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