Wash artichokes by plunging up and down in cold water. Cut off stem ends, and trim about 1/2 inch from top of each artichoke. Remove any loose bottom leaves. With scissors, trim away about one-fourth of each outer leaf. Brush top and edges of leaves with lemon juice to prevent discoloration.
Place artichokes in a large Dutch oven; add water to depth of 1 inch. Bring to a boil; cover, reduce heat, and simmer 25 minutes or until artichokes are almost tender. Spread leaves apart; scrape out the fuzzy thistle center (choke) with a spoon, and discard thistle. Set artichokes aside.
Coat a medium saucepan with cooking spray; add oil. Place over medium-high heat until hot. Add onion and celery; saute until tender. Add 1 cup water, rice, tomato, and 1 teaspoon Italian seasoning. Bring to a boil; cover, reduce heat, and simmer 20 minutes or until rice is tender and liquid is absorbed. Stir in olives.
Spoon rice mixture into artichoke cavities. Combine yogurt and flour. Spoon over rice mixture. Sprinkle with cheese and remaining 1/2 teaspoon Italian seasoning. Arrange artichokes in a 13- x 9- x2-inch baking dish. Cover and bake at 350° for 20 minutes. Bake, uncovered, 5 additional minutes or until thoroughly heated.