Estimating Photosynthesis and Concurrent Export Rates in C₃ and C₄ Species at Ambient and Elevated CO₂^1,2

Bernard Grodzinski^*, Jirong Jiao, and Evangelos D. Leonardos

Department of Plant Agriculture, University of Guelph, Ontario, Canada N1G 2W1

The ability of 21 C₃ and C₄ monocot and dicot species to rapidly export newly fixed C in the light at both ambient and enriched CO₂ levels was compared. Photosynthesis and concurrent export rates were estimated during isotopic equilibrium of the transport sugars using a steady-state ¹⁴CO₂-labeling procedure. At ambient CO₂ photosynthesis and export rates for C₃ species were 5 to 15 and 1 to 10 µmol C m² s¹, respectively, and 20 to 30 and 15 to 22 µmol C m² s¹, respectively, for C₄ species. A linear regression plot of export on photosynthesis rate of all species had a correlation coefficient of 0.87. When concurrent export was expressed as a percentage of photosynthesis, several C₃ dicots that produced transport sugars other than Suc had high efflux rates relative to photosynthesis, comparable to those of C₄ species. At high CO₂ photosynthetic and export rates were only slightly altered in C₄ species, and photosynthesis increased but export rates did not in all C₃ species. The C₃ species that had high efflux rates relative to photosynthesis at ambient CO₂ exported at rates comparable to those of C₄ species on both an absolute basis and as a percentage of photosynthesis. At ambient CO₂ there were strong linear relationships between photosynthesis, sugar synthesis, and concurrent export. However, at high CO₂ the relationships between photosynthesis and export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.

Plant Physiol. (1998) 117: 207-215
Copyright Clearance Center:   0032-0889/98/117/0207/09
© 1998 American Society of Plant Physiologists

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