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Re: CO2 and Photosynthesis
>
>
>Aquatic Plants Digest Wednesday, 14 June 1995 Volume 01 : Number 124
>
>In this issue:
>
> Re: Subject: UG heating coils & RUG filters
> African Biotype
> CO2 and Photosynthesis
> Re: substrate flow rate
> "Root Activator"
> Anaerobic substrates
> Anaerobic substrates
> Anaerobic substrates
> Anaerobic substrates
>
>See the end of the digest for information on subscribing to the
>Aquatic Plants mailing list and on how to retrieve back issues.
>
>----------------------------------------------------------------------
>
>From: George Booth <booth at hpmtlgb1_lvld.hp.com>
>Date: Tue, 13 Jun 1995 15:29:26 -0600
>Subject: Re: Subject: UG heating coils & RUG filters
Bob_Hoesch writes
>Date: Tue, 13 Jun 95 15:59:31 MST
>Subject: CO2 and Photossythesis
Bob Hoesch writes...
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.
>
Sorry, it's very muddled. With your permisssion I'd like to rewrite the
above.
My understanding is that aquatic plants have developed at least three
strategies for obtaining carbon. They use free dissolved CO2 from the
surrounding waters, they make use of atmospheric CO2 when possible,
and they obtain carbon 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. Iread this in
Aquatic Botany some years ago.
I don't know to which form of carbon the author was referring. You
were commenting upon CO2 in the substrate.
> 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.
This is what I read, although I have no idea what malate or malic acid is.
> 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).
Perhaps the malate is formed not from CO2 but from some other carbon
compound. As well I didn't mean to imply (if that was the case) that
malate is formed or stored in the roots.
> There is precious little mention of aquatic plant strategies in most
> plant physiology texts.
>
> Let's hear from some real physiologists out there.
Many of the questions we are attempting to answer in this newsletter
are under active investigation with research reported in a journal
called Aquatic Botany, available in some university libraries.
Dave
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