Barrier-zone formation in wounded stems of sweetgum

Abstract

The response of sweetgum trees (Liquidambarstyraciflua L.) to wounding was examined histologically. Seven stems were wounded artificially (by exposing the cambium or cutting deep into the xylem) or naturally (by fire or by frost cracks). After wounding, the cambium produced an anatomically distinct tissue which limited radial spread of discoloration and decay near the wounds. This tissue was characterized by traumatic gum canals, large amounts of longitudinal parenchyma, thin-walled fiber sclereids, callose deposits, staining of cell walls with phloroglucinol, and a weak or fading Maule reaction for lignin. This tissue extended no more than 60 cm longitudinally above and below the wounds and no more than 20 cm around the stem, less than 50% of the circumference of any tree. Gum canals and associated tissue were an effective barrier to the radial spread of discoloration and decay. Water-soluble extractives from the tissue of the annual ring formed after wounding inhibited growth of the decay fungus, Pleurotussapidus, invitro. This inhibition was not observed with extractives more than 20 cm longitudinally from the wound. These results indicate that the barrier zone formed by sweetgum in response to the stem wounds studied was limited to the vicinity of the wound and may be both an anatomical and chemical barrier. Thus, morphologically distinguishable barrier zones cannot alone account for protection of xylem formed after wounding from discoloration and decay.