Comparison Of Fruit Quality And Mechanisms For Differential Peel Color Of Yellowish Ponkan And Ponkan

Ponkan(Citrus reticulata Blanco),is one of the most important fresh edible citrus varieties across China.The peel color of Ponkan closely relates to commodity value,as well as colorful Ponkan cultivars meet the diversification of consumer pursuit.In recent years,we found a Ponkan cultivar with pale yellow peels,Yellowish Ponkan,which is a mutant of the major Ponkan variety called Ponkan,in Quzhou of Zhejiang Province.Compared with the Ponkan,this light yellow color of Yellowish Ponkan appears during the breaker stage and continues to mature stage.Yellowish Ponkan peel has lower total carotenoid content,but owns same carotenoids types with Ponkan.To compare the fruit quality and investigate the differences of peel colour,we used many methods such as high performance liquid chromatography(HPLC),molecular biology technology and RNA-sequencing,to obtain the following results:Fruit qualities of Yellowish Ponkan and Ponkan were analyzed in the whole growth and postharvest stage(S1-S5).We discovered that single Yellowish Ponkan mature fruit weight were slightly higher than Ponkan.From developmental stage to mature stage,the contents of characteristic citrus aroma like P-myrcene and D-limonene make no significant differences,but the total monoterpene derivatives of Yellowish Ponkan,as 340.81 ug/g was significantly higher than that of Ponkan as 190.46 ug/g in the mature stage S5.As sucrose is the main sugar in Ponkan fruit,the sucrose content of Yellowish Ponkan was slightly higher than that of Ponkan.Yellowish Ponkan has higher level of total soluble solids than Ponkan in S1-S5 stage.As citric acid is the main organic acid,the citric acid concentration of Yellowish Ponkan was twice higher than Ponkan in S4-S5.Due to the yellowish Ponkan peel total carotenoids is lower than that of Ponkan for 2-3 times showing pale yellow color,but the bright color gives consumers a fresh visual experience.In the S1-S5 stage,the chlorophyll content of Yellowish Ponkan was always higher than that of Ponkan,especially in breaker stage S2.The content of total carotenoids S1 was almost same in two cultivars,but it-undergone change in the stage of S2.During the whole developmental stage,the levels of phytoene,phytofluene,beta carotene,beta cryptoxanthin and violaxanthin in Ponkan peel elevated rapidly,in vivid contrast,these carotenoids in Yellowish Ponkan peel increased slowly even remained unchanged.The lutein content in the two cultivars was at the same level.It is noteworthy that the ABA content,carotenoid degradation products,the yellowish is always higher than that of Ponkan citrus,especially twice in stage S5.Analysis about MEP pathway showed that carotenoid biosynthesis genes DXS,DXR and PSY1 had identical expression level,but during S3-S5 development stage,the expression level of ZDS gene in Ponkan peel was slightly higher than that in Yellowish Ponkan during the development stage.Next analysis about the key genes in lutein biosynthesis did not detect a significant difference.The key genes NCED2 and NCED3 of the two degradation pathways were analyzed,and the expression level of the two lines increased sharply after the postharvest storage stage of S4-S5,but no significant difference was found.We analyzed RNA-Seq results of two Ponkan cultivars' peel in stage S2,and none of carotenoids biosynthesis genes showed significantly differences which consistent with the former results of QPCR.Interesting,10 up-regulated genes related to chlorophyll synthesis had been detected from 164 differential expressing genes in Yellowish Ponkan.At the same time,the selected ABA receptor PYL4,its expression in the Yellowish Ponkan was significantly lower than that of the Ponkan.