Lightly spoon flours into dry measuring cups; level with a knife. Combine water and potato in a saucepan; bring to a boil. Cook for 15 minutes or until tender. Place mixture in a blender; process until smooth. Reserve 1 1/2 cups potato liquid; discard the remaining potato liquid. Cool the potato liquid to warm (100° to 110°). Combine 1 cup bread flour, potato liquid, and yeast in a large bowl. Cover and let stand at room temperature 1 hour.
Add whole-wheat flour, honey, oil, and salt to yeast mixture. Turn dough out onto a floured surface. Knead until smooth and elastic (about 5 minutes); add enough of remaining bread flour, 1 tablespoon at a time, to prevent the dough from sticking to hands (the dough will feel tacky). Place dough into a 9 x 5-inch loaf pan coated with cooking spray. Cover and let rise 30 minutes or until doubled in size. (Press two fingers into dough. If indentation remains, the dough has risen enough.)
Preheat oven to 375°.
Uncover dough; brush egg white over loaf. Sprinkle dough with pumpkin seed and sunflower-seed kernels. Bake at 375° for 30 minutes or until loaf sounds hollow when tapped. Remove from pan; cool on a wire rack.