Compensatory roles of nitrogen uptake and photosynthetic N-use efficiency in determining plant growth response to elevated CO2: Evaluation using a functional balance model

We used a modified functional balance (FB) model to predict growth response of Helianthus annus L. to elevated CO2. Model predictions were evaluated a gainst measurements obtained twice during the experiment. There was a good agreement between model predictions of relative growth rate (RGR) responses to elevated CO2 and observations, particularly at the second harvest. The model was then used to compare the relative effects of biomass allocation t o roots, nitrogen (N) uptake and photosynthetic N use efficiency (PNUE) in determining plant growth response to elevated CO2. The model predicted that a rather substantial increase in biomass allocation to root growth had lit tle effect on whole plant growth response to elevated CO2, suggesting that plasticity in root allocation is relatively unimportant in determining grow th response. Average N uptake rate at elevated compared to ambient CO2 was decreased by 21-29 %. In contrast, elevated CO2 increased PNUE by approx. 5 0 % due to a corresponding rise in the CO2-saturation factor for carboxylat ion at elevated CO2. The model predicted that the decreased N uptake rate a t elevated CO2 lowered RGR modestly, but this effect was counterbalanced by an increase in PNUE resulting in a positive CO2 effect on growth. Increase d PNUE may also explain why in many experiments elevated CO2 enhances bioma ss accumulation despite a significant drop in tissue nitrogen concentration . The formulation of the FB model as presented here successfully predicted plant growth responses to elevated CO2. It also proved effective in resolvi ng which plant properties had the greatest leverage on such responses. (C) 2000 Annals of Botany Company.