Crop Science Journal of Natural Resources and Life Sciences Education
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Published in Crop Sci 23:1148-1153 (1983)
© 1983 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Nitrogen Response of Panicum Species Differing in CO2 Fixation Pathways. I. Growth Analysis and Carbohydrate Accumulation1

J. R. Wilson and R. H. Brown2

The apparent higher N use efficiency (NUE) of tropical than temperate grasses could be associated with the more efficient C4 cycle of CO2 fixation possessed by tropical grasses. This possible association between NUE and the CO2 fixation pathway was tested on five Panicum species with different pathways. P. maximum Jacq. (C4), P. prionitis Griseb. (C4), P. hylaeicum Mez (C3), P. laxum Sw. (C3), and also the intermediate (C3/C4) species, P. schenckii Hack. were grown at day/night temperatures of 35/30 and 24/ 19°C. Plants were grown initially in complete nutrient solutions and then in N-free solutions. Measurements of relative growth rate (Rw), leaf expansion, and leaf CO2 exchange rate (CER) made just before and during the N withholding period. Panicum maximum maintained a higher Rw, than the other species over a wide range of plant N status and had the highest maximum NUE of 6.1 kg of dry matter kg–lN day–1. Panicum prionitis did not show the same absolute advantage in Rw nor NUE over the C3 species. However, both C4 species generally showed a smaller relative decline in growth and CER at low N status than the C3 and C3/C4 species. Leaf area expansion was more sensitive to N stress than CER and there was some evidence that the C4 species were able to continue leaf development at lower leaf N concentrations than the C3 species. Growth temperature did not cause qualitative differences in the response to N, but NUE (kg kg–1 N day–1) was higher at 35/30°C than 24/19°C, except for P. schenckii. Morphological differences between species such as stem development also appeared to contribute to changes in Rw with development of N deficiency. It is concluded that the inherently higher efficiency of the C4 photosynthetic pathway does not necessarily lead to higher NUE of dry matter production of tropical grasses.

Key Words: C3, C4, C3/C4 grasses • Relative growth rate • Leaf expansion • Photosynthesis

1 Contribution of Dep. of Agronomy, Univ. of Georgia, Athens, GA 30602.

2 Plant physiologist, CSIRO, Div. of Tropical Crops and Pastures, Carmody Rd, St. Lucia, Old., Australia, 4067 and professor of agronomy, Univ. of Georgia, Athens, respectively.

Received for publication December 15, 1982.




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Copyright © 1983 by the Crop Science Society of America.