1. Faculty of Agriculture, Food and Natural Resources, The University of Sydney, NSW, 2006. Australia.
2. Corresponding author. Phone: +61 2 8627 1018. Fax: +61 2 8627 1099. E-mail: meredith.wilkes@sydney.edu.au
3. Visiting student from Institut Polytechnique LaSalle Beauvais, Beauvais Cedex, France.