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A Difference in N Use Efficiency in C₃ and C₄ Plants and its Implications in Adaptation and Evolution¹

Plant species that fix CO₂ by the C₄ cycle have higher rates of CO₂ uptake than species using the C₃ photosynthetic carbon reduction cycle. Greater CO₂ fixation capacity has been associated with reduced photorespiration, specialized leaf anatomy, and biochemical pathways that differ in C₃ and C₄ plants. The higher photosynthesis rate of C₄ 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 C₄ and C₃ species. It hypothesizes that C₄ plants have a greater N use efficiency (biomass production per unit of N in the plant) than do C₃ plants. This difference presumably results from the relatively smaller investment of N in the photosynthetic carboxylation enzymes of C₄ plants than of C₃ plants. Some adaptive and evolutionary implications of such a hypothesis as well as limitations of supporting data are discussed.

Key Words: Photosynthesis • Leaf anatomy • RuDP carboxylase • PEP carboxylase • Soluble protein • Fraction I protein

² Professor of agronomy.

Received for publication May 20, 1977.