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

Abstract

Plants of a Eucalyptus marginata clone (1JN30) were grown in aeroponics chambers that could be sealed to allow the manipulation of oxygen levels in the root environment. Roots were grown for varying periods of hypoxia (0, 2, 5, 11 or 29 days) before being inoculated with zoospores of Phytophthora cinnamomi. A similar set of roots was inoculated 3 days after the hypoxia treatments. Root extension was reduced at the end of all the hypoxia treatments. Six days after the hypoxia treatments, root extension had returned to normal 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. In the case of roots exposed to 2 days of hypoxia, the apical meristem appeared normal at the end of the treatment, but 3 days after the return to normal oxygen conditions many of the apical meristems had died and the roots had a clubbed appearance. Thus, E. marginata roots have an acclimatization period to hypoxia of between 2 and 5 days, after which they can tolerate hypoxia for extended periods. However, their ability to recover rapidly posthypoxia is reduced after 11 days' exposure. The percentage of inoculated roots that became infected and developed lesions was significantly reduced in roots inoculated immediately posthypoxia, compared with roots grown under normal oxygen conditions. As root growth recovered, lesion development also returned to normal. Thus, the rate of lesion development was related to the rate of root extension at the time of inoculation.