Principles of barrier formation of scab-infected apple fruits

Corky layers of scab-infected (Venturia inaequalis) apple fruits were sampled late in the season. At the affected fruit surface, phellem-like layers were formed whereas the underlying fruit cells exhibited a more proliferative division pattern. The question whether those dividing cells were capable to synthesize proanthocyanidins (PAs) was studied in vitro with callus tissues. Abscisic acid (ABA), acting as a stress hormone, stimulated proliferative callus growth. In the callus, epicatechin was the predominating flavanol basic unit. (+)-Catechin and oligomeric PAs were lower in amount. From the aspect of wound healing, it is of significance that even insoluble PAs were found in dividing callus cells. Low growth cycles of the callus cells were correlated with increasing amounts of bound PAs. Both, soluble and insoluble PAs were shown to complex with wall components of the parenchymatous fruit cells. This is an important aspect in view of achieving defence against pathogens by cell wall strengthening. The cell walls of the corky phellem being impregnated by suberin lacked the ability to complex with PAs.