Influence of High CO2Partial Pressure on Nitrogen Use Efficiency of the C4Grasses Panicum coloratum and Cenchrus ciliaris

Australia's tropical grasslands are dominated by C 4 grasses, characterized by their unique biochemistry and anatomy. Two naturalized C 4 grasses ( Panicum coloratum and Cenchrus ciliaris ) were used to investigate whether high CO 2 partial pressure [ p (CO 2 )] influences photosynthetic nitrogen use efficiency and plant nitrogen use efficiency (PNUE and NUE respectively). Plants were grown for 30 d with four levels of N at p (CO 2 ) of 38 or 86 Pa. PNUE was calculated from leaf CO 2 assimilation rates ( A ) and leaf N concentrations, and NUE from total leaf N content and plant dry mass. At each p (CO 2 ), PNUE and NUE were greater for C. ciliaris than for P. coloratum due to higher A and dry mass combined with lower leaf N concentrations. Elevated p (CO 2 ) increased PNUE of C. ciliaris only. This effect was due to lower leaf N concentrations (area basis). At high p (CO 2 ), NUE of C. ciliaris was also greater. This resulted from a 1.6-fold stimulation of dry mass by high p (CO 2 ). Although dry mass of P. coloratum was increased 1.2-fold by elevated p (CO 2 ), its NUE was unaffected. Leaf transpiration rates were halved at elevated p (CO 2 ), and we suggest that this factor plays a major role in the growth response of C 4 grasses to high p (CO 2 ).