Root growth and function of three Mojave Desert grasses in response to elevated atmospheric CO2 concentration

Root growth and physiological responses to elevated CO₂ were investigated for three important Mojave Desert grasses: the C₃ perennial Achnatherum hymenoides, the C₄ perennial Pleuraphis rigida and the C₃ annual Bromus madritensis ssp. rubens. Seeds of each species were grown at ambient (360 μl l⁻¹) or elevated (1000 μl l⁻¹) CO₂ in a glasshouse and harvested at three phenological stages: vegetative, anthesis and seed fill. Because P. rigida did not flower during the course of this study, harvests for this species represent three vegetative stages. Primary productivity was increased in both C₃ grasses in response to elevated CO₂ (40 and 19% for A. hymenoides and B. rubens, respectively), but root biomass increased only in the C₃ perennial grass. Neither above-ground nor below-ground biomass of the C₄ perennial grass was significantly affected by the CO₂ treatment. Elevated CO₂ did not significantly affect root surface area for any species. Total plant nitrogen was also not statistically different between CO₂treatments for any species, indicating no enhanced uptake of N under elevated CO₂. Physiological uptake capacities for NO₃ and NH₄ were not affected by the CO₂ treatment during the second harvest; measurements were not made for the first harvest. However, at the third harvest uptake capacity was significantly decreased in response to elevated CO₂ for at least one N form in each species. NO₃ uptake rates were lower in A. hymenoides and P. rigida, and NH₄ uptake rates were lower in B. rubens at elevated CO₂. Nitrogen uptake on a whole root-system basis (NO₃+NH₄uptake capacity × root biomass) was influenced positively by elevated CO₂ only for A. hymenoidesafter anthesis. These results suggest that elevated CO₂ may result in a competitive advantage forA. hymenoides relative to species that do not increase root-system N uptake capacity. Root respiration measurements normalized to 20 °C were not significantly affected by the CO₂treatment. However, specific root respiration was significantly correlated with either root C∶N ratio or root water content when all data per species were included within a simple regression model. The results of this study provide little evidence for up-regulation of root physiology in response to elevated CO₂ and indicate that root biomass responses to CO₂ are species-specific.