Abnormal behavior of chilling injury in postharvest papaya fruit is associated with sugar metabolism

In the most chilling-sensitive fruits and vegetables, usually, the lower the storage temperature, the more serious the symptoms of chilling injury (CI). As one of the special cases, papaya fruits at 1℃ show slighter CI symptoms than those at higher storage temperature. Such abnormal CI phenomenon has not received enough attention and its mechanism is not clear. The present study investigated the difference of CI severity and sugar metabolism in papaya fruits when stored between 1℃ and 6℃. The results showed that CI index in papaya fruits preserved at 1°C was markedly lower than that in fruit at 6°C, which was accompanied by higher content of glucose, fructose, and sucrose. In addition, compared to 6°C, 1°C promoted higher activities of sucrose synthase, sucrose-phosphate synthase, and neutral invertase, but lowered acid invertase activity. RT-qPCR analysis showed that 1°C upregulated the CpSPS expression and downregulated the CpAI expression when compared to 6°C. The present results indicate that higher chilling tolerance in papaya fruit at 1°C could be attributed to more accumulation of sucrose and reducing sugars in relation to more advantageous sugar metabolism. These results provided a basis for explaining the abnormal behavior of papaya fruits in response to varying low temperatures. PRACTICAL APPLICATION: For most chilling-sensitive fruits and vegetables, in the range of temperatures that induce chilling injury (CI), the lower storage temperature may lead to more severe CI. However, as one of the special cases, papaya fruits at 1℃ show slighter CI symptoms than those at higher storage temperature. The reason for this abnormal CI symptom in papaya fruits is that 1°C storage can regulated enzyme activities and changes in gene expression related to sugar metabolism, which could result in more accumulation of sucrose and slower degradation of hexose and contribute to alleviation of CI. Our results provided a basis for explaining the abnormal behavior of papaya fruit in response to varying low temperatures.