Maturity and cooling rate affects browning, polyphenol oxidase activity and gene expression of ‘Yali’ pears during storage

Abstract

Browning is the main physiological disorder of ‘Yali’ pear (Pyrus bretschneideri Rehd) during storage. In this study, the relationships between browning development in fruit from different harvest dates, and cooled either rapidly or slowly, with polyphenol oxidase (PPO) activity and isozymes, and PPO gene expression has been investigated. Development of browning was highest in late-harvest fruit in both core and flesh tissues and was higher in rapidly cooled than slowly cooled fruit. Mid-harvest fruit had the lowest browning incidence and PPO activity of core tissue was higher than in flesh and seeds, while the peak of PPO activity in mid-harvest fruit was the lowest. Six PPO isoenzymes were detected in fruit, three bands A, B and E in flesh and core tissues, three bands C, D and F in the seeds. The intensity of PPO isoenzyme staining of bands A and B in pulp and core was similar to that of PPO activity and browning incidence. PPO gene expression increased and then decreased in core tissues. Trends of expression were similar to those of PPO activity. Rapid cooling promoted the expression PPO. The results suggest PPO plays an important role in ‘Yali’ pear browning during storage.