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Growth of C₃ and C₄ Grasses Under Low N Levels¹

Plants with C₄ photosynthesis have higher photosynthetic and growth rates per unit of plant N than C₃ species. Most experiments in which nitrogen use efficiency (NUE) of C₃ and C₄ species have been compared have been short and conducted at high N levels. Four closely related tropical Panicum species, P. prionitis Griseb. (C₄), P. laxum Sw. (C₃), P. grumosum Nees (C₃) and P. spathellosum Doell (C₃/C₄), and a common forage, ‘Coastal’ bermudagrass Cynodon dactylon (L.) Pers. (C₄) were grown for 3 years in an artificial medium at low N levels in a greenhouse to determine growth and NUE. Growth rates were twice as high and the N concentration in live shoot tissue was lower for the C₄ than the C₃ or C₃/C₄ species. Concentration of N in roots at the end of the experiment was 4 g kg^–1 for the C₄ species compared to 6.1 to 8.3 g kg^–1 for the C₃ and C₃/C₄ species, and root weights for the C₄ species were twice or more than that of the other species. Dry matter produced per unit of N absorbed was twice as high in the C₄ as in the other species. It was concluded from this experiment that at very low soil N levels C₄ species are likely to produce more growth and be more competitive than C₃ species.

Key Words: Nitrogen use efficiency • Panicum species • Cynodon dactylon

² Professor of Agronomy.

Received for publication December 10, 1984.