Zhangfei Wu

To accurately evaluate the role of storage temperature in improving the quality of fresh-cut fruits and vegetables, the effects of two storage temperatures (5 °C and 15 °C) on the phenylpropanoid pathway and sucrose metabolism in fresh-cut melon (cv. Yugu) cubes were determined. A higher temperature (15 °C) expedited sucrose decomposition in the melon cubes at the early stage of storage, resulting in higher levels of glucose and fructose. This effect was corroborated by increased activities of acid invertase (AI), neutral invertase (NI), and sucrose synthase cleavage (SS-c), along with higher expressions of CmAI1/2, CmNI1/2, and CmSS1/2 in the melon cubes at 15 °C. Additionally, the higher activity and gene expression of hexokinase in melon cubes at 15 °C led to an increase in the utilization rate of sugars toward downstream metabolic pathways. Moreover, the melon cube storage at 15 °C elevated the activities and gene expressions of phenylalanine ammonia lyase (PAL), cinnamic acid 4-hydroxylase (C4H), and 4-coumaric acid: CoA ligase (4CL), thereby increasing the synthesis of phenolics. Sucrose showed a significant negative correlation with PAL, C4H, and 4CL, as well as with CmPAL5/7 and CmC4H1/3. However, hexokinase displayed a significant positive correlation with PAL, C4H, and 4CL, as well as with CmPAL1, CmPAL3-9, CmC4Hs, and Cm4CLs. These findings demonstrate that a higher-temperature storage of melon cubes can accelerate the phenylpropanoid pathway and sucrose metabolism by regulating the activity and gene expression of related enzymes, thereby inducing phenolic accumulation. These results also indicate that lower-temperature storage is not conducive to the conversion of sugars into phenolics in fresh-cut melon. Therefore, the temperature can be appropriately and briefly raised in the production and preservation process of fresh-cut melon to obtain higher levels of phenolics.

The effect of cutting on the molecular changes underlying sucrose metabolism and the phenylpropanoid pathway in melon fruit (cv. Yugu) during storage at 15 °C was investigated. Furthermore, the key metabolites, enzymes, and genes involved in sucrose and phenylpropanoid metabolism were determined. Results showed that the cutting of melon increased the activities of acid invertase (AI), neutral invertase (NI), and sucrose synthase-cleavage (SS-c) and the expressions of CmAI1/2, CmNI1/2, and CmSS1, while sucrose synthase-synthesis (SS-s) and sucrose phosphate synthase (SPS) activities and the CmSS2/3 and CmSPS1/2/4 gene expressions were suppressed. These led to sucrose decomposition and fructose and glucose accumulation in fresh-cut melon at the early stage of storage. Moreover, cutting increased the activity and gene expression of hexokinase, which accelerated the transformation of hexose in fresh-cut melon. In addition, cutting enhanced the activities of phenylalanine ammonia lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate-CoA ligase (4CL) and up-regulated the expressions of CmPAL1-9, CmC4H1-4, and Cm4CL1/2/3, which activated phenylpropanoid metabolism and resulted in phenolic accumulation in fresh-cut melon. These findings demonstrate that cutting of melon can enhance sucrose metabolism and phenylpropanoid pathway by regulating the activities and gene expressions of related enzymes. Therefore, cut-wounding promoted the conversion of sugars to supply the necessary substrates for phenolic accumulation in fresh-cut melon.