VIEW ARTICLE

Resistance

Cytochemical Responses of Pecan to Cladosporium caryigenum: Development of Specific Histological Indicators to Identify and Analyze In Situ Fungitoxic Phenols. Susan V. Diehl, Department of Plant Pathology and Weed Science, Mississippi State University, Drawer PG, Mississippi State, MS 39762; Clinton H. Graves, Jr.(2), and Paul A. Hedin(3). (2)Department of Plant Pathology and Weed Science, Mississippi State University, Drawer PG, Mississippi State, MS 39762; (3)Crop Science Research Laboratory, USDA-ARS, Mississippi State, MS 39762. Phytopathology 82:1033-1036. Accepted for publication 16 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1033.

The fungitoxic phenols, juglone, isoquercitrin, and condensed tannin, were purified from pecan leaves. We tested 10 phenolic-specific histological indicators to distinguish among these three compounds within the visible spectrum. The Hoepfner-Vorsatz stain was the best indicator for juglone because it shifted juglone’s absorbance maximum from 423 to 533 nm, at which there was no interference from other phenols. The Hoepfner-Vorsatz reagent shifted the isoquercitrin absorbance maximum from 353 to 399 nm, but there was interference from minor flavonoids. Condensed tannins had a bathochromic shift from 300 to 551 nm with no interference from other phenolic compounds when heated to 90 C in n-butanol-HCl. Standard curves for each compound were generated with different light paths on a microspectrophotometer. Leaf sections 30 µm thick and nut sections 20 µm thick provided the best absorbancy range with the lowest coefficients of variation. All three phenolic compounds were quantified in situ. This system permits examination of specific sites of infection for location and quantity of juglone, isoquercitrin, and condensed tannins, and estimation of the response to infection for each of these compounds.