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Pedro Aphalo Pedro.Aphalo at metla.fi
Thu Feb 10 07:00:48 EST 1994

> 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

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

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.

One can do more sophisticated analyses by extracting different organic
fractions from plant tissues: e.g. starch or sugars. As the turnover
of this compounds is relatively faster, one gets a 13C/12C measurement
integrated over a shorter time lapse.

Another posibility is to analyse wood from tree rings (in very long term
studies changes of the atmospheric composition become signifcant).

The theoretical basis of the method are well described by Farquhar et al. 
(Annual Review of Plant Physiology and Plant Molecular Biology, 40: 503-537,
1989). More practical aspects are covered by Ehleringer and Osmond (Stable 
isotopes. In: Plant Physiological Ecology. Field Methods and Instrumentation.
Eds. Pearcy et al. Chapman and Hall, London 1991).
I hope this is clear enough. Otherwise ask again, and I'll try to do better
next time. 8-)


 Pedro J. Aphalo

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

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

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