Inorganic nitrogen form: a major player in wheat and Arabidopsis responses to elevated CO2

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 C-3 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.