Nitrogen deficiency precludes a growth response to CO₂ enrichment in C₃ and C₄ Panicum grasses

We investigated the interaction of nitrogen (N) supply and CO₂ enrichment on the growth and photosynthesis of Panicum laxum (C₃), P. coloratum (C₄) and P. antidotale (C₄). Plants were grown at ambient CO₂ partial pressures (p(a)) of either 36 (low) or 71 (high) Pa, in potted soil supplied with 0 (low) or 60 (high) mg N kg^-1 soil-week^-1. Elevated CO₂ enhanced total plant dry mass of all three species by approximately 28% under high N supply, but had no effect on biomass accumulation under N deficiency. CO₂ enrichment resulted in reductions of CO₂ assimilation rates (A; measured at comparable p(a)) of P. laxum, indicating acclimation of photosynthesis. This acclimation, which was more pronounced under N stress, was unrelated to changes in leaf N or non-structural carbohydrate concentrations, because neither were affected by CO₂ enrichment. In the C₄ grasses grown at low N, A were fully saturated at the current ambient P(a), whereas at high N, A increased slightly when CO₂ was raised to 71 Pa. N deficiency reduced the initial slope of the CO₂ response curve of A in P. antidotale, and this effect was more pronounced at high CO₂. In conclusion, the preclusion of a growth response to CO₂ enrichment by N deficiency was correlated with a strong inhibition of A in the C₃ species, and the saturation of A at below current atmospheric P(a) in C₄ species.