Inhibition of whole plant respiration by elevated CO 2 as modified by growth temperature

Plants of alfalfa ( Medicago sativa ) and orchard grass ( Dactylus glomerata ) were grown in controlled environment chambers at two CO 2 concentrations (350 and 700 μmol mol ‐1 ) and 4 constant day/night growth temperatures of 15, 20, 25 and 30°C for 50–90 days to determine changes in growth and whole plant CO 2 efflux (dark respiration). To facilitate comparisons with other studies, respiration data were expressed on the basis of leaf area, dry weight and protein. Growth at elevated CO 2 increased total plant biomass at all temperatures relative to ambient CO 2 , but the relative enhancement declined ( P ≤0.05) as temperature increased. Whole plant respiration (R d ) at elevated CO 2 declined at 15 and 20°C in D. glomerata on an area, weight or protein basis and in M. sativa on a weight or protein basis when compared to ambient CO 2 . Separation of R d into respiration required for growth (R g ) and maintenance (R m ) showed a significant effect of elevated CO 2 on both components. R m was reduced in both species but only at lower temperatures (15°C in M. sativa and 15 and 20°C in D. glomerata ). The effect on R m could not be accounted for by protein content in either species. R g was also reduced with elevated CO 2 ; however no particular effect of temperature was observed, i. e. R g was reduced at 20, 25 and 30°C in M. sativa and at 15 and 25°C in D. glomerata. For the two perennial species used in the present study, the data suggest that both R g and R m can be reduced by anticipated increases in atmospheric CO 2 ; however, CO 2 inhibition of total plant respiration may decline as a function of increasing temperature