oxidation in C4

Jeffrey Kirby ez043438 at dilbert.ucdavis.edu
Sun Mar 15 19:45:43 EST 1998

Janice M. Glime (jmglime at MTU.EDU) wrote:

: If C4 plants benefit by being able to continue photosynthesis while 
: keeping their guard cells closed, what happens to the excess CO2 during
: that time?

Preliminary statement: I'm speculating without references. 

I assume by excess CO2 you mean CO2 that has entered the plant when its
stomata are open and that the plant has not yet fixed into a C4 compound. 
I believe it does one of two things: it either leaks out of the leaf
(closed stomata aren't perfect) or it goes into solution (as HCO3-) until
PEP carboxylase gets it.

:   My added question, can it be going to the closely aligned mitochondria,
: and if so, what prevents loss of photosynthate through photorespiration,
: especially at higher temperatures?

I'm not quite understanding this question. I think you mean the mito in
the mesophyll cells. Yes, CO2 can diffuse into the mito and nothing stops
it from diffusing out into the cytosol again.

I'm confused why the CO2 would go to the mito (other than by random
diffusion) and how this would increase photorespiration. Remember that
Rubisco is sequestered to the bundle sheath in C4 plants and PEP
carboxylase fixes CO2 in the mesophyll, the resulting C4 compounds are
transported to the BS, and the CO2 is rereleased. The resulting high [CO2]
in the BS favors CO2 fixation over photorespiration (use of O2 as a
substrate for Rubisco rather than CO2). 


-----------------------------                                      -----
        Jeffrey A. Kirby                         jakirby at ucdavis.edu


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