In small mixing bowl sprinkle lemon juice over apples which have been cored and diced. Toss lightly to combine and add celery, raisins, and mayo. Mix until thoroughly coated. Cover with plaxtic wrap and refrigerate for at least 30 minutes. To serve, line 4 salad plates with lettuce leaf; top each with an equal amount of apple mixture and sprinkle each with 1 t. sunflower seeds.