Drain pineapple, reserving 1 tablespoon juice (discard remaining juice or save for another use); set pineapple aside. In a small bowl, combine sour cream, mayonnaise, sugar, lemon juice and reserved pineapple juice. In a large bowl, combine the rice, apples, celery and pineapple. Add the dressing and toss to coat. Cover and refrigerate for 2 hours. Just before serving, stir in walnuts. Serve in a lettuce-lined bowl if desired.