Histochemical and microspectrophotometric analyses of early wound responses of resistant and susceptible Populus tremuloides inoculated with Entoleuca mammata (≡Hypoxylon mammatum)

Stem tissue of resistant and susceptible genotypes of Populus tremuloides Michx. wounded or wound-inoculated with Entoleuca mammata (Wahlenberg: Fr.) J.D. Rogers and Y.-M. Ju was prepared for histochemical and microspectrophotometric analysis. Samples were collected over a 96-h period. Parenchyma cell walls associated with the response zone of infected resistant and susceptible genotypes accumulated phenolic substances having lignin-like properties. Features of the lignified zone distinguished resistant from susceptible genotypes. This zone in the resistant genotype was uniformly lignified, while in the susceptible genotype, it was discontinuous. Wound callus developed in the infected resistant but not in the susceptible genotype. In the former, callus developed internal to the lignified zone, contained phenolic substances, and was visible 48 h after inoculation. In the susceptible, callus failed to develop. Wounded tissue of both genotypes displayed no distinguishing response characteristics. Both produced equivalent amounts of callus, accumulated similar levels of lignin-like substances, and deposited it in identical locations. It is concluded that resistant P. tremuloides limits infection by E. mammata by two distinct mechanisms: (i) by the development of an intact and localized lignified barrier zone and (ii) by the development of wound callus rich in phenolic substances. The susceptible is ineffective at developing either of these barriers.