1. Agronomy, Plant Physiology and Agroecology Division; International Rice Research Institute (IRRI); P.O. Box 3127, MCPO 1271 Makati City Philippines
2. Dep. of Agronomy; Univ. of Nebraska; Lincoln NE 68583 USA
3. Plant Breeding, Genetics, and Biochemistry Division, IRRI; Lincoln NE USA