Effects of hypoxia on root morphology and lesion development in Eucalyptusmarginata infected with Phytophthora cinnamomi

Plants of a Eucalyptus marginata clone (1JN30) were grown in aeroponics cha mbers that could be sealed to allow the manipulation of oxygen levels in th e root environment. Roots were grown for varying periods of hypoxia (0, 2, 5, 11 or 29 days) before being inoculated with zoospores of Phytophthora ci nnamomi. A similar set of roots was inoculated 3 days after the hypoxia tre atments. Root extension was reduced at the end of all the hypoxia treatment s. Six days after the hypoxia treatments, root extension had returned to no rmal for roots that had been exposed to 5 days of hypoxia, while for roots exposed to 11 or 29 days, extension was half the normal rate. Longitudinal sections of root tips after 5, 11 or 29 days of hypoxia indicated that the treatment caused a reduction in cell division, but not in cell expansion. I n the case of roots exposed to 2 days of hypoxia, the apical meristem appea red normal at the end of the treatment, but 3 days after the return to norm al oxygen conditions many of the apical meristems had died and the roots ha d a clubbed appearance. Thus, E. marginata roots have an acclimatization pe riod to hypoxia of between 2 and 5 days, after which they can tolerate hypo xia for extended periods. However, their ability to recover rapidly posthyp oxia is reduced after 11 days' exposure. The percentage of inoculated roots that became infected and developed lesio ns was significantly reduced in roots inoculated immediately posthypoxia, c ompared with roots grown under normal oxygen conditions. As root growth rec overed, lesion development also returned to normal. Thus, the rate of lesio n development was related to the rate of root extension at the time of inoc ulation.