Nitrogen assimilation and growth of wheat under elevated carbon dioxide.

Simultaneous measurements of CO(2) and O(2) fluxes from wheat (Triticum aestivum) shoots indicated that short-term exposures to elevated CO(2) concentrations diverted photosynthetic reductant from NO(3)(-) or NO(2)(-) reduction to CO(2) fixation. With longer exposures to elevated CO(2), wheat leaves showed a diminished capacity for NO(3)(-) photoassimilation at any CO(2) concentration. Moreover, high bicarbonate levels impeded NO(2)(-) translocation into chloroplasts isolated from wheat or pea leaves. These results support the hypothesis that elevated CO(2) inhibits NO(3)(-) photoassimilation. Accordingly, when wheat plants received NO(3)(-) rather than NH(4)(+) as a nitrogen source, CO(2) enhancement of shoot growth halved and CO(2) inhibition of shoot protein doubled. This result will likely have major implications for the ability of wheat to use NO(3)(-) as a nitrogen source under elevated CO(2).