1. Institute of Food Research, Norwich Laboratory, Norwich Research Park, Colney, Norwich, NR4 7UA, UK.
2. Corresponding author. Phone: +44(0) 1603 255000. Fax: +44(0)1603 507723. E-mail: alex.grant@bbsrc.ac.uk
3. Gist Brocades N.V., Research and Development, Wateringseweg 1, PO Box 1, 2600 MA, Delft, The Netherlands.
4. Department of Agricultural Sciences, University of Bristol, Institute of Arable Crops Research, Long Ashton Research Station, Bristol, BS18 9AF, UK.