1. Preheat oven to 350°.
2. Weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, xanthan gum, and next 3 ingredients; stir with a whisk.
3. Place granulated sugar and butter in a bowl; beat with a mixer at medium speed until blended. Beat in egg yolks. Beat in buttermilk, lemon rind, lemon juice, and canola oil until well blended. Gradually add flour mixture, beating at low speed until smooth.
4. Place egg whites in a small bowl; beat with a whisk until foamy. Add poppy seeds and egg whites to batter; beat at low speed just until blended.
5. Pour batter into a 9 x 5-inch loaf pan coated with cooking spray, and sprinkle with turbinado sugar. Bake at 350° for 50 minutes or until a wooden pick inserted in center comes out clean. Cool 15 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.