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.
--
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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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