Growth and Photosynthetic Carbon Metabolism in Tobacco Plants under an Oscillating CO2 Concentration in the Atmosphere

Abstract: The hypothesis for the present work was that photosynthetic acclimation to increased atmospheric CO 2 in Nicotiana tabacum could be prevented by an oscillating supply of CO 2 . This was tested by growing half of the plants (for the 20 day period after sowing) at 700 μmol mol ‐1 CO 2 (S+ plants) and half at 350 μmol mol ‐1 CO 2 (S‐ plants) and thereafter switching them every 48 h from high to low CO 2 and vice versa. These plants were compared with plants continuously kept (from sowing onwards) at 350 μmol mol ‐1 CO 2 (C‐ plants) and 700 μmol mol ‐1 CO 2 (C+ plants). Switching plants from high to low CO 2 and vice versa (S+ and S‐) did not improve plant growth efficiency, as hypothesized. The extra carbon fixed by the leaves under increased CO 2 in the atmosphere, supplied either continuously or intermittently, was mostly stored as starch and not used to build additional structural biomass. The differences in final plant biomass, observed between S+ and S‐ plants, are explained by the CO 2 concentration in the atmosphere during the first 20 days after sowing, the oscillation in CO 2 supply thereafter is playing a smaller role in this response. Switching plants from high to low CO 2 and vice versa, also did not prevent down‐regulation of photosynthesis, despite lower leaf sugar concentrations than in C+ plants. Nitrate concentration decreased dramatically in C+, S+ and S‐ plants. The leaf C/N ratio was highest in C+ plants (ranging from 8 to 13), intermediate in S+ and S‐ plants (from 7 to 11) and lowest in C‐ plants (from 6 to 8). This supports the view that the balance between carbohydrates and nitrogen may have a triggering role in plant response under elevated CO 2 . Carbon export rates by the leaves seem to be independent of total carbon assimilation, suggesting a sink limiting effect on tobacco growth and phototsynthesis under elevated CO 2 .