Place egg whites in a small mixing bowl; let stand at room temperature for 30 minutes. Meanwhile, in a large saucepan, combine the pudding mix, milk and sour cream until smooth. Cook and stir over medium heat until thickened and bubbly, about 5 minutes. Reduce heat; cook and stir 2 minutes longer.
Remove from the heat. Gradually whisk 1 cup hot filling into egg yolks; return all to the pan. Bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat. Gently stir in lemonade concentrate; keep warm.
Add lemon juice and cream of tartar to egg whites; beat on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form and sugar is dissolved.
Pour warm filling into pastry shell. Spread meringue over filling, sealing edges to pastry.
Bake at 350° for 15-20 minutes or until meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving. Yield: 6-8 servings.