Heat the margarine in a large saucepan and add the scallions and cucumber. Cook for about 2 - 3 minutes without coloring. Add the rice, broth and wine and return to a boil. Simmer the wine and broth mixture for about 10 minutes, stirring occasionally. Stir in the diced salmon and tarragon. Continue cooking for 5 minutes more, then switch off the heat. Cover and leave to stand for 5 minutes before serving.