Quantification of Lignin Formation in Almond Bark in Response to Wounding and Infection byPhytophthoraSpecies

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Abstract
A quantitative assay for lignin using thioglycolic acid was adapted for investigating lignification in inner bark tissue of almond [Prunus dulcis] trees in response to wounding and infection by several Phytophthora species. One-day-old wounds had 71% more ligninthioglycolic acid (LTGA) than unwounded tissue. LTGA of wounded tissue increased linearly (r2=0.95) over the 3-wk period following wounding. Cupric oxide oxidation of LTGA from 2-wk-old wounds confirmed that lignin was produced in response to wounding. The liginin content in variously aged wounds was negatively correlated (r=-1.00) to the length of cankers resulting after inoculation of similarly aged wounds with P. syringae. Substantially more LTGA was detected in the tissue of cankers caused by P. syringae than in nearby healthy tissue. Wounds inoculated with P. syringae, P. hibernalis, and P. cactorum had substantially more LTGA 3 days after inoculation than uninoculated wounds, whereas wounds inoculated with P. infestans, a nonpathogen of almond, had the same amount of lignin as the uninoculated wounds. The deposition of lignin in almond inner bark appeared to be a response to wounding and infection by pathogens.