Acquisition and allocation of carbon and nitrogen by Danthonia richardsonii in response to restricted nitrogen supply and CO₂ enrichment

Dry weight (DW) and nitrogen (N) accumulation and allocation were measured in isolated plants of Danthonia richardsonii (Wallaby Grass) for 37 d following seed imbibition. Plants were grown at ≈ 365 or 735 μ L L –1 CO 2 with N supply of 0·05, 0·2 or 0·5 mg N plant –1 d –1 . Elevated CO 2 increased DW accumulation by 28% (low‐N) to 103% (high‐N), following an initial stimulation of relative growth rate. Net assimilation rate and leaf nitrogen productivity were increased by elevated CO 2 , while N concentration was reduced. N uptake per unit root surface area was unaffected by CO 2 enrichment. The ratio of leaf area to root surface area was decreased by CO 2 enrichment. Allometric analysis revealed a decrease in the shoot‐N to root‐N ratio at elevated CO 2 , while the shoot‐DW to root‐DW ratio was unchanged. Allometric analysis showed leaf area was reduced, while root surface area was unchanged by elevated CO 2 , indicating a down‐regulation of total plant capacity for carbon gain rather than a stimulation of mineral nutrient acquisition capacity. Overall, growth in elevated CO 2 resulted in changes in plant morphology and nitrogen use, other than those associated simply with changing plant size and non‐structural carbohydrate content.