Correlation between the deposition of fluorescent compounds in papillae and resistance in barley against Erysiphe graminis hordei

Abstract

Events in the development of powdery mildew fungi and responses of barley epidermal cells were compared not only among combinations of barley and among races of mildew differing in virulence but also between epidermal cells adjacent and nonadjacent to stomata. Differences in fluorescence intensity and in the size of papillae and haloes were not observed until 12 h after inoculation; the fluorescence intensity and size of papillae increased rapidly and the differences became obvious 15 h after inoculation when the parasite penetrated into host cells. Failure of fungal penetration in incompatible combinations was mostly associated with papillae either with or without collapse of epidermal cells. The greater the resistance of barley, the greater was the rate of fungal cessation of growth the papilla stage and stronger was the fluorescence intensity of papillae. Haloes were larger in susceptible than in resistant hosts, but they usually did not form at sites where haustoria were produced. In incompatible combinations (with the .**GRAPHIC**. host epidermal cells adjacent to stomata (A cells) were less resistant than the nonadjacent ones (NA cells). More than 80% of haustoria formed were restricted to A cells, and about half of the conidia located in A cells formed haustoria. Many parasites of the remaining population in A cells ceased growth, followed by host cell collapse. In NA cells, most penetration pegs were terminated at the stage of papilla formation. The fluorescence intensity of papillae in NA cells was much stronger than that in A cells. Cytologically specific events of compatibility or incompatibility may be expressed at the papilla stage and that the fluorescent compounds deposited at the infection sites might represent chemical factors which are associated with resistance expressed at the stage of penetration, although it is not known whether the compounds as well as papillae themselves are a primary cause of resistance or a result of some unknown resistance mechanism.