C3-plant and C4-plant

David Kirschtel kirschte at msu.edu
Sun Jul 29 14:24:20 EST 2001

Cereoid* wrote:
> Pure speculation.
> Its worthless without one being able to refer back to the actual experiment
> itself, if it really existed, or being able to duplicate it.
> We still do not know where was it published or which species were used.

Sorry Cereoid, we need none of the above. Einstein's greatest
contribution to science was the _Gedanken_Experiment_ ("Thought
Experiment") and that's sufficient for our purposes here. So, let's continue....

The problem is, paradoxically, O2 concentration not CO2 and is the
result of photorespiration. RUBISCO has an affinity for O2 as well as
CO2. If O2 is bound to RuBP then it is oxidized, with CO2 being lost
from the Calvin-Benson cycle. This can result in a 20-50% loss of fixed
C from the plant in "normal" conditions. 

This is considered to be the selective force for the evolution C4
photosynthesis. C4 plants have spatially separated the O2-producing
Light Reactions from the C-fixing Dark Reactions of photosynthesis and
thus minimized/reduced the C-fixing inefficiency of photorespiration. 

So, in a sealed container with (presumptively) high O2 concentrations
the C3 plant would starve to death for lack of carbon. Meanwhile, the C4
plant would be "cannibalizing" the CO2 being lost from the C3 plant as a
result of photorespiration.

Original Question:
> I´ve recently heard of following experiment:
> If you cultivat a C3-plant and a C4-plant together in an airtight
> glassy container, the C3-plant will die after a short period of time.
> Could anybody explain to me for what reason / reasons the C3-plant
> will die.

  David Kirschtel, Ph.D. * kirschte at pilot.msu.edu * 517.432.0898
    112 N Kedzie Lab * Mich State Univ * E Lansing, MI * 48824
 First Year Online/Biology http://lecture.lite.msu.edu/~bio/fyol/
         [after 17 Sept: Biology Program, Hitchcock Hall,
       Univ. of Washington, Seattle WA, 98150 206.543.9120]

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