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 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 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 by 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 C-4 grasses to high p(CO2). (C) 2001 Annals of Botany Company.