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Cytology and Histology

Anatomical Response of a Susceptible Alfalfa Clone Infected with Verticillium albo-atrum. Barbara W. Pennypacker, Research associate, Department of Plant Pathology, The Pennsylvania State University; Kenneth T. Leath, research plant pathologist, USDA-ARS, U.S. Regional Pasture Research Laboratory, University Park, PA 16802. Phytopathology 76:522-527. Accepted for publication 9 December 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/Phyto-76-522.

Alfalfa plants, cloned from a Verticillium wilt-susceptible plant, were stubble inoculated with Verticillium albo-atrum and sampled periodically. Stem samples were embedded in paraffin and examined microscopically. Infected plants had discontinuous alterations in xylem vessel element development which were characterized by atypically narrow metaxylem vessel elements interspersed with undifferentiated cambial derivatives. The lack of cambial derivative differentiation caused mature xylem vessels to be interrupted by groups of immature cells. Protoxylem and some metaxylem vessel elements were obliterated by hypertrophied xylem parenchyma. Xylem vessel lumens and pit chambers were frequently plugged with gum and pectin, and the interior cell walls were often coated with gum, suberin, and occasionally pectin. Such deposits probably impede lateral water flow and, together with the disruption in xylem vessel element differentiation, the blockage of mature vessels by immature parenchyma, and the destruction of metaxylem vessel elements, result in xylem malfunction.