Thirteen indigenous and exotic Acacia species grown in Saudi Arabia were evaluated for their host status for Meloidogyne javanica in pot tests both in the growth chamber and under outdoor conditions. In both experiments, 21-day-old seedlings were transplanted individually into 15-cm-diam. plastic pots containing a steam-sterilized mixture of equal parts loam and sandy loam. Seedlings were inoculated with 5,000 M. javanica eggs/plant 30 days later. After 120 days, fresh root weight, disease index (1-9 scale), the number of eggs/pot (Pf), eggs/g fresh root, and a reproductive factor (Rf) were determined. Results of both the growth chamber and the outdoor tests were similar. Species were grouped into host suitability categories according to Rf, and they were also grouped into resistance categories based on the sum of gall index, gall size, and percentage of the root system that was galled. Only A. salicina was a poor host and was resistant to M. javanica. Acacia farnisiana, A. gerrardii subsp. negevensis var. najdensis, and A. saligna were excellent hosts and highly susceptible. Both A. nilotica and A. stenophylla were classified as good hosts and highly susceptible, while A. ampliceps, A. ehrenbergiana, A. gerrardii subsp. negevensis var negevensis...
The relationship between ecological variation and microbial genetic composition is critical to understanding microbial influence on community and ecosystem function. In glasshouse trials using nine native legume species and forty rhizobial strains, we find that bacterial rRNA phylotype accounts for 68% of inter-isolate variability in symbiotic effectiveness and 79% of host-specificity in growth response. We also find that rhizobial phylotype diversity and composition of soils collected from a geographic breadth of sites explains the growth responses of two acacia species. Positive soil microbial feedback between the two acacia hosts was largely driven by changes in diversity of rhizobia. Greater rhizobial diversity accumulated in association with the less responsive host species, Acacia salicina, and negatively affected the growth of the more responsive A. stenophylla. Together this work demonstrates correspondence of phylotype with microbial function, and demonstrates that the dynamics of rhizobia on host-species can feed back on plant population performance.
This thesis explores the hydrological factors that may contribute to the observed distribution
patterns of invasive willows (Salix) and native trees (Eucalyptus camaldulensis, E. largiflorens and
Acacia stenophylla) along the Lower River Murray (LRM) in southern Australia. An initial survey,
establishing the diversity and flowering biology of Salix taxa was carried out to ascertain the extent
of invasion, and the likelihood of hybridisation, which may accelerate invasion. S. babylonica,
S. fragilis, S. × chrysochoma and S. × rubens occur in the study region, each represented by a single gender. None were present on floodplains, but the most dominant taxon, S. babylonica,
occurred along the entire length of the main channel. No seed or seedlings were observed; hence
reproduction is likely to be asexual.
More detailed survey work was then carried out to characterise the distribution patterns of the
dominant S. babylonica and co-occurring natives (Eucalyptus camaldulensis, E. largiflorens and
Acacia stenophylla) along a hydrologic gradient produced by the extensive weir system in the
LRM. In weir pools, variation in daily water levels of weir pools is low (± 0.1 m) immediately
upstream of the weir...