1. Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK.
2. Corresponding author. Phone: +44 (0) 1382 562731. Fax: +44 (0) 1382 562426. E-mail: jswans@scri.sari.ac.uk
3. Centre UdL-IRTA, Av. Alcalde Rovira Roure 177, 25198 Lleida, Spain.