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 M ichx. wounded or wound-inoculated with Entoleuca mammata (Wahlenberg: Fr.) J.D. Rogers & Y.-M. Ju was prepared for histochemical and microspectrophoto metric analysis. Samples were collected over a 96-h period. Parenchyma cell walls associated with the response zone of infected resistant and suscepti ble genotypes accumulated phenolic substances having lignin-like properties . Features of the lignified zone distinguished resistant from susceptible g enotypes. This zone in the resistant genotype was uniformly lignified, whil e in the susceptible genotype, it was discontinuous. Wound callus developed in the infected resistant but not in the susceptible genotype. In the form er, callus developed internal to the lignified zone, contained phenolic sub stances, and was visible 48 h after inoculation. In the susceptible, callus failed to develop. Wounded tissue of both genotypes displayed no distingui shing 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 i nfection by E. mammata by two distinct mechanisms: (i) by the development o f an intact and localized lignified barrier zone and (ii) by the developmen t of wound callus rich in phenolic substances. The susceptible is ineffecti ve at developing either of these barriers.