Study On The Reasons For The Uneven Peel Coloration And Development Of Improvement Measures Of W Murcott Fruits

Carotenoids,as the main color-presenting pigments in citrus fruits,form color variation in composition and content of carotenoids in different tissues of fruits,which provides great convenience for us to investigate the color formation mechanism.W Murcott is a variety of citrus with orange-red flavedo and albedo,the flavedo of this variety is generally unevenly colored in production,and the current understanding of the mechanism of pericarp coloration of this variety is not yet deep,which limits the development of production control measures for its appearance quality.In this research,we analyzed the carotenoid composition and content of different tissues during the development of W Murcott fruits,and investigated the coloring mechanism of the fruit pericarp,especially the albedo,by combining the differences in gene expression of carotenoid metabolic pathways.At the same time,shading treatments such as shade netting,fruit whitening and bagging are carried out in the early stage of fruit color change,and compared the differences in fruit coloration under different treatments,the research clarifies the effect of light on citrus fruit coloration,aiming to provide theoretical support for use of the cultivation and biotechnological means to improve the appearance quality of citrus fruits.The main results of the study are as follows:1 Both the flavedo and albedo of W Murcott fruits accumulate the red pigmentβ-citraurinene resulting in an orange-red color.The accumulation of β-citraurinene in the flavedo is associated with the high expression of CCD4 b gene,while the pigment accumulation in the albedo may result from the penetration of carotenoids in the flavedoThe total carotenoid accumulation in the flavedo of W Murcott at maturity was determined by HPLC as high as 1222.67 μg/g DW,while the accumulation in the albedo was 249.67 μg/g DW,and the contents in both tissues were significantly higher than those in other test varieties.The red pigment β-citraurinene was only detected in the flavedo and albedo of W Murcutt and the flavedo of Lane Late navel orange,and the content of β-citraurinene in the flavedo of W Murcutt was significantly higher than that of Lane Late navel orange;the content of β-citraurinene in the albedo of W Murcutt was 19.98 μg/g DW,which was about 1/10 of the content in the flavedo of W Murcutt.Using fluorescence realtime quantitative detection,it was found that the expression level of the carotenoid metabolism degradation gene CCD4 b was significantly up-regulated in the flavedo of W Murcott compared with other citrus varieties,while the expression level of CCD4 b was lower in its albedo,and the expression levels of other genes related to carotenoid metabolism were also not detected to differ among varieties,leading to the speculation that the light orange-red color of the albedo is due to the osmosis of the flavedo.In addition,TEM observations showed that the number of colored corpora as well as plastid goblets in the plastids of the albedo of W Murcott was significantly higher than that of the Lane Late navel orange,indicating enhanced pigment storage capacity.2 Moderate shading in the field can promote uniform distribution of the flavedo of W Murcott fruitsBlack full shade bagging treatment was applied to W Murcott and other varieties during the pre-color change period.The results of HPLC showed that the total carotenoid content in the flavedo of W Murcott was reduced by about 44% compared with that of the fruits under natural cultivation,and the contents of β-citraurinene and β-cryptoxanthin were reduced by about 30% and 50%,respectively.In combination with field investigations,it was found that that complete shading accelerated peel degreening,chlorophyll degradation,and the conversion of chloroplasts to colored corpora,accelerating the process of peel yellowing,but carotenoid accumulation was significantly reduced compared to the control.Further experimental results with moderate shading(white bags,white coating and shade nets)showed that the contents of violaxanthin,β-citraurinene and β-cryptoxanthin in the flavedo of W Murcutt increased about 1.50,1.45 and 1.45 times,respectively,compared with the control,indicating that the moderate shading treatment promoted the accumulation of various carotenoid fractions in the fruit peel,which was more conducive to the formation of the red color trait of citrus fruit peel,and also moderated the sunburn scars caused by direct light,reduced the production of shaded fruits,greatly promoted the uniform coloration of fruit,and improved the fruit appearance and economic efficiency.