Regulation of the fermentative metabolism in apple fruit exposed to low-oxygen stress reveals a high flexibility

Storing 'Jonagold' apple (Malus x domestica Borkh. cv. 'Jonagold') closer to their anaerobic compensation point during dynamic controlled atmosphere storage may delay fruit ripening more effectively, but can also induce low-oxygen stress in the fruit leading to the induction of fermentation. Knowledge of the in vivo regulatory mechanism of the fermentative metabolism in plants is still limited. We exposed 'Jonagold' apple tissue slices to different oxygen partial pressures (0, 0.3, 0.5, 1, 5 and 21 kPa O-2) and temperatures (1 degrees C and 18 degrees C) at harvest and after 6 months of storage. A combination of experimental techniques and a kinetic modelling approach was used to translate the in vitro measured enzyme activity into in vivo activities and to provide new insights in the regulation of the fermentative metabolism. The regulation of the fermentative metabolism appeared to be highly flexible as it depends largely on the conditions applied. Temperature is known to increase the overall metabolic reaction rate, which appeared to have a large influence on the induction and regulation of the fermentative metabolism. Furthermore, the model indicated that the in vivo regulation of the fermentative metabolism was different under anoxic (0 kPa O-2) and hypoxic conditions (0.3, 0.5 and 1 kPa O-2) and that an extra regulation was present after 6 months of storage. Even though fermentation was clearly induced under all the experimental conditions applied, the flux through the fermentative metabolism started to decrease again after prolonged low-oxygen stress.