1. Horticulture Research International, East Malling, West Malling, Kent ME19 6BJ, U.K.
2. School of Mathematical Sciences, Laver Building, University of Exeter, North Park Road, Exeter EX4 4QE, U.K.
3. DME/ESALQ, University of SãTo Paulo, 13418‐900 Piracicaba, SP, Brazil