1. Department of Plant Sciences; Norwegian University of Life Sciences; PO Box 5003, NO-1432 Ås Norway
2. Nofima AS; PO Box 210, NO-1431 Ås Norway
3. School of Agriculture Policy & Development; Reading University; PO Box 237, Whiteknights Reading, RG6 7BE UK
4. Rothamsted Research; Harpenden, AL5 2JQ UK