Nitrogen Response of Panicum Species Differing in CO2 Fixation Pathways. II. CO2 Exchange Characteristics1

The C4 cycle of CO2 fixation may allow for more efficient use of N in CO2 assimilation and dry matter production than the C3 cycle. The association of high N use efficiency with C4 photosynthesis was tested on six Panicum species which differ in CO2 fixation pathway. Panicum maximum (C4), P. prionitis (C4), P. hylaeicum (C3), P. laxum (C3), P. milioides (C3/C4), and P. schenckii (C3/C4) were grown at day/night temperatures of 35/30 and 24/ 19°C and gas exchange measurements were made at 35, 30, and 24°C. Plants were grown in complete nutrient solutions and then N was withdrawn and measurements of carbon dioxide exchange rates (CER) and transpiration were made just before and during the N depleted from the highest levels attained for all the C3 and C3/C4 species. For P. maximum and P. prionitis the CER response was curvilinear with optimum leaf N levels of about 30 to 35 g kg−1, except for P. maximum grown at 24/19°C in which an optimum was not reached. The C4 species, P. maximum, exhibited the highest average CER/N value of 24 mmol CO2 kg−1N s−1 for plants grown at either 24/19 or 30/35°C. For all other species CER/N was lower than for P. maximum, and CER/ N was lower in plants grown at 24/19 than at 35/30°C. The lower CER/N at 24/19°C was associated with higher specific leaf weights. The C4 species, P. prionitis, had the lowest CER/N values of 4.0 and 5.5 mmol CO2 kg−1N s−1 for plants grown at 24/ 19 and 35/30°C, respectively; due perhaps to its very thick leaves with specific leaf weights three to five times higher than the other species. Deficiency of N decreased both leaf conductance and mesophyll conductance in all species, but for plants grown at 35/30°C reduction of mesophyll conductance was much greater than leaf conductance. It is concluded that N use efficiency is not necessarily higher in C4 than in C3 species and that the advantage apparently conferred by the C4 metabolism may be negated by morphological or other leaf characteristics.