Influence of Low Oxygen Levels in Aeroponics Chambers on Eucalypt Roots Infected with Phytophthora cinnamomi

Phytophthora cinnamomi is a major pathogen of Eucalyptus marginata (jarrah) in Western Australia (23). It can kill trees of all ages through infection of roots (10,27) or by girdling the trunks of 1to 7year-old trees in situations were the collar is ponded (12,19). Field observations have indicated that periods of waterlogging (low oxygen) can result in the rapid death of trees from P. cinnamomi infection (7,12). Flooded soils are assumed to increase disease severity by increasing the mobility of zoospores and by adversely affecting host physiology, resulting in predisposition or poor regeneration of damaged roots (18,29). Jarrah roots exposed to waterlogging produced tyloses that blocked the xylem vessels of the roots and prevented water movement (8). Glasshouse studies with 3to 4-month-old jarrah seedlings have shown that waterlogging, simultaneously with inoculation or after inoculation with zoospores of P. cinnamomi, increased the number, but not the size, of lesions compared with nonwaterlogged seedlings (9). However, seedlings waterlogged before inoculation had very few lesions compared with those waterlogged infected simultaneously with or after inoculation (9). This observation is contrary to observations of predisposition to Phytophthora root rots following waterlogging in rhododendrons (4), walnuts (18), cherries (28), alfalfa (17), and safflower (13). Soil-based studies do not allow the precise control of soil oxygen levels or observation of the effect of low oxygen on root development and subsequent infection development. As the impediment of gaseous exchange caused by static water around roots and the subsequent decrease in oxygen levels is the main cause of root damage in waterlogged soils (16), hydroponic and aeroponics systems can be adapted to study the effect of low oxygen on root development. Hydroponic systems are commonly used in the investigation of physiological and biochemical changes in roots and shoots under anoxic and hypoxic conditions (1,3,11). However, jarrah does not produce roots as rapidly as do crop species, and hydroponic systems do not allow for spot inoculation of roots with zoospores. In contrast, aeroponics systems are simple to maintain for long periods and allow for the nondestructive or sequentially destructive observations during disease development (20,21,26,30). In an aeroponics system, the effect of low oxygen on root development can be separated from the effect of chemical and biochemical reductions and changes to microbial metabolism that also occur in the soil under waterlogged conditions (16). The aim of this study was to develop a system to allow the investigation of root and lesion development in E. marginata infected with P. cinnamomi under precisely controlled oxygen conditions.