CO2 enrichment inhibits shoot nitrate assimilation in C3 but not C4 plants and slows growth under nitrate in C3 plants

The CO2 concentration in Earth's atmosphere may double during this century. Plant responses to such an increase depend strongly on their nitrogen status, but the reasons have been uncertain. Here, we assessed shoot nitrate assimilation into amino acids via the shift in shoot CO2 and O-2 fluxes when plants received nitrate instead of ammonium as a nitrogen source (Delta AQ). Shoot nitrate assimilation became negligible with increasing CO2 in a taxonomically diverse group of eight C-3 plant species, was relatively insensitive to CO2 in three C-4 species, and showed an intermediate sensitivity in two C-3-C-4 intermediate species. We then examined the influence of CO2 level and ammonium vs. nitrate nutrition on growth, assessed in terms of changes in fresh mass, of several C-3 species and a Crassulacean acid metabolism (CAM) species. Elevated CO2 (720 mu mol CO2/mol of all gases present) stimulated growth or had no effect in the five C-3 species tested when they received ammonium as a nitrogen source but inhibited growth or had no effect if they received nitrate. Under nitrate, two C-3 species grew faster at sub-ambient (similar to 310 mu mol/mol) than elevated CO2. A CAM species grew faster at ambient than elevated or sub-ambient CO2 under either ammonium or nitrate nutrition. This study establishes that CO2 enrichment inhibits shoot nitrate assimilation in a wide variety of C-3 plants and that this phenomenon can have a profound effect on their growth. This indicates that shoot nitrate assimilation provides an important contribution to the nitrate assimilation of an entire C-3 plant. Thus, rising CO2 and its effects on shoot nitrate assimilation may influence the distribution of C-3 plant species.