Annals of Botany

To model the effect of increasing atmospheric CO₂ on semi-arid grasslands, the gas exchange responses of leaves to seasonal changes in soil water, and how they are modified by CO₂, must be understood for C₃ and C₄ species that grow in the same area. In this study, open-top chambers were used to investigate the photosynthetic and stomatal responses of Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) grown at 360 (ambient CO₂) and 720 µmol mol^-1 CO₂ (elevated CO₂) in a semi-arid shortgrass steppe. Assimilation rate (A) and stomatal conductance (g_s) at the treatment CO₂ concentrations and at a range of intercellular CO₂ concentrations and leaf water potentials (_leaf) were measured over 4 years with variable soil water content caused by season and CO₂ treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase (V_c,max), and ribulose bisphosphate regeneration capacity (J_max) were reduced in P. smithii grown in elevated CO₂, to the degree that A was similar in elevated and ambient CO₂ (when soil moisture was adequate). Photosynthetic capacity was not reduced in B. gracilis under elevated CO₂, but A was nearly saturated at ambient CO₂. There were no stomatal adaptations independent of photosynthetic acclimation. Although photosynthetic capacity was reduced in P. smithii growing in elevated CO₂, reduced g_s and transpiration improved soil water content and _leaf in the elevated CO₂ chambers, thereby improving A of both species during dry periods. These results suggest that photosynthetic responses of C₃ and C₄ grasses in this semi-arid ecosystem will be driven primarily by the effect of elevated CO₂ on plant and soil water relations.