1. Plant Research International, P.O. Box 16, NL–6700 AA Wageningen, The Netherlands (L.H.S., G.M.S., J.W.M., H.A.C.B., D.B., W.J.); and
2. State Institute for Quality Control of Agricultural Products (RIKILT), P.O. Box 230, NL–6700 AE Wageningen, The Netherlands (I.J.W.E., A.L.)