1. Department of Horticulture, Danish Institute of Agricultural Sciences, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark; Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark; Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark; and Polymer Chemistry, Graduate School of Bioagriculture Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan