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Rice (Oryza sativa) and wheat (Triticum aestivum) are the two most commercially important crops, accounting for more than 40% of global food production. They were domesticated in different climates and differ largely in their growth environments: Rice is tropically cultivated in hot, wet climates, whereas wheat tends to be grown in cooler temperate climates. However, both crops have been bred in similar directions. For example, the introduction of semidwarf traits into rice and wheat from Chinese and Japanese varieties in the 1960s made a great contribution to increasing yield in both species (Evans, 1997). Since the semidwarf cultivars can use large inputs of nitrogen (N) fertilizer without lodging, the introduction of dwarfing genes allowed the production of varieties with high leaf N content and enhanced sink capacity. Large inputs of N fertilizer in turn have drawn much attention to the environmental impact of N …