Subject: C4 vs. C3 (a little botany)

From: [email-masked] (Brendan A. Niemira)
Date: Tue, 14 Jun 1994 12:22 est

As someone else (Emil, perhaps?) has said, good old <Zea mays>, corn to the
Yanks, maize to the rest of the world, is a C4 grass.  The C3 and C4 compounds
aren't really all that important, but any introductory plant physiology text
will explain it.
The significant fact about C4 plants is that they are designed to grow in some
pretty nasty climates.  C4 metabolism, coupled with Crassulacean Acid Metabolism,
allows growth in deserts, on mountains, tundra, etc.  The poor C3 plants must
necessarily capture and subsequently incorportate their CO2 while the sun 
shines, so they open their pores to get the CO2 during the day; the heat and
wind dries them out and they die.  Those clever C4 plants, however, open their
pores at NIGHT and capture the CO2 when the air is cooler, so they won't dry
out.  The next day (with pores closed), they use the light to incorporate the
CO2.  C4 metabolism is a more expensive [energy-consuming] physiological 
process than the C3 metabolism, but it allows the C4 plant to capture more
CO2 overall than C3 plants.  If you put a C4 (corn) in a sealed chamber with
a C3 (potato), the C4 will suck up all the CO2, and the C3 plant will wither
and die.
Interesting fact:
The C4 plants are more effecient overall because at low CO2 concentrations,
(about 180-250 ppm, Earth's atmospheric CO2 conc. up until a few years ago)
the more expensive method is more cost-effective.  However, growth chamber
studies have shown that at higher CO2 concentrations (about 380-400 ppm), the
more expensive method isn't needed, and the C3 plants are more efficient,
overall, and therefore more competitive.  Current CO2 concentration is 375 ppm
and rising.  Screw global warming, we're talking about ecological collapse 
due to differential CO2 fixative capacity.  Ooops.. should have posted to
sci.ecology.conspiracy
Brendan A. Niemira
Dept. Botany and Plant Pathology
Michigan State University
.........

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Brendan A. Niemira                      [email-masked]
        All opinions expressed are entirely my own.
"You know your Shelley, Bertie."
"Oh, am I?"                      P. G. Wodehouse
                                 THE CODE OF THE WOOSTERS

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From: [email-masked] (Steinn Sigurdsson)
Date: 14 Jun 94 11:47:40

In article <[email-masked]> [email-masked]  (Brendan A. Niemira) writes:

 ...

My understanding was that C4 mechanism became competitive because
geochemical processes have been depleting the atmospheric CO2 on
very long time scales. Go back O(10^7) years and CO2 concnetrations
were ideal for C3. In fact there was a series of articles recently
in Nature about future CO2 depletion due the erosion of the Himalayas
and the impending collapse of C3 plants and associated ecosystems.
For a while it was the frontrunner for "first natural process to
terminate life on Earth" - time scale was 1-2 \times 10^8 years
as I recall, much shorter than, for example, solar evolutionary
time scale.

*  Steinn Sigurdsson   			Lick Observatory      	*
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