1. Dep. of Agronomy, Faculty of Agriculture at Kamphaeng Saen Kasetsart Univ., Kamphaeng Saen Campus Nakhon Pathom 73140 Thailand
2. Dep. of Applied Radiation and Isotopes, Faculty of Science Kasetsart Univ. Bangkok Thailand 10900
3. ETH Zurich, Institute of Agricultural Sciences Universitaetstrasse2 Zurich 8092 Switzerland