ESTIMATING PHOTOSYNTHESIS AND CONCURRENT EXPORT RATES IN C-3 AND C-4 SPECIES AT AMBIENT AND ELEVATED CO2

The ability of 21 C-3 and C-4 monocot and dicot species to rapidly exp ort newly fixed C in the light at both ambient and enriched CO2 levels was compared. Photosynthesis and concurrent export rates were estimat ed during isotopic equilibrium of the transport sugars using a steady- state (CO2)-C-14-labeling procedure. At ambient CO2 photosynthesis and export rates for C-3 species were 5 to 15 and 1 to 10 mu mol C m(-2) s(-1), respectively, and 20 to 30 and 15 to 22 mu mol C m(-2) s(-1), r espectively, for C-4 species. A linear regression plot of export on ph otosynthesis rate of all species had a correlation coefficient of 0.87 . When concurrent export was expressed as a percentage of photosynthes is, several C-3 dicots that produced transport sugars other than Suc h ad high efflux rates relative to photosynthesis, comparable to those o f C-4 species. At high CO2 photosynthetic and export rates were only s lightly altered in C, species, and photosynthesis increased but export rates did not in all C(3)species. The C-3 species that had high efflu x rates relative to photosynthesis at ambient CO2 exported at rates co mparable to those of C-4 species on both an absolute basis and as a pe rcentage of photosynthesis. At ambient CO2 there were strong linear re lationships between photosynthesis, sugar synthesis, and concurrent ex port. However, at high CO2 the relationships between photosynthesis an d export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.