Interactive effects of growth-limiting N supply and elevated atmospheric CO2 concentration on growth and carbon balance of Plantago major

To assess the interactions between concentration of atmospheric CO2 and N supply, the response of Plantago major ssp. pleiosperma Pilger to a doubling of the ambient CO2 concentration of 350 mu l l(-1) was investigated in a range of exponential rates of N addition. The relative growth rate (RGR) as a function of the internal plant nitrogen concentration (Ni), was increased by elevated CO2 at optimal and intermediate N-i. The rate of photosynthesis, expressed per unit leaf area and plotted Versus N-i, was increased by 20-30% at elevated CO2 for N-i above 30 mg N g(-1) dry weight. However, the rate of photosynthesis, expressed on a leaf dry matter basis and plotted versus N-i, was not affected by the CO2 concentration. The allocation of dry matter between shoot and root was not affected by the CO2 concentration at any of the N addition rates. This is in good agreement with theoretical models, based on a balance between the rate of photosynthesis of the shoot and the acquisition of N by the roots. The concentration of total nonstructural carbohydrates (TNC) was increased at elevated CO2 and at N limitation, resulting in a shift in the partitioning of photosynthates from structural to nonstructural and, in terms of carbon balance, unproductive dry matter. The increase in concentration of TNC led to a decrease in both specific leaf area (SLA) and Ni at all levels of nutrient supply, and was the cause of the increased rate of photosynthesis per unit leaf area. Correction of the relationship between RGR and Ni for the accumulation of TNC made the effect of elevated CO2 on the relationship between RGR and Ni disappear. We conclude that the shift in the relationship between RGR and Ni was due to the accumulation of TNC and not due to differences in physiological variables such as photosynthesis and shoot and root respiration, changes in leaf morphology or allocation of dry matter.