1. Department of Agricultural Sciences, Imperial College at Wye, University of London, Wye, Kent TN25 5AH, UK
2. Department of Botany, Faculty of Chemistry and Biology, The Norwegian University of Science and Technology, N-7491 Trondheim, Norway
3. Horticulture Research International Wellesbourne, Wellesbourne, Warwickshire CV35 9EF, UK
4. Institute of Arable Crop Research Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK