Preheat oven to 180 degrees C. Grease and flour a 20x30cm pan. Sift together the flour, baking powder and salt. Set aside.
In a large bowl, cream together the margarine and sugar until light and fluffy. Add the egg whites, eggs and vanilla and beat until smooth. Add the flour mixture alternately with the apple sauce and root bee.r Beat well, then spread batter into pan.
Bake in the preheated oven for 30 to 40 minutes, or until a toothpick inserted into the cake comes out clean. Allow to cool. Ice if desired.