Anatomy and lignin content in relation to resistance of Dactylis glomerata to Stagonospora leaf spot

Parallel veins of orchardgrass leaves were examined in relation to width of leaf spots incited by Stagonospora arenaria. Trials of 12 greenhouse-inoculated plants and 12 naturally infected, field-grown single-cross lines that ranged widely in mean leaf spot size gave similar results. Veins with large vascular bundles (enlarged metaxylem vessels present) alternated regularly with veins having small vascular bundles (enlarged metaxylem vessels absent). Vascular bundles usually were connected to the lower and/or upper epidermis by girders of lignified sclerenchyma and/or girders of thin-walled cells. Mean leaf spot size was not related to width of small or large vascular bundles, distance between vascular bundles, or percentage of veins with large vascular bundles. Edges of leaf spots usually stopped at parallel veins, but wider leaf spots crossed one or more veins. Leaf spots crossed veins having small vascular bundles more frequently than veins having large vascular bundles. Differences in vein crossing were related to the frequency of girders. About 86-88% of large vascular bundles had two girders (to the upper and lower epidermis), but only about 10-50% of small vascular bundles had two girders. The proportion of small vascular bundles with two girders was greater in resistant plants and lines than in susceptible plants and lines. Regression indicated that 40-55% of heritable variation in mean leaf spot width may be accounted for by frequency of small vascular bundles having two girders. Leaf spot size was not associated with constitutive lignin content of uninoculated leaves in two greenhouse trials of 14 single-cross F(1) lines. Inoculated leaves had higher lignin content than controls, indicating that ligninlike compounds were synthesized during infection.