Growth and photosynthetic carbon metabolism in tobacco plants under an oscillating CO2 concentration in the atmosphere

The hypothesis for the present work was that photosynthetic acclimation to increased atmospheric CO2 in Nicotiana tabacum could be prevented by an oscillating supply of CO2. This was tested by growing half of the plants (for the 20 day period after sowing) at 700 mu mol mol(-1) CO2 (S+ plants) and half at 350 mu mol mol(-1) CO2 (S- plants) and thereafter switching them every 48 h from high to low CO2 and vice versa. These plants were compared with plants continuously kept (from sowing onwards) at 350 mu mol mol(-1) CO2 (C- plants) and 700 mu mol mol(-1) CO2 (C+ plants). Switching plants from high to low CO2 and vice versa (S+ and S-) did not improve plant growth efficiency, as hypothesized. The extra carbon fixed by the leaves under increased CO2 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 CO2 concentration in the atmosphere during the first 20 days after sowing, the oscillation in CO2 supply thereafter is playing a smaller role in this response. Switching plants from high to low CO2 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 CO2. 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 CO2.