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Plant species that fix CO 2 by the C 4 cycle have higher rates of CO 2 uptake than species using the C 3 photosynthetic carbon reduction cycle. Greater CO 2 fixation capacity has been associated with reduced photorespiration, specialized leaf anatomy, and biochemical pathways that differ in C 3 and C 4 plants. The higher photosynthesis rate of C 4 species also results in more dry matter production per unit of water transpired. This paper reviews published reports of productivity and N content of some C 4 and C 3 species. It hypothesizes that C 4 plants have a greater N use efficiency (biomass production per unit of N in the plant) than do C 3 plants. This difference presumably results from the relatively smaller investment of N in the photosynthetic carboxylation enzymes of C 4 plants than of C 3 plants. Some adaptive and evolutionary implications of such a hypothesis as well as limitations of supporting data are discussed.
R. H. Brown (Sun,) studied this question.