Anatomical Changes in the Stems of Azadirachta Indica (Meliaceae) Infected by Pathogenic Fungi

Abstract

Development of gum ducts and chemical nature of occluding material was studied by histological and histochemical methods in the differentiating xylem of Azadirachta indica A.Juss. In response to injury and subsequent fungal infection the stem developed vascular occlusions and gum resin cavities. Fungal hyphae were observed in all the cell types of xylem including vessel elements and associated parenchyma cells. Chemical nature and forms of the occluding material in gum ducts/cavities differed from that of wound exudates. The occluding material was rich in proteins, lipids, polysaccharides, lignin and phenolics while the material exuded from wounds contained only lipids and proteins. The epithelial cells of the gum ducts showed intense staining for nucleic acids, proteins and insoluble polysaccharides as compared to the neighbouring cells. Some of the epithelial cells accumulated phenolics, which were later released into the duct lumen after dissolution of cell walls. Axial parenchyma cells near the cavities/ducts were free from starch while the vessels were plugged with gum-like material and tyloses. On the other hand, xylem parenchyma cells away from the infection site showed heavy accumulation of starch and negligible amounts of proteins and lipids. The boundary layer was characterised by the presence of few vessels and an abundance of axial and ray parenchyma with heavy accumulation of phenolics, particularly a catacholic type of tannin.