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P Aphalo pedro at castle.ed.ac.uk
Fri Feb 11 01:58:07 EST 1994

Paul Conklin (psconk at acpub.duke.edu) wrote:
: In response to the question:

: >>Hi everyone, I may have accidently deleted the answer to this 
: >>question, but I am still wondering why a C12/C13 usage ratio for 
: >>RUBISCO would tell one about stomatal opening. Ellen 

: Pedro Aphalo wrote in part:

: >I do not remember seeing the original posting. I'll try to answer:

: >13C and 12C are both stable isotopes of C. The abundance of 13C is
: >about 1% of the total terrestrial C. 13CO2 is heavier than 12CO2
: >and this makes their diffusivities slightly different. Furthermore,
: >some enzimes like Rubisco show some "preference" for one of the
: >isotopes.

: >Delta 13C tells about stomatal opening in C3 plants, because the 
: >discrimination between 12C and 13C is different for Rubisco and
: >gaseous diffusion, and because 13C/12C is fairly constant in the
: >atmosphere. Consequently by measuring 13C/12C in bulk dry plant
: >tissue one can estimate Ci and water use efficiency throughout the
: >past life of a plant. Or in other words, how much the flux of CO2
: >was limited by diffusion.

: Having found this confusing myself, let me add a further note of 
: clarification.  If the stomates are wide open, diffusion is not 
: very limiting and the air in the leaf closely resembles the air 
: outside.  Rubisco then "chooses" between 12C and 13C and the 
: resulting tissue reflects Rubisco's "preference" only.  If the 
: stomates are closed, diffusion is quite limiting.  The air in the 
: leaf reflects the "preference ratio" of diffusion and looks 
: different from outside air.  Rubisco then "chooses" its CO2 from 
: this modified air.  The result is the combination of diffusion 
: AND Rubisco discrimination, which is different from what Rubisco 
: produces by itself.  In the ultimate limit of a plant using up 
: evey molecule of CO2 the stomates let in the resulting tissue 
: would show the diffusion ratio only. 

: Apologies if I've explained the obvious, but I don't deal with 
: this every day and find it useful to replay this thought 
: experiment each time I encounter this topic.

You have made it obvious with your explanation. Which is
quite a different thing! ;-)

: Paul Conklin
: psconk at acpub.duke.edu


 Pedro J. Aphalo

 Finnish Forest Research Institute
 Suonenjoki Research Station
 SF-77600 Suonenjoki, Finland

 Janet: Pedro.Aphalo at fi.metla
 World: Pedro.Aphalo at metla.fi

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