Combine 1 1/2 cups sugar, flour, egg yolks, plum pulp, and water in a medium-size non-aluminum saucepan. Cook over medium heat, stirring constantly, until thickened and bubbly. Pour into pastry shell.
Beat egg whites (at room temperature) and cream of tartar until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Spread meringue over filling, sealing to edge of pastry. Bake at 350° for 12 minutes or until meringue is golden brown.