1. Instituto de Recursos Geneticos y ProductividadColegio de PostgraduadosMéxico56230México
2. CSIRODivision of Plant Industry and Cooperative Research Centre for Plant ScienceP.O. Box 475 1600Canberra CityA.C.T.2601Australia
3. Research School of Biological SciencesAustralian National Univ. and Cooperative Research Centre for Plant ScienceP.O. Box 475Canberra CityA.C.T.2601Australia