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CO2 and Photosynthesis
From: ac554 at freenet_carleton.ca (David Whittaker) Date: Sat, 10 Jun
1995 04:43:36 -0400
Charlie Bay said:
>>>I had thought that it would also be a good way to do CO2 injection
>>>right into the substrate, but I now think that if I keep the
>>>flow-rate low (like I think it should be), I won't be able to get
>>>enough PPM CO2 injected into the substrate.
Bob Hoesch said:
>>..... CO2 is used only by the leaves, and then only when the
>>lights are on. Photosynthesis is the process whereby the carbon
>>in carbon dioxide is .......(blah blah)
David Whittaker said:
>I'm not exactly sure what your point is Bob. Are you implying that CO2
>is not taken up by the root system? My understanding is that plants
>have at least three strategies for obtaining CO2. They use free
>dissolved CO2 in the surrounding waters, they make use of atmospheric
>CO2 when possible, and they obtain it through the root system in the
>substrate. There, concentrations can be up to 100 times that of the
>water. The carbon is stored as malate (malic acid) during both the day
>and night, and converted during the day for use in photosynthesis. I
>assume that it is piped up to the leaves, just as oxygen is pumped
>down into the root system. I think that I read
>this in Aquatic Botany some years ago.
I was ASSUMING that CO2 is not taken up by the root system. Maybe it is,
but I've never heard of this. My plant physiology text has no mention of
it (but the treatment of aquatic plants is practically nonexistent).
I'm confused by your explanation of 3 modes of uptake. Plants don't
actively "take up" CO2---it passively diffuses through the stomata in the
leaves, so it would seem that the uptake in water or in air would be via
the same mechanism. Have aquatic plants evolved more efficient strategies
for living in CO2 impoverished environments? Receptors for CO2 perhaps?
If so I'd love to hear more. I'm skeptical.
Is CO2 taken up by the roots at all?
Let me back up a bit. Most plants utilize CO2 in a "C3" manner, meaning
that it is incorporated first into a 5 carbon sugar, which is then broken
down into 2-3 carbon compounds (3-phosphoglycerate). These are then sent
into the Calvin Cycle. Then into all the metabolic pathways which build
the rest of the carbon compounds found in plants.
Some plants which are adapted to high light and high temperature
environments can utilize a "C4" strategy. Corn, for example. In this
process, CO2 is first fixed into 4-carbon compounds such as malate, then
sent into the Calvin cycle. It seems unlikely that aquatic plants would
have evolved this strategy, but I don't know the facts here.
In any event: Perhaps aquatic plants can take up CO2 via the roots and
translocate it to the leaves. I've never heard of this either. But I see
no way that CO2 can be incorporated into malate in the roots (i.e., in the
absence of C4 pathways, or of any photosynthesis at all).
There is precious little mention of aquatic plant strategies in most
plant physiology texts.
Let's hear from some real physiologists out there.
Bob Hoesch
National Fish & Wildlife Forensics Laboratory
Ashland, OR
Bob_Hoesch at fws_gov