Studies On The Effects Of Bagging On Carotenoid Biogenesis And Quality Of Pummelo Fruits

‘Majiayou' is a local variety of pummelo with excellent quality which planted in Guangfeng City,Shangrao County,county of Jiangxi Province.It is a red flesh pomelo with tender quality,fragrant taste and unique flavor.As the main cultivar in Guangfeng,it supports the local industry.Although bagging is a widely used cultivation technique in China,it is not a very common method in citrus planting.Moreover,the mechanism of this action has not been thoroughly studied.During cultivation,It was found that after bagging the fruit,not only the surface of ‘Majiayou' become more smooth,but the pulp of it become deeper red.In this way,the project taking the bagging ‘Majiayou' as a starting point to study the effects of bagging on the quality of pomelo fruit.Based on this,the content differences of carotenoids,volatile substances,hormones and primary metabolites in ‘Majiayou' during different developmental stages of bagging and non-bagging fruit were compared and analyzed by means of HPLC and GC-MS.The possible regulatory pathways involved in bagging were explored by transcriptome sequencing in order to elucidate the effect of bagging on the quality of ‘Majiayou' and its molecular mechanism.The mechanism will provide an important theoretical basis for further study on the regulation mechanism of environmental signals on carotenoids and other metabolic pathways,as well as the application of bagging in citrus production practice.The main results are as follows:1.the Effects of bagging on ‘Majiayou' fruit quality(1)The effects of bagging on the conventional qualities of ‘Majiayou': Through the analysis of the conventional qualities of bagged and unbagged fruits,it was found that there was no significant difference between bagged and unbagged ‘Majiayou' fruits in terms of weight,peel thickness,soluble solids,titratable acid and VC.However,bagging treatment significantly affected fruit firmness.By measuring the firmness of peel and pulp of bagged and unbagged ‘Majiayou' at harvest time,it was found that the firmness of peel and pulp after bagging was significantly lower than that of unpacking treatment,and there was no significant difference in the firmness of pulp.(2)the Effects of bagging on the color difference of ‘Majiayou': After bagging,thepeel of ‘Majiayou' fades rapidly,the fruit surface turned yellow gradually,and turned pale yellow around 190 days after flowering;the process of green fading of ‘Majiayou'without bagging was longer,and the color difference CCI was still negative about 190 days after flowering,indicating that the fruit surface was greenish.Meanwhile,the CCI value of the pulp of ‘Majiayou' was positive after bagging,which was significantly higher than that of the pulp of ‘Majiayou' without bagging,indicating that the pulp of fruit turned red after bagging.(3)the Effects of bagging on carotenoid accumulation of ‘Majiayou' at different developmental stages: Lycopene and beta-carotenoid were mainly accumulated in‘Majiayou'.After bagging,the accumulation rate of lycopene in the pulp of ‘Majiayou'increased,and reached the maximum at 150 days after anthesis,which was significantly higher than that of non-bagged ‘Majiayou'.After that,the content of lycopene in the pulp of bagged and non-bagged ‘Majiayou' decreased gradually,and the decline rate of bagged pulp was relatively small.Therefore,at the harvest time(210 days after flowering),bagging treatment accumulated more lycopene than non-bagging treatment.The change trend of beta-carotenoids during the development of bagged and unpacked fruits was similar to that of lycopene.(4)the Effects of bagging on the accumulation of volatile substances in ‘Majiayou' at different developmental stages: 69 kinds of volatile substances were detected in pummelo peel.The total volatile substances in pummelo peel bagged(1344.65 ug/g)were significantly lower than those in unpacked pummelo peel(4897.63 ug/g)at 210 days after anthesis by semi-quantitative method.However,there was no significant difference in the content of volatile substances between bagging and non-bagging treatments.(5)the Effects of bagging on the content of sucrose and organic acid in ‘Majiayou'pulp at different developmental stages: Compared with the non-bagging treatment,the rate of sucrose accumulation and organic acid degradation in ‘Majiayou' increased after bagging.However,there was no significant difference in total sugar and total acid content between bagged and unpacked ‘Majiayou' pulp at harvest time.(6)the Effects of bagging on hormone content of ‘Majiayou' at different developmental stages: During fruit development,JA content in the peel of bagged‘Majiayou' was higher than that of unpacked ‘Majiayou',and ABA content was higher than that of unpacked fruit at the later stage of fruit development.2.Identification of differentially expressed genes and their metabolic pathways in response to bagging by transcriptome analysis.In order to further study the molecular mechanism of bagging affecting fruit quality,a new generation of high-throughput sequencing method,Illumina Hi Seq TM,was used to sequence the transcriptomes of bagged and unpacked fruits at different developmental stages.FPKM method was used to calculate the gene expression level and 311non-repetitive differentially expressed genes were identified.By calculating the FPKM values of different genes at different developmental stages of ‘Majiayou',we can see that the increase of PSY expression and the decrease of CCD1 expression are the main reasons for the red flesh of the bagged fruit.The results can also be verified by RT-PCR.GO analysis showed that differentially expressed genes were mainly involved in the development of chloroplasts,the transmission of photosynthetic electron chains,plant stress resistance,hormone response and signal transduction,as well as the response to light intensity and light quality,among which more differentially expressed genes were involved in hormone response and stress resistance.Therefore,it is speculated that bagging treatment caused an adverse environment for ‘Majiayou',enabling the pulp to secrete more stress-resistant hormones to resist adversity,and accelerated fruit development and maturation under the regulation of hormones.Among hormone response related genes,there are many transcription factors responding to ethylene.It is speculated that bagging creates a closed environment for fruit,which leads to the accumulation of more ethylene in the microenvironment of bagging,and the accumulation of ethylene accelerates the ripening and color transformation of fruit.