A COMPARATIVE-STUDY OF HISTOLOGICAL AND ULTRASTRUCTURAL ALTERATIONS INDUCED BY PYTHIUM-ULTIMUM OR ITS METABOLITES ON GERANIUM (PELARGONIUM)ROOTS

Degradation events induced on geranium (Pelargonium) root cells by inf ection with Pythium ultimum were studied and compared with the effect of a treatment with partially purified culture filtrates of the pathog en. Following inoculation, penetration of host roots by the pathogen w as restricted to the outer region of the cortex while pronounced cellu lar alterations were observed in all parts of the root. Cortical and e ndodermal cells appeared distorted as compared to the untreated contro ls, and cells of the most external layers were collapsed. Endodermal c ells had a denser cytoplasm than normal and often contained opaque inc lusions. Similar cytological alterations were found to be induced in r oots treated with fungal filtrates. Ultrastructural damages induced by both infection and toxin treatment included retraction of the plasma membrane and cytoplasmic disorganization or breakdown. In addition api cal cells were characterized by increased vacuolation and by the accum ulation of electron-dense deposits. Deposition of opaque material was also observed in endodermal cells and adjacent intercellular spaces. D espite marked cell distortion associated with a folding of cell walls, cytochemical labelling of pectin and cellulose did not indicate hydro lysis of these main cell wail components. The similarities between the alterations seen in each zone of the root tissues, in both P. ultimum -infected and fungal metabolite-treated plants, suggest that the obser ved cell damage is largely induced by similar biochemical processes. T oxic, diffusible metabolites are likely to be produced in planta by P. ultimum during the pathogenic interaction and appear to contribute to the expression of symptoms on geranium roots.