1. United States Department of Agriculture, Agricultural Research Service, Plant Science Research Unit, 3127 Ligon Street, Raleigh, NC 27607, USA;
2. Crop Science Department, North Carolina State University, 3127 Ligon Street, Raleigh, NC 27607, USA,
3. Department of Crop and Soil Science, Oregon State University, 109 Crop Science Building, Corvallis, OR 97331‐3002, USA;
4. Department of Biology, Box 90338, Duke University, Durham, NC 27708, USA