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Abstract

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. [References: 47]