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

Australia's tropical grasslands are dominated by C₄grasses,characterized by their unique biochemistry and anatomy. Twonaturalized C₄grasses (Panicum coloratum and Cenchrus ciliaris)were used to investigate whether high CO₂partial pressure p(CO₂)] influences photosynthetic nitrogen use efficiency andplant nitrogen use efficiency (PNUE and NUE respectively). Plantswere grown for 30 d with four levels of N at p(CO₂) of 38 or86 Pa. PNUE was calculated from leaf CO₂assimilation rates (A)and leaf N concentrations, and NUE from total leaf N contentand plant dry mass. At each p(CO₂), PNUE and NUE were greaterfor C. ciliaris than for P. coloratum due to higher A and drymass combined with lower leaf N concentrations. Elevatedp (CO₂)increased PNUE of C. ciliaris only. This effect was due to lowerleaf N concentrations (area basis). At high p(CO₂), NUE of C.ciliaris was also greater. This resulted from a 1.6-fold stimulationof dry mass by high p(CO₂). Although dry mass of P. coloratumwas increased 1.2-fold by elevated p(CO₂), its NUE was unaffected.Leaf transpiration rates were halved at elevated p(CO₂), andwe suggest that this factor plays a major role in the growthresponse of C₄grasses to high p(CO₂). Copyright 2001 Annalsof Botany Company Panicum coloratum, Cenchrus ciliaris, nitrogen use efficiency, elevated CO₂, leaf N concentration, growth, photosynthesis