nmol/mg DW.3.At 4 °C,the relative content of linoleic acid and linolenic acid,the unsaturation of fatty acids and the unsaturated index all show a downward trend,and the relative contents of oleic acid,palmitic acid and stearic acid increase.The phospholipid（PA）,lysophosphatidylcholine（LPC）,lysophosphatidylethanolamine（LPE）,lysophosphatidylinositol（LPI）and digalactooligosaccharide（DGDG）in the phospholipid fraction show an upward trend.Phosphatidylcholine（PC）,phosphatidylethanolamine（PE）,lysophosphatidylinositol（PI）and total phospholipids show an overall downward trend.PLD activity is stimulated during low temperature storage,and PLDα1,PLDα2,PLDα3,and PLDδ₁ genes are activated at low temperature,and the expression level is up-regulated.PLDδ₂ is inhibited and expressed at low temperature,and the expression level is down-regulated.According to the sensitivity of each index to low temperature stress,it is determined that linoleic acid and linolenic acid are the indicator fatty acids of banana plum.Furthermore,the PA,PC,MGDG are markers of chilling damage,and PLDα3,PLDδ₁,PLDδ₂ are the landmark of cold-damaging genes.4.The Ethylene absorbent and 1-MCP treatment can effectively inhibit the increase of chilling injury index of banana plum fruits which store at 4°C,inhibit the relative conductivity,MDA content,LOX enzyme and PLD enzyme activity,and reduce the degree of membrane lipid peroxidation.It reduces the relative content of palmitic acid and stearic acid,inhibits the relative content of linolenic acid,linoleic acid,and increases the relative content of coldresistant indicator fatty acids in the early stage of storage.It ensures the fluidity of the cell membrane and improves the coldness of the banana plum fruit.At the same time,the two treatments also inhibites the increase of PA,LPC,LPE,LPI and DGDG content,and slowed the decline of PC,PE,PI and total phospholipid content.The expression of PLDα3 and PLDδ₁ is inhibited,and the up-regulation of PLDδ₂ is stimulated.In summary,both treatments can effectively alleviate the chilling symptoms of banana plum fruit,but 1-MCP treatment is better.