The metabolism of soluble carbohydrates related to chilling injury in peach fruit exposed to cold stress

In order to elucidate the metabolism of soluble carbohydrates related to chilling injury (CI) in peach fruit (Prunus persica L. Batsch cv. Yulu) exposed to cold storage, we systematically investigated the changes in soluble carbohydrate content, transcript levels for genes encoding related enzymes, and the ascorbic acid (AsA) regeneration cycle in fruit stored at 0 °C and 5 °C for 28 days. At 0 °C, sucrose levels gradually increased up to day 21 before declining, but no significant changes of reducing-sugar levels (glucose and fructose) were observed during this time. However, the levels of sucrose decreased sharply and reducing-sugar levels increased during storage at 5 °C. Thus, fruit stored at 0 °C showed significantly higher levels of sucrose and lower levels of glucose and fructose than fruit stored at 5 °C. This difference was primarily due to lower transcription levels for neutral invertase 2 (NI/2), acid invertase (AI) and sucrose synthase (SS) and lower activities of NI and AI, and higher levels for sucrose phosphate synthase 1 (SPS/1). Both groups of fruit displayed an early, dramatic increase in the transcription levels of sorbitol and sorbitol dehydrogenase (SDH). Additionally, stable membrane permeability, a lack of CI symptoms and a regular AsA cycle were observed throughout the test period in fruit stored at 0 °C. On the other hand, membrane permeability increased sharply in fruit stored at 5 °C, which caused severe internal browning and stimulated AsA cycle activity later in storage. These results suggest that the higher levels of sucrose, resulting from the balance between its degradation and biosynthesis, contribute to membrane stability and enhanced chilling tolerance in peach fruit. Additionally, higher levels of glucose may have provided more reducing power and served as a substrate for AsA biosynthesis, which resulted in enhanced AsA cycle activity in chilling-sensitive fruit.