CO2 uptake mechanisms

> 		   Bob Hoesch writes:
>      From: ac554 at freenet_carleton.ca (David Whittaker) Date: Sat, 10 Jun
>      1995 04:43:36 -0400
>      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.
Many aquatic plants can take up CO2 in the form of the bicarbonate ion.  They keep the CO2 
and spit out hydroxide.  These are plants that live in crowded conditions where there 
isn't much flow-through of water.  Given bright light, they can raise the pH up to 10.  
Elodea, Najas, and Ceratophyllum can do this.  

>      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).
G. Evylin Hutchinson, in Fundamentals of Limnology, Vol. III, cites some evidence that 
aquatic plants can get CO2 from the substrate by way of the air channel system that 
extends throughout the plant.  The CO2 apparently diffuses through the air channels.  
Hutchinson states that a gas can diffuse through a gas about 100,000 times as fast as it 
can diffuse through water.  

Paul Krombholz