All available evidence suggests that C4 plants have evolved from ancestors possessing the C3 pathway of photosynthesis and this has occurred independently many times in taxonomically diverse groups (3,21). At present, the precise evolutionary transition, at the anatomical, physiological, and biochemical levels, from a C3 to a C4 plant is not clear. It is generally believed that studies of C3-C4 intermediate species might provide insight into the evolution of C4 photosynthesis. In addition, since most of the world's important crops are C3 plants, there has been considerable interest in improving their productivity by screening for mutants with reduced rates of photorespiration or by incorporating C4 characteristics into C3 plants (3,19,20). Thus, the search for naturally '