oxidation in C4

[Subbaiah C. Chalivendra]

>> 

>> 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?

>> 


To briefly answer the question,


C4 plants, like C3 plants, do not photosynthesize when their stomata
are closed. But unlike C3 plants, they have specialized anatomy and
biochemistry which allow them to concentrate atmospheric CO2 by a
O2-insensitive primary carboxylation step. The CO2 is released
enzymatically and refixed in a different cell layer using the same
enzymes as the C3 plants have. Because the CO2/O2 ratios are low in
cells <underline>where</underline> the secondary CO2 fixation occurs,
photorespiration is minimized. 


The plants that continue photosynthesis while keeping their guard cells
closed are termed as CAM (Crassulacean <bold>Acid</bold> Metabolism)
plants. Like C4 plants, these species also fix CO2 in 2 steps. But,
unike C4 plants, the CAM do not possess specialized anatomy and so the
carobxylation steps are not spatially separated. In stead, CAM plants
have a temporally separated pathway that allows them to store CO2
(acquired by a O2-insensitive primary carboxylation step from the
atmosphere during the night by keeping their stomata open) as
carboxylic <bold>acids in their vacuoles</bold>. The collection of
atmospheric CO2 in the cool and dark period allows them to keep their
stomata closed during the day time and minimize water loss. During the
day (or the light period), the fixed CO2 is released and refixed using
light energy and the same enzymes as the C3 plants do. Because the
CO2/O2 ratios are low in cells <underline>when</underline> the
secondary CO2 fixation occurs, photorespiration is minimized.




I am not sure if I made complete sense. More detailed information is
available in Salisbury and Ross and other Plant Physiology texts.


Subbaiah
Subbaiah Chalivendra

S-27 Turner Hall

Dept. Crop Sciences, 1102 S. Goodwin Ave

University of Illinois

Urbana, IL 61801-4730


Phone: 217 333-9743 or 

	   333-7585          


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