PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

The response of nitrate assimilation to elevated CO₂ in wheat shoots over a range of photon flux densities.

Searles, Peter¹, Smart, David¹, Bloom, Arnold¹, ¹

ABSTRACT- Carbon fixation by C₃ plants initially increases by approximately 30% when CO₂ concentration is doubled. This increase is often much less over longer periods due to a decline in overall protein content, and the resulting decrease in Rubisco and other Calvin cycle enzymes. One hypothesis to explain this phenomenon is that elevated CO₂ may inhibit the photoassimilation of nitrate. Our previous investigations have indicated competition for reductant between CO₂ and nitrate is involved in the inhibition of nitrate photoassimilation, and that the inhibition may be light dependent. We assessed this possibility by simultaneously monitoring fluxes of carbon dioxide, oxygen, and water vapor from intact wheat shoots (Triticum aestivum cv. veery 10) using a custom-designed gas exchange system. As expected, plants grown at 350 ppm CO₂ increased CO₂ uptake by 30% when exposed to 700 ppm CO₂. The assimilatory quotient (AQ), the ratio of CO₂ consumption to O₂ evolution, was greater in wheat shoots at 700 ppm CO₂ than at 350 ppm CO₂ under near-saturating photon flux densities. Little or no difference was seen in AQ at low photon flux densities. The AQ serves as a measure of nitrate photoassimilation with AQ increasing as nitrate photoassimilation decreases. The increase in AQ under elevated CO₂ indicates that increasing global atmospheric CO₂ concentrations may inhibit nitrate photoassimilation in C₃ plants.

KEY WORDS: elevated CO₂ , gas exchange, wheat, nitrate