Chronological analysis of cytological responses in powdery-mildewed barley leaves

Abstract

The cytological responses against Erysiphe graminis f.sp. hordei, race I, of barley epidermal cells from either highly resistant, resistant, or susceptible barley leaves were chronologically examined by light and fluorescent microscopy. Small papillae and aggregations of epidermal cytoplasm around the papillae were observed beneath appressorial tips of the fungus in all barley leaves 12–13 h after inoculation. In all barley leaves, if papillae increased in size and fluoresced 18 h after inoculation, the mass of aggregated cytoplasm disappeared and there was no fungal penetration. If successful penetration was accomplished 15 h after inoculation in highly resistant and resistant barley, the aggregated cytoplasm fluoresced and increased in amount and fluorescent intensity until 18 h. Eventually, all the cytoplasm of the affected epidermal cells became granulated and showed strong fluorescence 48 h after inoculation. In resistant barley, about 10% of conidia resulted in successful penetration of the nonfluorescent papillae and in small, fluorescent and distorted haustoria; in these cases, the fluorescence of the aggregate in the invaded epidermal cells was delayed. In resistant and susceptible barley, 30 and 70%, respectively of conidia yielded penetration hyphae which successfully penetrated the nonfluorescent papillae and resulted in normal haustoria 48 h after inoculation. Aggregation of epidermal cytoplasm around the penetration sites in these was also observed 15 h after inoculation but was not noticeably fluorescent and had dispersed 48 h after inoculation. These results suggest that in barley leaves compatibility or incompatibility is established by 15 h after inoculation, incompatibility being expressed by the aggregation and fluorescence of the epidermal cytoplasm.