VIEW ARTICLE

Resistance

Pinocembrin: An Antifungal Compound Secreted by Leaf Glands of Eastern Cottonwood. Louis Shain, Associate professor, Department of Plant Pathology, University of Kentucky, Lexington 40546; Joseph B. Miller, principal laboratory technician, Department of Plant Pathology, University of Kentucky, Lexington 40546. Phytopathology 72:877-880. Accepted for publication 20 November 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-877.

Pinocembrin (PC), 5,7-dihydroxyflavanone, was identified as a major constituent of leaf resin of eastern cottonwood (Populus deltoides). Bioassays of PC at pH 6 against spore germination of four fungal pathogens of cottonwood suggested that it is most active against Melampsora medusae (toxic at 16 ppm), least active against Septoria musiva (no inhibition at 32 ppm), and intermediate in activity against Cytospora chrysosperma (toxic at 32 ppm) and Marssonina brunnea (9% of control germination at 32 ppm). Similar tests at pH 8 suggested that ionized (bathochromic-shifted) PC (γ_max = 324 nm) is substantially less inhibitory than its nonionized form (γ _max = 288 nm). The concentration of PC in water droplets collected from young leaves after 2.5 and 30 min was 19 and 45 ppm, respectively. The total amount of PC on leaf positions 1–5 starting with the first unfolded leaf below the branch apex differed significantly on field-grown leaves (eg, 346–10 mg × 10^–3), but not greenhouse-grown leaves (eg, 735–542 mg × 10^–3). As leaves expanded through positions 1–5, the amount of PC per unit leaf area decreased significantly in both field- and greenhouse-grown leaves. This indicates that a sufficient amount of PC is present on the surface of young, expanding leaves to contribute substantially to their resistance to M. medusae and to a lesser degree to M. brunnea. As leaves age, however, the concentration of PC is depleted as a result of weathering, leaf expansion, and insufficient replenishment. Clones that consistently retain high levels of PC on older expanded leaves would be expected to retain also greater resistance to sensitive pathogens. Indications of variability in the concentration of PC on older leaves were obtained in a 15-clone comparison.