Histological analyses of the host response of two aspen genotypes to wounding and inoculation with Hypoxylon mammatum

Ramets derived from root sprouts of two quaking aspen (Populus tremuloides) parents were either mechanically wounded or wounded and inoculated with mycelium of Hypoxylon mammatum. Female aspen parents were considered either resistant or susceptible based on a 30-year field trial of their open-pollinated progenies that had 21% and 93% mortality, respectively, caused by H. mammatum. Each stem was wounded twice at the base and in the nonlignified zone. Samples were taken 5 mm above the site of wounding at 7 and 14 days. Histochemicals used were phloroglucinol-HCl and Sudan Black B to detect lignin and suberin, respectively. Thin sections of the resistant aspen ramets showed intensive staining phloroglucinol-HCl, which was localized along both sides of the wound margin. Collenchyma callus was well developed by day 7 and was derived from the phloem tissue internal to the cell walls staining positive for phloroglucinol-HCl, near the vascular cambium. In contrast, the tissues of susceptible aspen ramets were less stained, the stained cells were less scattered and less collenchyma callus had formed than on the resistant ramets. The presence of the fungus increased the amount of staining observed with phloroglucinol-HCl and delayed wound closure in both aspen genotypes. No distinguishing pattern could be identified between the two aspen genotypes with respect to suberin formation. When comparing stem response to phloroglucinol-HCl with respect to the tissue type on the same stem (succulent versus woody), the inoculation of older woody tissue resulted in a more defined host response and thus were better in separating the two aspen genotypes.