| The color and luster of the peel,as the two most important appearance qualities of citrus fruits,affect consumers’ purchasing desire.In production,early maturing(harvested in October)navel oranges and late maturing sweet oranges(harvested after May)often fail to show their expected color due to the high temperature during the harvest season,seriously affecting the economic value of citrus fruits.The research team found in the early stage that appropriate stress treatments such as changing temperature and natural biomaterials can effectively improve the fruit color of some early maturing citrus varieties,especially increasing the carotenoid content in the fruit.However,there is little research on the wax that determines glossiness.Based on this,this study used the late ripening rejuvenated valencia orange and the early ripening honey navel orange as materials,and used the existing postharvest low-temperature and biotreatment technologies of the research group to compare and analyze the differences in carotenoid and wax content of citrus peel under different postharvest treatment conditions,and screened a candidate gene that might0 regulate the two metabolic pathways based on transcriptome data and gene expression analysis.The main results are as follows:1.Postharvest treatment promotes citrus peel color change and carotenoid accumulationBy taking photos and measuring the color difference value,it was found that there was no significant color change in the summer orange green fruits under low temperature treatment.Biotreatment can significantly promote the skin of the valencia orange green fruits to turn orange yellow;Both low-temperature treatment and biotreatment can promote the skin of honey navel orange fruit to turn bright red,and biotreatment is more efficient.The determination of related carotenoid components and content revealed that the content of violaxanthin and β-cryptoxanthin has significantly increased.At the 15 th and 20 th days of biotreatment,there was a significant increase in the content of violaxanthin.In the honey navel orange fruit,the content of β-citraurin was 18 times higher than that of the control group in biotreatment group at the 20 th day of treatment;The content of the lowtemperature treatment group was 3.82 times higher than that of the control group at the 20 th day of treatment.The results of transmission electron microscopy showed a significant increase in the number of plastid globules in the peel of honey navel oranges treated after harvest.The above results indicate that low temperature treatment has no significant effect on the color transformation of summer orange returning green fruits,but can promote the color transformation of honey navel orange fruits.Biotreatment can significantly promote the color transformation of valencia orange returning green fruits and honey navel orange fruits,and is conducive to the accumulation of related chromogenic carotenoids.2.Postharvest treatment changed the wax morphology and structure of the citrus peel and promoted the accumulation of wax.This study used scanning electron microscopy to observe the surface wax morphology and structure of citrus fruits before and after treatment,and measured their wax content.The results showed that under untreated conditions,the surface wax crystals of valencia orange and honey navel orange fruits appeared as flakes.After low-temperature treatment,the surface wax layer of summer orange fruit becomes loose and layered,while the wax peeling on the surface of honey navel orange fruit is not severe,and the wax layer is evenly covered.On the contrary,after biotreatment,the wax layer on the surface of valencia orange fruit is tightly bound and does not peel off much.The wax crystals on the surface of honey navel orange fruit increase in quantity and are unevenly distributed,appearing rough and stacked near the stomata.Analyze the wax components and content of the fruit peel,it was found that the main components of peel wax in two citrus materials were alkanes,alcohols,aldehydes,and acids.At the 15 th day of low-temperature treatment in valencia orange,the alcohol content significantly decreased compared to the control group,however,the content of alcohols in fruit wax on the 5th day of biological treatment and the content of aldehydes in fruit wax on day 20 of biological treatment were significantly increased compared with the control group respectively.Compared with the control group,the contents of waxy components in honey navel orange fruit were significantly reduced in each period before the 15 th day of biological treatment and low temperature treatment,while at the 20 th day,low-temperature treatment significantly increased the content of alcohols and acids in the wax.Biotreatment also significantly increased the content of alcohols,aldehydes,and acids compared to the control group.The above results indicated that both postharvest low temperature and biotreatment have altered the waxy structure and waxy component content of citrus peel,but the treatment effect varies depending on genotypes.The results of correlation analysis showed that there was a certain degree of negative correlation between the total carotenoids and the contents of each component and the wax content in the valencia orange turned green fruits under the two treatments and the honey navel orange fruits under the low temperature treatment,while the content of carotenoids and components in biologically treated honey navel orange fruit was highly positively correlated with the accumulation of fatty acids in wax.3.Obtaining a candidate gene that may coordinate regulation of carotenoid and wax metabolism.In order to explore the regulation mechanism of postharvest processing on carotenoid and wax metabolism,Cs09550,an unknow gene highly related to metabolic activities,was screened based on early transcriptome data,and its expression was measured.Compared with the control group,the expression level of this gene has no significant change in each period of low temperature treatment and biological treatment in valencia orange,only the expression level in the 5th day of low temperature treatment is significantly increased compared with the control group;compared with the control group,the expression level of this gene was significantly increased on the 5th day of honey navel orange low temperature treatment,and the expression level of the low temperature treatment group and the biological treatment group on the 20 th day were both significantly increased.The expression level of the gene is correlated with the accumulation level of carotenoids and waxes,and it is speculated that it may be highly correlated with the metabolism of carotenoids and waxes.In summary,this study mainly explored the effects of low temperature and postharvest biological treatment on the appearance quality of citrus fruits,revealed the accumulation law and correlation of carotenoids and waxes under the two treatment conditions,It was found that biological treatment can more significantly promote the color change and accumulation of colored carotenoids in citrus fruits,change the waxy structure and content of the fruit surface,and obtain a candidate gene that may synergistically regulate two metabolic pathways,the research results provide a theoretical basis for the subsequent research and development of fruit color quality improvement technology and the analysis of the coordinated regulation mechanism of the two metabolic pathways. |
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