REVERSIBILITY OF PHOTOSYNTHETIC ACCLIMATION OF SWISS-CHARD AND SUGAR-BEET GROWN AT ELEVATED CONCENTRATIONS OF CO2

Although leaf photosynthesis and plant growth are initially stimulated by elevated CO2 concentrations, increasing insensitivity to CO2 (accl imation) is a frequent occurrence. In order to examine the acclimation process, we studied photosynthesis and whole plant development in swi ss chard (Beta vulgaris L. Koch ssp. ciela) and sugarbeet (Beta vulgar is L. ssp. vulgaris) grown at either ambient or twice ambient concentr ations of CO2. In an initial controlled environment study, photosynthe tic acclimation to elevated CO2 levels was observed in both subspecies 24 days after sowing (DAS) but was not observed at 42 and 49 DAS for sugarbeet or at 49 DAS for swiss chard. Although sugarbeet and swiss c hard differed in root size and morphology, this was not a factor in th e onset of photosynthetic acclimation. The reversal of photosynthetic acclimation that was observed in older plants grown at elevated CO2 co ncentrations was associated with a rapid increase in root development (i.e. increased root: shoot [R/S] ratio), increased sucrose levels in sinks (roots) and no differences in total soluble leaf protein of eith er subspecies relative to the ambient CO2 condition. In a second set o f experiments, swiss chard and sugarbeet were grown in outdoor Plexigl ass chambers at different times of the year (i.e. summer and early fal l). Average 24-h temperature was 30.7 and 19.4 degrees C for the summe r and fall plantings, respectively. In agreement with the controlled e nvironment study, lack of photosynthetic acclimation, determined from the response of photosynthesic rate to internal CO2 concentration, was correlated with increased root biomass and sucrose concentration rela tive to the ambient condition. However, photosynthetic acclimation was observed depending on the season, i.e. summer (swiss chard) or fall ( sugarbeet), suggesting that acclimation was affected by environmental factors, such as temperature. Data from both experiments suggest that continued long-term photosynthetic stimulation may be dependent upon t he ability of increased CO2 to stimulate new sink development which wo uld allow full utilization of the additional carbon made available in a high CO2 environment.