1. Weed Research Laboratory; Nanjing Agricultural University; Nanjing 210095 Jiangsu P.R. China
2. College of Life Science and Technology; Henan Institute Science and Technology; Xinxiang 453003 Henan P.R. China
3. Investigación y Desarrollo en Agricultura Tropical, S.A., Tambor; Alajuela 4050 Costa Rica