VIEW ARTICLE

Fluorescing Materials Associated with Vein Blackening and Necrosis in Leaves of Black Rot-Resistant Cabbage. T. Staub, Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; P. H. Williams, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 62:858-866. Accepted for publication 1 February 1972. DOI: 10.1094/Phyto-62-858.

The veins and hydathodes of resistant and susceptible cabbage leaves were inoculated with aqueous suspensions of Xanthomonas campestris. An accumulation of a blue-green fluorescing material (E_max = 500 to 510 nm) in the lumen of xylem vessel elements prior to vein darkening was observed only in lines containing the f gene for resistance to X. campestris. The fluorescence within the lumen disappeared as the infected vessels darkened and fluorescence appeared in the parenchymatous tissues of the hydathodes. In leaves of susceptible plants, a red-brown fluorescence which was unstable to ultraviolet light accumulated in the cell walls of the bundle sheath sclerenchyma. Following infiltration of leaf panels with 10⁸ cells/ml of X. campestris, the rates of bacterial multiplication and lesion development were similar in resistant and susceptible plants, indicating that the intercostal parenchyma is not directly involved in resistance. The necrotic responses of cabbage leaf parenchyma to X. campestris were identical to those elicited by incompatible or heterologous plant pathogens, X. malvacearum, X. vasculorum, Pseudomonas syringae, and P. solanacearum. A blue-green fluorescence could be observed in the bundle sheath of veins surrounding the necrotic panels induced by both homologous and heterologous organisms. Only slightly more fluorescence appeared in resistant than in susceptible plants. A histological comparison of hydathodes infected with X. campestris from resistant and susceptible plants showed extensive maceration of xylem vessel terminals and surrounding parenchyma in susceptible plants, whereas the corresponding tissues in resistant plants were not macerated.