Barrier Zone Formation: A Mechanism of Tree Defense Against Vascular Pathogens

Abstract

Barrier zones of axial parenchyma, were common in sapwood of trees which had re- covered from vascular infections. They were a consistent feature in a representative sample of vascular wilt diseases when trees expressed some resistance. Vascular wilt diseases caused by Ceratocystis ulmi, C. coerulescens, Verticil- lium albo-atrum, and Fusarium oxysporum were studied. Barrier zone formation is described as a basic, non-specific -defense mechanism against vascu- lar pathogens in trees. When barrier zones of living xylem elements form in response to in- jury or infection, survival of the vascular cam- bium is favoured. Hence the definition of bar- rier zones in trees has been expanded; a barrier zone is a protective tissue which forms in re- sponse to infection as well as to mechanical wounding, and serves to isolate necrotic sap- wood from living cambium. Barrier zones are specialized tissues; not zones of defective mis- aligned undifferentiated cells which has been the traditional view of wound wood.