Influence of high CO2 partial pressure on nitrogen use efficiency of the C4 grasses Panicum coloratum and Cenchrus ciliaris

Australia's tropical grasslands are dominated by C4 grasses, characterized by their unique biochemistry and anatomy. Two naturalized C4 grasses (Panicum coloratum and Cenchrus ciliaris) were used to investigate whether high CO2 partial pressure [p(CO2)] 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(CO2) of 38 or 86 Pa. PNUE was calculated from leaf CO2 assimilation rates (A) and leaf N concentrations, and NUE from total leaf N content and plant dry mass. At each p(CO2), 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(CO2) increased PNUE of C. ciliaris only. This effect was due to lower leaf N concentrations (area basis). At high p(CO2), NUE of C. ciliaris was also greater. This resulted from a 1.6-fold stimulation of dry mass by high p(CO2). Although dry mass of P. coloratum was increased 1.2-fold b elevated p(CO2), its NUE was unaffected. Leaf transpiration rates were halved at elevated p(CO2), and we suggest that this factor plays a major role in the growth response of C4 grasses to high p(CO2).