1. a Plant Biology and Biogeochemistry Department , Riso National Laboratory , PBK‐124, P.O. Box 49, Roskilde , DK‐4000 , Denmark
2. b Plant Biology and Biogeochemistry Department , Riso National Laboratory , PBK‐124, P.O. Box 49, Roskilde , DK‐4000 , Denmark Phone: Fax: E-mail:
3. d Landskontoret for Planteavl , Udkærsvej 15, Aarhus N , DK‐8200 , Denmark