Root growth and function of three Mojave Desert grasses in response to elevated atmospheric CO sub(2) concentration

Root growth and physiological responses to elevated CO sub(2) were investigated for three important Mojave Desert grasses: the C sub(3) perennial Achnatherum hymenoides, the C sub(4) perennial Pleuraphis rigida and the C sub(3) annual Bromus madritensis ssp. rubens. Seeds of each species were grown at ambient (360 mu l l super(-1)) or elevated (1000 mu l l super(-1)) CO sub(2) 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 C sub(3) grasses in response to elevated CO sub(2) (40 and 19% for A. hymenoides and B. rubens, respectively), but root biomass increased only in the C sub(3) perennial grass. Neither above-ground nor below-ground biomass of the C sub(4) perennial grass was significantly affected by the CO sub(2) treatment. Elevated CO sub(2) did not significantly affect root surface area for any species. Total plant nitrogen was also not statistically different between CO sub(2) treatments for any species, indicating no enhanced uptake of N under elevated CO sub(2). Physiological uptake capacities for NO sub(3) and NH sub(4) were not affected by the CO sub(2) 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 CO sub(2) for at least one N form in each species. NO sub(3) uptake rates were lower in A. hymenoides and P. rigida, and NH sub(4) uptake rates were lower in B. rubens at elevated CO sub(2). Nitrogen uptake on a whole root-system basis (NO sub(3)+NH sub(4) uptake capacity [mult] root biomass) was influenced positively by elevated CO sub(2) only for A. hymenoides after anthesis. These results suggest that elevated CO sub(2) may result in a competitive advantage for A. hymenoides relative to species that do not increase root-system N uptake capacity. Root respiration measurements normalized to 20 degree C were not significantly affected by the CO sub(2) treatment. However, specific root respiration was significantly correlated with either root C[ratio]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 CO sub(2) and indicate that root biomass r