Will rising atmospheric CO₂ concentration inhibit nitrate assimilation in shoots but enhance it in roots of C₃ plants?

Bloom et al. proposed that rising atmospheric CO ₂ concentrations ‘inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein of C ₃ plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades’. Previously we argued that the weight of evidence in the literature indicated that elevated atmospheric [CO ₂] does not inhibit NO ₃ ⁻ assimilation in C ₃ plants. New data for common bean (Phaseolus vulgaris) and wheat (Triticum aestivum) were presented that supported this view and indicated that the effects of elevated atmospheric [CO ₂] on nitrogen (N) assimilation and growth of C ₃ vascular plants were similar regardless of the form of N assimilated. Bloom et al. strongly criticised the arguments presented in Andrews et al. Here we respond to these criticisms and again conclude that the available data indicate that elevated atmospheric [CO ₂] does not inhibit NO ₃ ⁻ assimilation of C ₃ plants. Measurement of the partitioning of NO ₃ ⁻ assimilation between root and shoot of C ₃ species under different NO ₃ ⁻ supply, at ambient and elevated CO ₂ would determine if their NO ₃ ⁻ assimilation is inhibited in shoots but enhanced in roots at elevated atmospheric CO ₂.