c12/C13

[Paul Conklin]

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.

Paul Conklin
psconk at acpub.duke.edu