Histopathology and histochemistry of sugar beet seedlings resistant and susceptible to Aphanomyces cochlioides

Abstract

Hyphae were observed within diseased cortical tissue of hypocotyl lesions of both the susceptible and the resistant varieties but were not detected in symptomless tissues in advance of lesion margins. Hyphae ramified extensively intercellularly and, to a lesser extent, intracellularly. Only in severely diseased beets were hyphae found within the stele. Protoplasts of cortical cells in proximity to hyphae appeared to lose turgor early in pathogenesis; then the cells collapsed and cortical tissue was apparently macerated. Histochemically, cell wall constituents (pectin, hemicellulose, non-cellulosic polysaccharides, lignin, suberin, and cutin) were little changed in diseased beets as compared with healthy beets. Peroxidase and (at times) phenolase activity increased and phenols and quinones accumulated in the stele of diseased plants (adjacent to lesions) to levels above those found in healthy plants. Possibly these phenols and quinones may inhibit colonization of stelar tissues. No differences in accumulation or localization of these substances in the resistant as compared with the susceptible beet varieties were detected by histochemical methods.