1. Department of Crop Science, North Carolina State University, Box 7629, Raleigh, North Carolina 27695-7629, Agricultural Research Servive, U.S. Department of Agriculture, Department of Botany, North Carolina State University, Box 7625, Raleigh, North Carolina 27695-7625, and Department of Statistics, North Carolina State University, Box 8203, Raleigh, North Carolina 27695-8203