The human needs for food, fiber, and raw materials rapidly are outstripping supply (10). Worldwide losses of crops from plant diseases, insect pests, and adverse weather conditions total billions of dollars annually. It is axiomatic that improvements in crop productivity can be made only through 2 basic approaches: 1) by modifying plant genotypes to fit their environment better; and 2) by changing the environment to ameliorate the effects of environmental stresses.