INHIBITION OF WHOLE-PLANT RESPIRATION BY ELEVATED CO2 AS MODIFIED BY GROWTH TEMPERATURE

Plants of alfalfa (Medicago sativa) and orchard grass (Dactylus glomer ata) were grown in controlled environment chambers at two CO2 concentr ations (350 and 700 mumol mol-1) and 4 constant day/night growth tempe ratures of 15, 20, 25 and 30-degrees-C for 50-90 days to determine cha nges in growth and whole plant CO2 efflux (dark respiration). To facil itate comparisons with other studies, respiration data were expressed on the basis of leaf area, dry weight and protein. Growth at elevated CO2 increased total plant biomass at all temperatures relative to ambi ent CO2, but the relative enhancement declined (P less-than-or-equal-t o 0.05) as temperature increased. Whole plant respiration (R(d)) at el evated CO2 declined at 15 and 20-degrees-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 CO2. Separation of R(d) into respiration req uired for growth (R(g)) and maintenance (R(m)) showed a significant ef fect of elevated CO2 on both components. R(m) was reduced in both spec ies but only at lower temperatures (15-degrees-C in M. sativa and 15 a nd 20-degrees-C in D. glomerata). The effect on R(m) could not be acco unted for by protein content in either species. R(g) was also reduced with elevated CO2; however no particular effect of temperature was obs erved, i.e. R(g) was reduced at 20, 25 and 30-degrees-C in M. sativa a nd at 15 and 25-degrees-C in D. glomerata. For the two perennial speci es used in the present study, the data suggest that both R(g) and R(m) can be reduced by anticipated increases in atmospheric CO2; however, CO2 inhibition of total plant respiration may decline as a function of increasing temperature.