Resistance index and browning mechanism of apple peel under high temperature stress

Apples are one of the most important economic crops worldwide. Because of global warming and an aggravation of environmental, abnormally high temperatures occur frequently in fruit-growing season and seriously affect normal fruit growth and reduce fruit quality and yield. We took five-year-old ‘Ruixue’ (Qinfu 1 × Pink Lady; CNA20151469.1) fruits as test materials, and the ambient temperature during fruit development was monitored. The results showed that during the fruit-growing season, especially during the rapid growth stage (July to August), the maximum daily temperature exceeded 30 °C and lasted for more than 40 days. To determine the effects of high temperature stress on the apple fruit resistance, we treated expanding, veraison, and maturity-period fruits at different temperatures. It was found that the fruits of the expanding period showed strong resistance to high temperature stress, whereas during veraison and maturity, fruit resistance to high temperature stress decreased, and the fruit peel browning phenotype appeared. Meanwhile, the content of malonaldehyde (MDA), hydrogen peroxide (H2O2), and superoxide anion (O2.-) in the peel gradually increased with increasing temperature. The content of total phenols, flavanol, and flavonoids in the peel decreased substantially at 45 °C. Moreover, it was found that polyphenol oxidase gene (MdPPO1) was most sensitive to high temperature stress in apple. Furthermore, transient and stable MdPPO1 overexpression significantly promoted peel browning. The transgenic materials were more sensitive to high temperatures, and browning was more severe compared to non-genetically modified organism (WT). Stable MdPPO1 knockout calli obtained via clustered regularly interspersed short palindromic repeats (CRISPR/Cas9) gene knockout technology reduced the browning phenotype, and the resultant fruits were not sensitive to the effects of high temperature stress. Thus, MdPPO1 expression may be a key factor of high temperature–related changes observed in the browning phenotype that provides a scientific theoretical basis for the selection of high temperature–resistant varieties and apple cultivation and management in the future.