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We have proposed that rising atmospheric CO₂concentrations inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein in shoots of C₃plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades. Although hundreds of studies support this proposal, several publications in 2018 and 2019 purport to present counterevidence. The following study evaluates these publications as well as presents new data that elevated CO₂enhances root nitrate assimilation in wheat andArabidopsiswhile it inhibits shoot nitrate assimilation. 

Most plants, contrary to popular belief, do not waste over 30% of their photosynthate in a futile cycle called photorespiration. Rather, the photorespiratory pathway generates additional malate in the chloroplast that empowers many energy-intensive chemical reactions, such as those involved in nitrate assimilation. Thus, the balance between carbon fixation and photorespiration determines the plant carbon–nitrogen balance and protein concentrations. Plant protein concentrations, in turn, depend not only on the relative concentrations of carbon dioxide and oxygen in the chloroplast but also on the relative activities of magnesium and manganese, which are metals that associate with several key enzymes in the photorespiratory pathway and alter their function. Understanding the regulation of these processes is critical for sustaining food quality under rising CO2 atmospheres. 

Critical for predicting the future of primary productivity is a better understanding of plant responses to rising atmospheric carbon dioxide (CO2) concentration. This review considers recent results on the role of the inorganic nitrogen (N) forms nitrate (NO3–) and ammonium (NH4+) in determining the responses of wheat and Arabidopsis to elevated atmospheric CO2 concentration. Here, we identify four key issues: (i) the possibility that different plant species respond similarly to elevated CO2 if one accounts for the N form that they are using; (ii) the major influence that plant–soil N interactions have on plant responses to elevated CO2; (iii) the observation that elevated CO2 may favor the uptake of one N form over others; and (iv) the finding that plants receiving NH4+ nutrition respond more positively to elevated CO2 than those receiving NO3– nutrition because elevated CO2 inhibits the assimilation of NO3– in shoots of C3 plants. We conclude that the form and amount of N available to plants from the rhizosphere and plant preferences for the different N forms are essential for predicting plant responses to elevated CO2.