Observations on the Responses of Lentil Root Cells to Hyphae of Fusarium oxysporum

The infection of lentil roots by Fusarium oxysporum Schlecht and the responses of the host cells to invading hyphae were examined by light microscopy. Hyphae from inoculum placed on the zone of cell elongation entered the roots at the juncture of epidermal cells within 8 h after inoculation. Although swollen hyphal apices were observed on the epidermal cells, root penetration occurred without formation of these structures or appressoria. The sheath of material found on the surface of uninoculated roots was absent from inoculated roots penetrated by hyphae. Prior to penetration, the epidermal cells became irregular in shape and their cytoplasm appeared to be plasmolysed or granular. Hyphae were observed in the cortex 10—12 h after inoculation and non–penetrated cortical cells were distinctly lobate. Often these lobed cells had a broad, peripheral band of diffuse cytoplasm. When hyphae were first observed in the cortical cells, the walls were ruptured and only slightly stained or unstained by toluidine blue. The inability of such walls to bind the stain may have been the result of the removal of wall components by fungal enzymes. Although extensive proliferation of hyphae was evident throughout the cortex after 24 h of incubation, the endodermis and vascular cylinder were free of hyphae for at least 72 h. Hyphae from inoculum placed on the root hairs or the root apex failed to penetrate the roots during the first 24 h of incubation. The cytological results herein are discussed in relation to the infection of field plantings by this pathogen.