Fire blight resistance in Pyrus: hydroquinone formation as related to antibiotic activity

Aqueous extracts of pear leaf blade and woody tissues, which are fire blight resistant, exhibited greater antibiotic activity than did extracts of petioles and leaf midribs, and bark, which are susceptible. Increases in antibiotic activity with incubation time were associated with increases in the amount of hydroquinone formed and a concurrent decrease in arbutin. Largest increases in antibiotic activity occurred in extracts of tissues which exhibited the greatest amount of antibiosis in the tissue bioassay test. The levels of other phenolics did not change significantly, even when tissue fragments were present in the extracts. Leaf age did not appreciably affect the diffusion from leaf discs of compounds necessary for antibiotic activity. β-Glucosidase was not inactivated in leaf discs thawed for long periods. There is an insufficient amount of free hydroquinone in Pyrus tissues to affect antibiotic activity or fire blight resistance. Free hydroquinone occurs in large amounts in extracts obtained with hot methanol. These findings continue to support the validity of tissue bioassay as a method for measuring the relative β-glucosidase activity in Pyrus tissues.