CO2 uptake mechanisms

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> --------------------------------------------------------------------
>    *  To: Aquatic-Plants at actwin_com
>    *  Subject: CO2 and Photosynthesis
>    *  From: Bob_Hoesch at mail_fws.gov
>    *  Date: Tue, 13 Jun 95 15:59:31 MST
> --------------------------------------------------------------------
>      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).
G. Evelyn Hutchinson, in Fundamentals of Limnology, Vol III, cites some evidence that CO2 
can diffuse by way of air channels from the roots to the green parts of aquatic plants.  

>      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.

>      Bob Hoesch
>      National Fish & Wildlife Forensics Laboratory
>      Ashland, OR
>      Bob_Hoesch at fws_gov

A number of species of aquatic plants can absorb the bicarbonate ion, keep the CO2, and 
excrete hydroxide.  They tend to live in crowded conditions where there is not much 
flow-through of water, and, in good light, they can raise the pH to 10.  They often 
precipitate calcium carbonate on their leaves.  Elodea, Najas, and Ceratophyllum are 
examples.  Another group can't do this, and apparently are able to only absorb free CO2.  
This information should be in any textbook of limnology. 
Paul Krombholz