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

The C 4 cycle of CO 2 fixation may allow for more efficient use of N in CO 2 assimilation and dry matter production than the C 3 cycle. The association of high N use efficiency with C 4 photosynthesis was tested on six Panicum species which differ in CO 2 fixation pathway. Panicum maximum (C 4 ), P. prionitis (C 4 ), P. hylaeicum (C 3 ), P. laxum (C 3 ), P. milioides (C 3 /C 4 ), and P. schenckii (C 3 /C 4 ) 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exchange measurements were made at 35, 30, and 24°C. Plants were grown in complete nutrient solutions and then N was withdrawn and measurements of carbon dioxide exchange rates (CER) and transpiration were made just before and during the N depleted from the highest levels attained for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 C 3 and C 3 /C 4 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 C 4 species, P. maximum , exhibited the highest average CER/N value of 24 mmol CO 2 kg −1 N 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 C 4 species, P. prionitis , had the lowest CER/N values of 4.0 and 5.5 mmol CO 2 kg −1 N 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 C 4 than in C 3 species and that the advantage apparently conferred by the C 4 metabolism may be negated by morphological or other leaf characteristics.