Preheat oven to 350 degrees F. Coat a 12-cup Bundt pan, preferably nonstick, with cooking spray and dust with flour (or use cooking spray with flour).
Toast poppy seeds in a small dry skillet over medium heat, stirring constantly, until fragrant, 3 to 4 minutes. Transfer to a plate to cool.
Whisk whole-wheat flour, all-purpose flour, poppy seeds, baking powder, baking soda and salt in a medium bowl. Combine buttermilk, oil, vanilla, lemon zest and lemon juice in a glass measuring cup.
Beat eggs, egg whites and sugar in large bowl with an electric mixer on high speed until thickened and pale, about 5 minutes.
Fold the dry ingredients into the egg mixture with a rubber spatula, a third at a time, alternating with 2 additions of the buttermilk mixture. Scrape the batter into the prepared pan, spreading evenly.
Bake the cake until the top springs back when touched lightly and a toothpick inserted in the center comes out clean, 35 to 40 minutes. Let cool in the pan for 5 minutes, then turn out onto a wire rack.
To prepare the glaze: Sift 3/4 cup confectioner's sugar into a small bowl; mix with lemon juice and water to create a thin glaze. Poke 1-inch deep holes all over the cake with a skewer. Coat the warm cake with glaze using a pastry brush. Let cook completely. To serve, set the cake on a serving plate and dust with confectioner's sugar.