Study On The Main Characteristics And Formation Mechanism Of Fruit Quality Of 'Zaomi' Ponkan

Ponkan is one of the table citrus in China,accounting for more than 30%of annal citrus production.Due to concentrated maturity period,new varieties with different maturity period is one of the main goals for Ponkan breeding.Recently,we selected a novel Ponkan cultivar basing on a spontaneous mutant of‘Xinnu'Ponkan and assigned a name‘Zaomi'Ponkan.‘Zaomi'exhibited many excellent traits including early maturity period,high sugar content and fewer seeds,and it relieved the problem of concentrated marketing.‘Zaomi'has played an important role in optimizing industry structure of Ponkan in Hunan Province.Explorations of quality characteristics and formation mechanism of‘Zaomi'have important significance not only for citrus breeding but also for optimizing planting techniques of‘Zaomi'.For‘Zaomi'Ponkan,this research identified its main quality characteristics,explored quality changes at fruit development and post-harvest storage stages,analyzed the physiological and molecular mechanisms involved in the main quality traits,and evaluated the mutant genes that might associate with excellent traits.Basing on what we get from these investigations,techniques matching with‘Zaomi'plantation were proposed or advised.The main results of this research are as follows:1.Relative to‘Xinnu',‘Zaomi'fruit ripened at 20 days earlier,.showed a thinner rind and had no/few seeds in which?4 seeds per fruit could be found.Both cultivars shared patterns in sugar and acid accumulations but the composition contents differed.After 190 days after full flowering(DAFB),‘Zaomi'showed significant higher total soluble solids(TSS)and sugars,in which sucrose increase at fruit enlargement period contributed outstandingly.Citrate in‘Zaomi'fruit was higher significantly at early fruit developing stage relative to‘Xinnu'while the difference disappeared at late fruit developing stage.;.Corresponding to sugars accumulated,expressions of sucrose synthetase gene(CrSS),sucrose phosphate synthase gene(CrSPS),and sucrose transfer protease genes(CrSUC1?CrSUC3 and CrSUC4)were significantly up-regulated in‘Zaomi'.These differentially expressed genes are probably related to higher sugar metabolism and accumulation of‘Zaomi'Ponkan.2.With just a single-fruit package and no preservative under room temperature,fruit micro-environment maintained at about 90%relative humidity and fruit water loss rate was less than 4%.Fruit TSS of‘Zaomi'maintained around 13~o Brix during storage period which was significantly higher than that of'Xinnu',although their harvesting TSS showed no difference.The acid content of‘Zaomi'fruit harvested at Nov.10th maintained at about 0.4%during storage,significantly lower than that of‘Xinnu'.Different peel coloring was found when Ponkan was harvested at different maturity degree,but the differences disappeared with prolonged storage time.In addition,the rot rate of‘Zaomi'increased significantly with delayed harvesting.Therefore,‘Zaomi'Ponkan is suitable for early harvest in production.It is suggested that‘Zaomi'Ponkan harvested at mid-November and‘Xinnu'Ponkan harvested at early December.3.Fruit peel color differed greatly between two cultivars from 140 to 210 DAFB.Dramatic degradation of chlorophylls in peel resulted to early coloring of‘Zaomi'Ponkan.Both cultivars shared patterns in pigment accumulations but the composition contents differed.From 140 to160 DAFB,total chlorophyll content in‘Xinnu'peel was nearly 2 times higher than that of‘Zaomi',however,total carotenoid showed no difference during this period.Violaxanthin,?-cryptoxanthin and xanthophyll were main carotenoids when Ponkan fruit was undergoing color-transferring.At 200 DAFB,three compositions accounted for 57.8%of total carotenoid content in both Ponkan peels while chlorophyll in‘Xinnu'peel was 6 times higher than that of‘Zaomi'.Analysis showed that down-regulated expression of chlorophyll synthesis genes(CrCHLH?CrPORA),and up-regulated expression of chlorophyll degradation-related genes(CrChlase?CrPa O and CrRCCR)probably lead to earlier chlorophyll decrease in‘Zaomi'.4.At fruit color-transferring period,exogenous ethephon treatment accelerated de-greening of‘Zaomi'fruit in vitro and vivo,while exogenous ABA did not affect fruit coloring.It could be concluded that difference in endogenous ethylene among varieties may participate in earlier color-transferring of‘Zaomi'fruit.Frurther analysis showed that expressions of ethylene biosynthetic genes CrACS2 and CrACO of‘Zaomi'were up-regulated during peel coloring process but genes related to ethylene signal transduction such as CrETR1 and CrERF1 were up-regulated early.Differential expression of these genes may affect chlorophyll accumulation and accelerate fruit coloring of‘Zaomi'Ponkan.5.Compared with‘Xinnu'Ponkan,‘Zaomi'Ponkan showed bunch of changes in flower including slight delay of anthesis,petals enlargement,significant reduction of stamens and abnormal anthers.,Anthers drying out and darking in color were found in‘Zaomi'flowers.Further,pollen grains of‘Zaomi'exhibited not only structure deformity but also reductions of pollen grains number,vigor and germination rate relative to‘Xinnu'Ponkan.All these results suggested that‘Zaomi'Ponkan was male sterile.Furthermore,pollen tube growth of‘Zaomi'was inhibited with self-pollination and self-incompatibility could be a direct cause of seedless fruit when‘Zaomi'was in isolated plantation..Pollination cultivars impacted fruit setting and comprehensive fruit characteristics for both‘Zaomi'and‘Xinnu'.Fruit setting rates of both cultivars were significantly higher using male parent Clementine and‘Nanfeng'mandarin than that with self-pollination.In addition,‘Zaomi'Ponkan showed smaller fruit size with self-pollination while high sugar trait of fruit was stable among kinds of pollination stratigies.6.Genome sequences showed high similarity between‘Zaomi'and‘Xinnu'.Among genome SNP and InDel mutations,most mutations were found in non-coding region but fewer mutations were in coding region.Basing on InDels,38 frameshift mutation genes were obtained These genes functions in fertility,pigment metabolism,gene transcription regulation,growth and development,disease resistance and plant stress response.