Differences in Occurrence of Fruit Sunburn Between Satsuma Mandarin and Ponkan Tangerine and Possible Mechanisms
Sunburn is a common physiological disorder in citrus production,but the underlying mechanism remains largely unknown.The study was carried out at Fenghuang Mountain,Banyue Town,Dangyang City,Hubei Province in 2021.Satsuma mandarin(Citrus unshiu Marc.)and Ponkan(C.reticulata)were used as experimental materials to investigate the difference in sunburn and potential mechanisms by examining peel injury degree,performing cytological observation,measuring antioxidant content and gene expression levels.The results showed that Satsuma mandarin was more susceptible to sunburn than Ponkan.Significant difference in the incidence rates of sunburn was observed between the two varieties,43.12％for Satsuma mandarin and 22.52％for Ponkan.Sunburn seriously influenced the compactness of pericarp cells and resulted in the accumulation of lignin.Satsuma mandarin fruits were found to accumulate greater level of reactive oxygen species(ROS).In addition,nobiletin,a critical metabolite in the polymethoxyflavone metabolic pathway,was significantly enriched in Ponkan fruit peel.Consistently,transcript levels of CrCHS2,CrCHI and CrFNS,encoding crucial enzymes involved in biosynthesis of flavonoid,and CrOMT2 in the polymethoxyflavone pathway were significantly induced in the sunburnned Ponkan fruit peel.Taken together,these findings indicate that the main reason for the difference in sunburn between Satsuma mandarin and Ponkan is that the latter contains more antioxidant that can effectively decrease the ROS accumulation,thus leading to mitigation of sunburn-derived damage.
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