The Effect Of Ca+GA₃ Treatment On Preventing Fruit Cracking In Ponkan And Its Mechanism

Ponkan（Citrus reticulata Blanco cv.Ponkan）,a perennial plant of the genus Citrus in the family Rutaceae（Rutaceae）.Ponkan is known for its colorful,crisp flesh,rich in nutrition and popular.Fruit cracking is a physiological disease that commonly occurs in Ponkan fruit development,often appearing in the fruit expansion period,the cracked area is easy to breed bacteria,rot and mold,cracked fruit will have a serious impact on the economic benefits of Ponkan.Although the mechanism of fruit cracking in Ponkan has not been fully revealed,many studies have shown that Ca deficiency is one of the important reasons for fruit cracking.In this study,Ca+GA₃was sprayed on the fruit during the growth period of Ponkan,and the effect of Ca+GA₃treatment on fruit quality and other physiological indexes and transcriptome data was investigated to study the changes of Ponkan pericarp after Ca+GA₃treatment and to lay the theoretical foundation for Ponkan fruit cracking research.The following conclusions were reached in this experiment:1.Cracked fruits appeared at 105 days after the flowering stage of Ponkan,and Ca+GA₃treatment greatly reduced the cracking rate of Ponkan.It was observed by tissue section that the cell arrangement was tight and orderly before the occurrence of fruit splitting,and there was basically no cavity,but the cell arrangement was irregular and the cell collapsed after the occurrence of fruit splitting.2.It was found that Ca+GA₃treatment could increase the fruit size,increase the fruit weight and thicken the skin of Ponkan,and at the same time,it would reduce the cracking rate.Ca+GA₃treatment can also improve fruit quality,as Ca+GA₃treatment can increase the soluble solids（TSS）,vitamin C（VC）and titratable acid（TA）of flesh.TSS,VC and TA of the meat were higher than those of CK,and the solid-to-acid ratio was also reduced.3.Based on the transcriptome data,it was found that genes with significant differences in expression in Ca+GA₃treatment Ponkan fruit pericarps were mainly involved in the metabolism of phenyl propane,pentose and glucuronide interconversion pathway and reactive oxygen species metabolism,which provided a basis for further study on the mechanism of fruit cracking in Ca+GA₃treatment.4.The physiological and biochemical characteristics of Ponkan fruit development were observed and analyzed to reveal the mechanism of Ponkan fruit crack formation.The increase of reactive oxygen species（ROS）in Ponkan fruit pericarp,the decrease of antioxidant enzyme activity and the increase of O₂·and H₂O₂content led to the accumulation of reactive oxygen species,which stimulated the activity of antioxidant enzymes,made the antioxidant enzymes scavenging ability insufficient,broke the equilibrium system,led to the deepening of membrane lipid peroxidation,and intensified the occurrence of fruit cracking.Superoxide anion（O₂·）production rate,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）contents,and peroxidase（POD）and polyphenol oxidase（PPO）activities,as well as increased superoxide dismutase（SOD）and catalase（CAT）activities,which reduced the damage of ROS on the pericarp.5.Ca+GA₃treatment of Ponkan pericarps decreased the activities of phenylalanine ammmonialyase（PAL）,4-coumarate:Co A ligase（4CL）,cinnamate-4-hydroxylase（C4H）,and cinnamic acid.C4H)activity was decreased and lignin content was reduced.Ca+GA₃treatment reduced water soluble pectin（WSP）content,and pectin methyl esterase（PME）,pectin lyases（PL）,poly Galacturonic acid（PG）and cellulase（CX）activities;as well as increased the original pectin and cellulose contents.