Analysis on the pericarp browning and related enzyme activities in different pomegranate cultivars

Abstract: 【Objective】Fruit browning of pomegranate is a physiological disease, which influences its com⁃mercial value. Polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonialyase (PAL) arethe regulatory enzymes of fruit browning. Phenolic compounds are the substrate of enzymic browning.There are abundant germplasm resources of pomegranate in China. The difference in browning and relatedfactors among different pomegranate cultivars has not been reported. The purpose of the study was to explore the differences in percarp browning and its correlations to related enzyme activities, total phenoliccontents and antioxidant capacity in different pomegranate cultivars at mature stage.【Methods】Twentydominant pomegranate cultivars were used as the experimental materials in the study. The browning de⁃gree of the pericarp was determined by colorimetric analysis with absorption at 410 nm. The activities ofPPO, POD and PAL were determined with ultraviolet spectrophotometry, which measured the optical ab⁃sorption at wavelengths of 410 nm, 470 nm and 290 nm, respectively. The Folin-Ciocalteu method wasused for total phenol content determination, which involved measurement of optical absorption at a wave⁃length of 750 nm. The DPPH radial scavenging activity was determined colorimetrically at 517 nm. Datawere generated from triplicate experiments and analyzed using one-way ANOVA and Duncan’s multiplecomparison tests. The software used for statistical analysis was SPSS 19.0.【Results】The browning degree,total phenolic contents and DPPH radial scavenging activity of the twenty pomegranate cultivars were inthe ranges of 0.278-0.369 mg·g^-1, 108.3-123.7 mg·g^-1 and 73.6%-82.4%, respectively. The browningdegree, total phenolic contents and DPPH radial scavenging activity were all highest in‘Shuijingtian’pericarp, which was 0.369, 123.7 mg·g^-1 and 82.4%, respectively. Except for‘Heyinhuapi’,‘Taihang⁃hong’,‘Mantianhong’,‘Huaibeifenpi’and‘Huaibeimanaozi’, the difference in browning degree between‘Shuijingtian’and the remaining fourteen cultivars was significant (P＜0.05). The difference in total phe⁃nolic content was not significant between‘Shuijingtian’and‘Heyinhuapi’,‘Mantianhong’,‘Huaibeifenpi’or‘Huaibeimanaozi’, but was significant between‘Shuijingtain’and the other fifteen cultivars(P＜0.05). There was significant difference in the DPPH radial scavenging activity among differentpomegranate cultivars (P＜0.05). The activities of PPO and POD were highest in‘Shuijingtian’, whichwere 297.4 U·min^-1·g^-1 and 147.6 U·min^-1·g^-1, respectively, and lowest in‘Taishanjinhong’, which were215.3 U·min^-1·g^-1 and 126.321 U·min^-1·g^-1, respectively. The PAL activity was in the range of 157.2-182.6 U·min^-1·g^-1 and was highest in‘Taihanghong’and lowest in‘Taishanjinhong’. The PAL activityof‘Taihanghong’was about 1.16 times that of‘Taishanjinhong’. The PPO activity in‘Shuijingtian’and‘Taishanjinhong’had significant difference from that of the other cultivars (P＜0.05). The POD activityin‘Shuijingtian’and PAL activity in‘Taihanghong’had significant difference from those in all the othercultivars (P＜0.05) except for‘Mantianhong’and‘Heyinhuapi’. There was significant difference (P＜0.05) in POD and PAL activities between‘Taishanjinhong’and all the other cultivars but‘Yanzhihong’.Correlation analysis indicated that pericarp browning degree was significantly and positively correlatedwith the activities of PPO, POD and PAL, total phenolic content and DPPH radial scavenging activity. Thecorrelation coefficient of browning degree and PPO activity was 0.920, which was higher than those be⁃tween browning degree and PPO activity (r=0.896) and POD activity (r=0.881). POD activity had signifi⁃cant positive correlations with PAL activity, total phenolic content and DPPH radial scavenging activity,with a correlation coefficient of 0.809, 0.912 and 0.868, respectively. PAL activity had significant positivecorrelations with total phenols content and DPPH radial scavenging activity, and their correlation coeffi⁃cients were 0.837 and 0.718, respectively. There was a significant positive correlation between total phe⁃nolic content and DPPH radial scavenging activity, with a correlation coefficient of 0.939. The eigenvalueof the first principal components was 5.423, with cumulative variability of 90.4%. Basically, the first prin⁃cipal components could reflect the original index information. The first principal components includedbrowning degree, activity of PPO and POD, total phenolic content and DPPH radial scavenging activity ofpomegranate pericarp. The greater the score value of the first principal components, the greater the valuesof browning degree, activities of PPO and POD, total phenols content and DPPH radial scavenging activi⁃ty. The distributions of browning degree and PPO activity were overlapping. The second principal compo⁃nents mainly included PAL activity. The score of the first principal components and the overall score wereboth highest in‘Shuijingtian’and lowest in‘Taishanjinhong’.【Conclusion】There were differences inpericarp browning, its related enzyme activities, total phenolic contents and DPPH radial scavenging activity among different pomegranate cultivars at maturation stage, and they displayed significant correlations.PPO was the dominant enzyme related to pericarp browning in pomegranate. The activities of POD andPAL were significantly and positively correlated to pericarp browning. The phenolic compound content inpomegranate pericarp was closely related with its antioxidant capacity.‘Taishanjinhong’‘Yanzhihong’and‘Linxuanerhao’were resistant to browning and worth exploitation and utilization.