LEVELS OF A HYDROJUGLONE GLUCOSIDE IN DEVELOPING PECAN LEAVES IN RELATION TO SCAB SUSCEPTIBILITY

Polyphenols were analyzed in expanding buds and developing leaves of p ecan [Carya illinoensis (Wangenh.) C. Koch] cultivars with varying res ponses to Cladosporium caryigenum (Ell. et Lang. Gottwald), the organi sm causing scab. Plant tissue extracts were examined by high-performan ce liquid chromatography using a water : methanol gradient to separate polyphenolic components on a C-18 reversed phase column. A diode-arra y detector was used to identify profile components by retention times and computer matching of ultraviolet spectra to standard compounds in a library. Concentrations of these polyphenols were compared throughou t the growing season in leaves of pecan cultivars with low ('Elliott') , intermediate ('Stuart'), and high ('Wichita') susceptibility to scab ; during susceptibility to infection by Cladosporium caryigenum from 1 6 cultivars; and in 'Wichita' leaf discs with and without scab lesions . The major polyphenolic constituent of tissues for all cultivars was identified as hydrojuglone glucoside, which was detected in intact bud s and leaves throughout the growing season. Hydrojuglone glucoside con centration increased concomitantly with leaf expansion and then declin ed slowly. Juglone was barely, if at all, detectable, regardless of le af age. No correlation was found between cultivar susceptibility to pe can scab and the levels of either juglone or hydrojuglone glucoside in the healthy leaves of 16 cultivars. Leaf tissue with scab lesions had significantly higher juglone and hydrojuglone glucoside levels than l eaf discs without scab lesions. Chemical names used: 4-8-dihydroxy-1-n aphthyl B-D-glucopyranoside (hydrojuglone glucoside); 1,5-hydroxy-napt hthoquinone (juglone).