Physiological Characteristics Of The Sunburn Disease Occurrence Occurrence In Ponkan Fruit And The Mechanism Of The Peel Colour Change

Ponkan（Citrus reticulata Blanco cv.Ponkan）is one of the excellent varieties in Fujian Province of China.Its fruit is sweet,easy to peel,loved by consumers.However,the occurrence of Ponkan fruit sunburn physiological diseases has become more and more common in the summer in environment of long-term high temperature and strong direct light in summer,resulting in serious impacts on the edibility and economic value of the fruits.In this experiment,we investigated the effects of major meteorological factors on the incidence and degree of sunburn in Ponkan fruit,and investigated the effects of different degrees of sunburn on fruit quality,peel cell structure,peel reactive oxygen species,antioxidant enzyme activity and related gene expression,and analysed gene expression,and used transcriptome sequencing and targeted metabolome sequencing techniques to analyse gene expression and metabolites in the peel of normal and sunburned fruit to investigate the key gene expression differences and metabolite composition of peel colour changes.It will be helpful to understand the colour change pattern of sunburned fruit peel and the anti-sunburn effect of photoprotective pigments,and further analyse the physiological and molecular mechanism changes of sunburn disease of Ponkan fruit,and provide theoretical basis for the prevention of sunburn disease of Ponkan fruit.The main findings were as follows:（1）By measurement of Ponkan orchard air temperature,light intensity,Ponkan fruit surface temperature and light intensity,fruit sunburn incidence and degree,it was found that both air temperature and light intensity were higher in July,August and September than in other months in 2022,with the highest air temperature and light intensity of the year occurring in August at 38.8°C and 2454.65μmol·m^-2·s^-1.41.01℃and1599.00μmol·m^-2·s^-1 were defined for the threshold temperature and light intensity of sunburn disease in Ponkan.The Ponkan fruit surface temperature,light intensity and incidence of sunburn differed between the different sides of the Ponkan tree canopy.The western and southern sides of the canopy received the highest light intensity and lasted the longest,and the fruit surface temperature was higher than the other sides,resulting in a more severe degree and incidence of sunburn than the other sides;the Ponkan surface temperature and light intensity of Ponkan fruit grown on the western slope were higher than on the southern slope,and the degree and incidence of sunburn were also higher on the southern slope.（2）The quality of Ponkan fruit with different degrees of sunburn was measured,showing that as the degree of sunburn increased,the peel gradually shrank and sunken,the juice sac shrank and shriveled,the peel cells gradually contracted,and the oil cells collapsed.The thickness of the peel gradually increased and the edibility and juice yield decreased significantly,leading to a reduction in the quality of Ponkan fruit.（3）The H₂O₂ and O₂??contents and the enzyme activities of SOD,POD,CAT,GPX and APX as well as the changes of related gene expression of Ponkan fruits at different degrees of sunburn were measured.The enzymatic activities of SOD,GPX and APX showed a trend of increasing and then decreasing with the increase of sunburn,while the enzymatic activities of POD and CAT showed a trend of decreasing with the increase of sunburn,and the expression levels of antioxidant enzyme-related genes were consistent with the trend of antioxidant enzyme activities.The total antioxidant capacity of the peel also showed a trend of increasing and then decreasing with increasing sunburn.（4）Transcriptome sequencing technology and targeted metabolome determination technology were used to analyze the gene expression differences between sunburn and normal peel of Ponkan.According to the annotation and differentially expressed genes,the key regulatory genes of chlorophyll,carotenoid and anthocyanin metabolic pathways were analyzed.The results showed that the chlorophyll pathway synthesis genes were down-regulated and the chlorophyll degradation related genes were up-regulated in sunburn peel.The key genes PSY and LCY-B of carotenoid pathway synthesis were up-regulated,and the key gene VDE of lutein cycle was significantly up-regulated.The key synthetic genes of anthocyanin metabolic pathway were down-regulated.Determination of peel pigment content revealed a decrease in chlorophyll content,an increase in carotenoid content and a decrease in anthocyanin content in the sunburned peel of Ponkan.Further determination of carotenoid composition in Ponkan peels using targeting metabolism technique identified phytoene、β-cryptoxanthin laurate、zeaxanthin dimyristate、lutein myristate、lutein dimyristate、antheraxanthin、lutein dilaurate and zeaxanthin as the major carotenoids in the sunburned peel.