1. H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8, 1TL, UK.
2. IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK.
3. Corresponding author. Phone (44) 1275 549330. Fax (44) 1275 394281 E-mail: arthur.tatham@bbsrc.ac.uk