koning at ECSUC.CTSTATEU.EDU
Tue Apr 22 13:30:16 EST 1997
At 2:00 PM -0400 4/22/97, Doug Jensen wrote:
> I have some personal confusion regarding the adaptiveness of CAM
>photosynthesis. It is usually portrayed as a pathway that decreases
>photorespiration, and, as such, it is presented an alternative to C-4
>photosynthesis. To decrease photorespiration, there should be a mechanism to
>decrease [oxygen] in the region of the chloroplasts. I don't see that. It
>appears to me that the primary purpose of CAM is water conservation.
> Am I missing something here?
I think you understand this pathway correctly!
The plant faces declining CO2 levels through the
day as the malate (or whatever) stored in the
vacuole is depleted. At the same time oxygen
levels are increasing as stomata are closed and
light reactions release it into the trapped gas
spaces. Reducing photorespiration might have been
a selective force for the evolution of C-4, but
I don't think the same is true of CAM.
Worse, to operate the C-4 pump at night, the CAM
plant must run respiration to supply the PEP.
This is like spinning your wheels...it may look
and maybe even sound impressive, but it is not
very efficient...not much forward momentum. Day
photosynthesis decreased by photorerspiration and
night respiration used to drive photosynthesis...
it isn't efficient at all.
This makes CAM plants among the SLOWEST growing
species on earth. On the other hand, the ability
to close the stomata in daytime and still run
photosynthesis while gaining carbon dioxide at
night when it is cool via open stomata permits
CAM plants to be VERY competitive in extremely
As far as I can think, the stomatal reversal is
the adaptive feature and C-4 photosynthesis provided
the survival mechanism to permit it. Thus, water
availability was the likely selective pressure for
the evolution of CAM from C-4 in my opinion.
Ross Koning | koning at ecsu.ctstateu.edu
Biology Department | http://koning.ecsu.ctstateu.edu/
Eastern CT State University | phone: 860-465-5327
Willimantic, CT 06226 USA | fax: 860-465-4479
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