Histological studies were performed on a compatible and an incompatible interaction between chickpea (Cicer arietinum L.) plants and the fungus Ascochyta rabiei (Pass.) Labr. The time course of infection, development on leaflets and stems of susceptible (ILC 1929) and resistant (ILC 3279) plants was monitored by light or scanning electron microscopy with the aim to compare histological changes as the basis for further work on biochemical changes in this plant‐pathogen interaction. Spores of A. rabiei began to germinate from 12 hpi on and developed a polar germ tube; fungal colonization, secretion of a mucilaginous exudate and appressoria formation (1–3 dpi) were identical on both cultivars. Leaves of susceptible plants were invaded by the fungus directly through the cuticle, the fungus then spread subepidermally followed by a rapid collapse of the leaf tissue (4–6 dpi). Development of leaf spots and fungal pycnidia could be observed 6–8 dpi. The resistant cultivarrapidly responded (24–48 hpi) to fungal infection and cells of the palisade parenchyma exhibited autofluorescence. In later stages of the infection (4–5 dpi) fluorescent areas developed to small necrotic spots all over the leaflet. These necrotic areas, were the result of cell death and a subsequent change in the leaf structure and were characterized by the accumulation of phenolic compounds. Leaves of the resistant cultivar were invaded by the fungus to less than 5%.

The origin of arterio-venous anastomoses, connecting the efferent filament artery (EFA) with the central venous sinus (CVS) of gill filaments can be well discerned by scanning electron microscopy in the rainbow trout. Corresponding vessels between the afferent filament artery and the CVS could not be detected with the techniques applied. AVA-specific endothelial cells are characterized by their bulky shape and their microvillous surface. The general morphology of AVA's in Salmo gairdneri is very similar to that of AVA's in Tilapia mossambica (Vogel et al., 1974) but they are much longer in the trout. No filament whorls have been encountered in AVA endothelia of Salmo gairdneri.

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