Compartmentalization of decay in carnations resistant to Fusarium oxysporum f sp dianthi

The anatomy of well-developed defense responses in carnation to Fusarium oxysporum f. sp. dianthi was studied in light of the compartmentalization of decay in trees (CODIT) model. Compartmentalization of the fungus was achieved by occlusion of vessels with gums (wall 1) and circumscription of the affected area with reaction zones (walls 2 to 4). Reaction zones were characterized by thickening, lignification, and suberization of parenchyma cell walls; suberization of fiber walls; and proliferation of xylem parenchyma cells bordering the reaction zone. A cork layer was formed from the proliferating cells. Wall thickenings stained for methyl ester groups (indicative of pectins or xylans) and lignin aldehydes, but not for cellulose, callose, or lipids. Induced lignin stained like cork rather than vessel or fiber lignin. Gums stained for pectin and lignin. Pressure exerted by proliferating parenchyma caused the stem to burst. Cells at crack margins stained for lignin and suberin. No essential differences existed between vascular and extravascular reaction zones, which sometimes merged fluently. Compartmentalized tissues were replaced by vascular regeneration within the proliferating xylem parenchyma on the side of the medulla. Defense responses focused for protection of regenerated cambium and xylem (wall 2) rather than the original cambium (wall 4), as in trees.