Soil and Plant Water Relations Determine Photosynthetic Responses of C3 and C4 Grasses in a Semi‐arid Ecosystem under Elevated CO2

摘要

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 (gs) at the treatment CO2 concentrations and at a range of intercellular CO2 concentrations and leaf water potentials (ψleaf) were measured over 4 years with variable soil water content caused by season and CO2 treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase (Vc,max), and ribulose bisphosphate regeneration capacity (Jmax) 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 ψ_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.