Evaluation of sunburn resistance and screening of physiological indexes related to sunburn of the fruits in 120 pomegranate accessions

Abstract:【Objective】Sunburn is a physiological disease of pomegranate fruits, which seriously affects the appearance and quality of the fruits. In order to understand the sunburn resistance of pomegranate fruits, we investigated the incidences of sunburn and sunburn indexes of 120 pomegranate accessions. Based on the sunburn resistance of pomegranate accessions, 15 accessions with different sunburn resis- tance were selected to study the relationship between the sunburn resistance and the antioxidant activity.【Methods】The sunburn incidences and sunburn indexes of 120 pomegranate accessions, planted in Gangji Eco-Agricultural Demonstration site, Anhui Academy of Agricultural Sciences, were investigat- ed, and the corresponding sunburn resistance were evaluated. The antioxidant capacity was determined by FRAP (Ferric reducing antioxidant power) method. The contents of total phenol, total flavonoid, an- thocyanin, carotenoid, ascorbic acid (AsA) and glutathione (GSH) content of sunburn pericarp were de-tected. SPSS 25.0 software was used to do the correlation analysis and linear regression analysis be- tween total antioxidant capacity and antioxidant contents.【Results】There were significant differences in the sunburn rates and the sunburn indexes of 120 pomegranate germplasm resources. The most abundant pomegranate accessions had the sunburn index from 20 to 40, which represented resistant acces- sions. There were 40 varieties with sunburn index from 40 to 60, which represented susceptible acces- sions. There were 25 accessions with sunburn index ranging from 0 to 20, which represented highly re- sistant accessions. There were 10 accessions with sunburn index over 60, which represented high sensi- tivity to sunburn. The relationship between the sunburn resistance ability and the antioxidant activity of 15 accessions with high resistance and sensitive accessions was studied. The results showed that there were significant differences in the total antioxidant capacity of 15 accessions, which increased with the increase of sunburn index. AHHB-31 and AHHB-27 had the strongest total antioxidant capacity, which were 416.22 and 414.20 mmol·g-1, respectively. While AHHB-65 had the weakest total antioxidant capacity (131.05 mmol·g-1). The contents of total phenol, total flavonoid, anthocyanin, carotenoid, ascorbic acid and glutathione in the pericarp of 15 pomegranate accessions were also different in germplasm level. In general, the antioxidant contents of pomegranate pericarp increased with the sunburn index in a certain range. Exceeding this threshold, the content of antioxidant in highly susceptible accessions was reduced. The total antioxidant capacity of the pomegranate peel and the sunburn index was signifi- cantly and positively correlated (p < 0.01). The correlation analysis between antioxidant substances and sunburn index showed that ascorbic acid and total phenol content had significantly positive correlation with the sunburn index of fruit (p < 0.01), and the correlation coefficients were 0.750 and 0.629, respec- tively. The anthocyanin, glutathione and total flavonoid contents had correlation coefficients of 0.624, 0.538 and 0.506, respectively (p < 0.01). There was no correlation between the sunburn index and the carotenoid content. Furthermore, the correlation between the total antioxidant capacity and the contents of total phenol, total flavonoid, anthocyanin, carotenoid, ascorbic acid and glutathione were analyzed, separately. The results showed that the total antioxidant capacity was significantly correlated with the contents of total phenol, glutathione, ascorbic acid, total flavonoid, anthocyanin and carotenoid content (p < 0.01), indicating that these antioxidants could greatly reflect the total antioxidant capacity of sun- burn fruits. Among them, the correlation coefficient between the total antioxidant ability and the total phenol, glutathione and ascorbic acid contents were higher than those between the total antioxidant abil- ity and total flavonoid, anthocyanin and carotenoid contents.【Conclusion】The sunburn resistance of pomegranate accessions were significantly different. Generally, the total antioxidant capacity can reflect the sunburn resistance of pomegranate fruits. The antioxidant ability of pomegranate fruits depends on the contents of total phenols, ascorbic acid and glutathione. The results not only provide a basis for ge- netic improvement of sunburn-resistant varieties, but also provide an insight to studying the anti-sun- burn mechanism of pomegranate.