Plants are not dumb: The role of photorespiration and respiration in plant assimilation of nitrate to protein

Background/Question/Methods

Plant nitrogen concentrations generally decline under elevated CO₂ atmospheres. The presenter has proposed that CO₂ inhibits nitrate assimilation into protein in the shoots of C₃ plants. Several independent meta-analyses have now concluded that this is the explanation most consistent with observations from hundreds of studies.

This presentation will show the results of experiments on Arabidopsis and wheat grown in controlled environmental chambers. Ten independent methods examined the influence of CO₂ on nitrate assimilation. These methods included shoot CO₂ and O₂ gas exchange, chlorophyll fluorescence, stable isotope fractionation, isotope labelling, nitrate depletion from the medium, plant nitrate and organic nitrogen accumulation, genotypes with altered nitrate reductase capacity, and growth analysis.

Results/Conclusions

All ten methods affirmed that CO₂ inhibited nitrate assimilation in Arabidopsis and wheat during the day and night. Several physiological mechanisms were responsible for this inhibition. (1) Photorespiration, contrary to popular opinion, is not a wasteful process; rather it can empower the first step of nitrate assimilation. (2) Bicarbonate interferes with nitrite transport into chloroplasts. These mechanisms have broad implications:

(1)    C₃ carbon fixation is far more efficient than previously thought, and this may explain why C₃ species still comprise about 96% of the world’s flora despite 23 million years of atmospheric CO₂ concentrations below 400 ppm.

(2)    Plant responses to rising CO₂ vary as a result of the differences in the reliance of plants on ammonium and nitrate as nitrogen sources.

(3)    Declining protein concentrations in plants that are reliant on nitrate are responsible for increasing pest damage as herbivores consume more plant material to meet their nutritional needs.