To prepare fruit, combine first 7 ingredients in a large bowl; toss gently. Cover and chill.
To prepare cake, coat a 12-cup tube pan with cooking spray, and dust with 2 tablespoons flour.
Lightly spoon remaining 3 cups flour into dry measuring cups; level with a knife. Combine flour, poppy seeds, baking soda, and salt in a medium bowl, stirring with a whisk. Place 1 3/4 cups sugar and butter in a large bowl; beat with a mixer at medium speed until well blended. Add eggs, 1 at a time, beating well after each addition. Combine buttermilk and 1/4 cup juice. Add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Spoon batter into prepared pan. Bake at 350° for 43 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack. Serve with fruit mixture.