Increased susceptibility of Eucalyptus marginata to stem infection by Phytophthora cinnamomi resulting from root hypoxia

Eucalyptus marginata growing on rehabilitated bauxite mines may be exposed to waterlogging (hypoxia) at the roots, as well as ponding around the stems at the soil surface. This paper examines whether these conditions may pred ispose stems of E. marginata to infection by Phytophthora cinnamomi. Plants of E. marginata clones resistant and susceptible to P. cinnamomi were grow n in an aeroponics system that could be sealed to allow the manipulation of oxygen levels in the root zone to simulate waterlogging. Plants grown unde r normal oxygen conditions were compared with those whose root zone was exp osed to hypoxia (2 mg O-2 L-1) before, during or after the stems were inocu lated with zoospores of P. cinnamomi. Inoculation was achieved by construct ing receptacles around the stems that could hold water and zoospores. Stomatal conductance increased in plants whose roots had been exposed to hy poxia. This effect lasted for at least 2 weeks after the resumption of norm al oxygen conditions. P. cinnamomi entered and colonized ponded stems; howe ver, there were no visible lesions on stems 14 days after inoculation. For any given clone of E. marginata, the extent of colonization was significant ly greater in stems whose root zone had been exposed to hypoxia than in con trol stems. The activity of the plant defence-associated enzymes PAL, 4-CL and CAD and the concentrations of soluble phenolics were higher in the stem s of plants whose roots were exposed to hypoxia, but the increase in activi ty in response to colonization was much greater for plants whose roots were under normal aerobic conditions. The greatest difference between colonized and noninoculated plants was observed at the colonization front. Peroxidas e activity increased after tissues were colonized, rather than preceding th e colonization as seen with the other enzymes. The stress induced by root hypoxia remained after roots were returned to no rmal oxygen conditions. Plants with root hypoxia showed greater stem coloni zation by P. cinnamomi and seemed less able to recognize the pathogen and s witch on rapid defence responses.