PPOX and infection: Certain environmental stresses during grain filling, such as infection by pathogen, wheat pre-germination, or disruption of the immature caryopsis, might cause POD and PPO release and catalyze the oxidation of phenols to form quinones, which are highly reactive and produce insoluble polymers associated with discoloration symptoms (Barz and Koster, 1981; Cochrane, 1994).