Nitrogen assimilation and growth of wheat under elevated carbon dioxide

Simultaneous measurements of CO2 and O-2 fluxes from wheat (Triticum, aestivum) shoots indicated that short-term exposures to elevated CO2 concentrations diverted photosynthetic reductant from NO3- or NO2- reduction to CO2 fixation. With longer exposures to elevated CO2, wheat leaves showed a diminished capacity for NO3- photoassimilation at any CO2 concentration. Moreover, high bicarbonate levels impeded NO2- translocation into chloroplasts isolated from wheat or pea leaves. These results support the hypothesis that elevated CO2 inhibits NO3- photoassimilation. Accordingly, when wheat plants received NO3- rather than NH4+ as a nitrogen source, CO2 enhancement of shoot growth halved and CO2 inhibition of shoot protein doubled. This result will likely have major implications for the ability of wheat to use NO3- as a nitrogen source under elevated CO2-.