DIRECT AND INDIRECT INHIBITION OF SINGLE LEAF RESPIRATION BY ELEVATEDCO2 CONCENTRATIONS - INTERACTION WITH TEMPERATURE

Two herbaceous perennials, alfalfa (Medicago sativa L. cv. Arc) and or chard grass (Dactylus glomerata L. cv. Potomac), were grown at ambient (367 mumol mol-1) and elevated (729 mumol mol-1) CO2 concentrations a t constant temperatures of 15, 20, 25 and 30-degrees-C in order to exa mine direct and indirect changes in nighttime CO2 efflux rate (respira tion) of single leaves. Direct (biochemical) effects of CO2 on nightti me respiration were determined for each growth condition by brief (<30 min) exposure to each CO2 concentration. If no direct inhibition of r espiration was observed, then long-term reductions in CO2 efflux betwe en CO2 treatments were presumed to be due to indirect inhibition, prob ably related to long-term changes in leaf composition. By this criteri on, indirect effects of CO2 on leaf respiration were observed at 15 an d 20-degrees-C for M. sativa on a weight basis, but not on a leaf area or protein basis. Direct effects however, were observed at 15, 20 and 25-degrees-C in D. glomerata; therefore the observed reductions in re spiration for leaves grown and measured at elevated relative to ambien t CO2 concentrations could not be distinguished as indirect inhibition . No inhibition of respiration at elevated CO2 was observed at the hig hest growth temperature (30-degrees-C) in either species. CO2 efflux i ncreased with measurement and growth temperature for M. sativa at both CO2 concentrations; however, CO2 efflux in D. glomerata showed comple te acclimation to growth temperature. Stimulation of leaf area and wei ght by elevated CO2 levels declined with growth temperature in both sp ecies. Data from the present study suggest that both direct and indire ct inhibition of respiration are possible with future increases in atm ospheric CO2, and that the degree of each type of respiratory inhibiti on is a function of growth temperature.