Peroxidase: Changes in soluble and bound forms during maturation and ripening of apples

Peroxidase (POD) enzymes, both soluble and bound, have been implicated in the ripening process of a number of fruits. However, their roles are poorly understood. The aim of this study was to define clearly the classes of POD activity in apple parenchyma tissues on the basis of their solubility in salt solutions, to determine the subcellular localisation of ionically-bound POD activity, and to determine whether soluble- or ionically-bound POD activities exhibit any relationships with maturation and ripening. The cortical parenchyma of apples (Malus domestica Borkh cv. Cox's orange pippin) was investigated for changes in POD activity during maturation and post-harvest storage at 0, 4, and 8°C at 100% relative humidity (RH), and controlled atmosphere (CA) storage (4°C in 1.25% O₂ and 0.5% CO₂ ). The parenchyma cells contained both soluble and ionically-bound POD activity. The ionically-bound POD was located in the cell wall in the middle lamella, plasmodesmata and cell corners, and could be extracted quantitatively with 0.4 M CaCl₂ at neutral pH to leave a POD-free residue. During maturation, both forms of activity increased on a 'per apple' basis, but decreased as a function of fresh weight. Under all conditions of storage, soluble POD exhibited a peak of activity at approximately the mid-point of ripening-related softening; this event may constitute a biochemical marker. In contrast, ionically-bound POD generally decreased during ripening, except under CA storage. CA storage inhibited ripening-related softening after an initial loss of firmness. However, softening resumed after transfer of apples to normal atmosphere storage at 8°C. This may provide a new insight into controlling tissue softening.