Role of hydroxyl radical in modification of cell wall polysaccharides and aril breakdown during senescence of harvested longan fruit.

The effects of reactive oxygen species (ROS), especially hydroxyl radical (OH), on cellular wall disassembly in vitro and in vivo, and aril breakdown in longan fruit, were investigated. OH caused the disassembly of cell wall material (CWM) in vitro, demonstrated by the increases of total sugars and uronic acid released, and the downshift in molecular mass of CDTA soluble pectin (CSP) and Na2CO3 soluble pectin (CO3-SP). In addition, OH resulted in decreased CO3-SP content and increased CSP content in CWM suspension, suggesting the conversion of CO3-SP to CSP. Application of exogenous OH accelerated aril breakdown in longan fruit while the process was delayed by l-cysteine·HCl, a ROS scavenger. Furthermore, lower CWM content and decreased molecular mass of pectins were observed in OH-treated fruit. These results indicated that OH contributed to the degradation of cell wall polysaccharides, resulting in aril breakdown in longan fruit.