Combine flour and salt in a bowl; cut in shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Add ice water, 1 tablespoon at a time, tossing with a fork until moist. Press mixture gently into a 4-inch circle on heavy-duty plastic wrap; cover with additional plastic wrap. Roll dough, still covered, into an 11-inch circle. Chill 5 minutes or until plastic wrap can be easily removed.
Remove plastic wrap, and fit dough into a 9-inch pie plate coated with cooking spray. Fold edges of dough under, and flute. Pierce bottom and sides of dough with a fork. Bake at 450° for 10 minutes or until lightly browned. Let cool completely on a wire rack. Reduce oven temperature to 325°.
Place egg yolk in a bowl; stir well, and set aside. Combine 1-1/4 cups sugar and cornstarch in a nonaluminum saucepan; stir well. Stir in 1 cup water and lemon juice; bring to a boil.Cook 1 minute or until thick, stirring constantly with a whisk. Gradually add hot cornstarch mixture to egg yolk, stirring constantly with whisk. Return mixture to pan. Cook over medium heat 2 minutes, stirring constantly. Remove from heat; stir in margarine and lemon rind. Spoon into pastry shell. Cover surface of filling with plastic wrap; set aside.
Beat egg whites and cream of tartar with clean, dry beaters at high speed of a mixer until foamy. Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaksform. Uncover pie, and spread meringue evenly over hot filling, sealing to edge of crust. Bake at 325° for 25 minutes or until meringue is golden. Let cool completely on a wire rack. Serve at room temperature. Store loosely covered in refrigerator.