To All
A comparison of Opuntia productivity to C3 and C4 plants, under both
natural and cultivated conditions, was done in the early 90's by Park
Nobel
as part of a long term investigation of the physiology of Opuntia. I am
writing from a place where I don't have access to the papers (but I do
have
the references:
Nobel, PS, E Garciamoya, and E Quero. 1992. The high annual
productivity
of certain agaves and cacti under cultivation. Plant, Cell and
Environment
15(3). pp329-335.
Nobel, P. S. (1991). Achievable productivities of certain CAM plants -
basis for high values compared with C3 and C4 plants. New Phytologist
119:183-205.).
>From what I can remember, the productivity of CAM plants was remarkably
high. Remember what happened in Australia! Your assumptions throughout
here seems to be that productivity in deserts is limited by physical
factors and that may not be true.
The real problem here is that we are anthropomorphizing the idea of
stress. The idea needs to be one that can be applied to specific
situations, not cofining it to a generalization like "deserts are
stressful
environments." Stress seems possible in all environments, native or
otherwise. Are not some of our native oaks under much stress now in
their
native habitat with the addition of sudden oak death and gypsy moths?
Perhaps stress needs to be tied to phylogeny as much as to environment.
As for the theoretical basis for engineering or selecting for
drought-tolerance, there seems to be much that might be done to me.
Drought adaptations found in some lineages might be transplanted into
other lineages through engineering. Fundamental changes might be
considered, such as engineering rubisco's ability to discriminate between
CO2 and O2. I am not a plant physiologist but I think lots of genetic
variation with regard to productivity under drought conditions exists
and,
if I am right, then there is a basis for hoping that a particular plant
species might be induced to maintain productivity at low levels of water
availability.
I may be wrong (and perhaps I missed the beginning of this thread) but I
seem to detect a worry that we are trying to expand biofuel agriculture
onto land now not utilized for agriculture. This seems like a separate
issue to me and one that deserves its own thread.
Phil Ganter
Biological Sciences
Tennessee State University
On 12/21/11 12:42 AM, "Wayne Tyson" <[email protected]> wrote:
Ecolog:
Additional responses to Merran:
I very much appreciate Merran's thoughtful response. I have made similar
observations, but I do not know of any studies which have settled this
matter. I remain open to enlightenment.
I will attempt to do justice to Merran's contributions, but am doubtful
that
I can accomplish that in one or two emails. I hope that Merran and
others,
particularly some apparently highly qualified individuals who have
contacted
me off list with some very provocative ideas.
In terms of the evolution of C4 plants from C3's and the abundance of the
former in the tropics, I see further fertile fields for research. This
may
open a whole additional can of worms, but might it be that C4 evolved via
a
mutation that ALSO worked in more mesic circumstances rather than arising
only in xeric environments? But this is too much and too distracting for
now, and perhaps worthy of a spin-off thread? Later.
"A saguaro is bigger than a sagebrush, but it took longer for it to get
that
way?" --Merran
Exactly!
And with respect to KY bluegrass and buffalo grass, I presume that
productive potential (quantity) is not as important as persistence under
stress. But are not "lawns" under continuous luxury-consumption
(quantity)
conditions by definition? I presume that Merran will stress his buffalo
grass, and not have to mow down the excess biomass as much (quality, in
Merran's eyes, not quantity, which is irrelevant in that context. And
context is everything, eh?
Let us not neglect RATE calculations, especially if we're going to get
picky
(nothing wrong with that)? I forgot to mention unit/time, and thanks to
Merran for correcting my oversight.
I hope someone will clear up the confusion about how "selection hasn't
allowed plants to create the same biomass with less water" as Merran also
points out. Any if it has, which plants they are and how much more
efficient
they are in producing more units of biomass IN LESS TIME OR THE SAME TIME
as
less efficient non-drought-tolerant plants.
WT
----- Original Message -----
From: "Merran" <[email protected]>
To: <[email protected]>
Sent: Tuesday, December 20, 2011 8:16 PM
Subject: Re: [ECOLOG-L] Plant Physiology Drought tolerance Re: [ECOLOG-L]
course and symposium on plant breeding for drought tolerance
> Isn't drought tolerance defined by a plant's water use efficiency? C4
> plants have the ability to fix 2 or 3 times more carbon with the same
> amount of water not because they use less water in photosynthesis, but
> because they limit photorespiration and the amount of water lost
> through
> their stomatas. So they do fix more more carbon with less water, but
> unless the climatic conditions are perfect I don't think the advantage
> is
> really that great. I'm fairly sure that the tropics have a greater
> abundance of C4 plants than the American deserts, and saltbushes (C4,
> right?) are not usually that much larger than sagebrushes.. There must
be
> other limiting factors.
>
> It's my understanding as well that CAM photosynthesis is not the same
> as
> C4
> photosynthesis -- I've read that it is a different, even more
> efficient process. It occurs in desert succulents and allows the
> plants
> to
> open their stomatas only at night, thus losing far less water to
> transpiration. The CO2 is stored as an acid and metabolised the next
day.
> These plants can breath in up to 40 times more Carbon dioxide than C3
> plants with the same water loss.
> However efficient these plants are, they are also very slow-growing
> -- something that I have never fully understood. I think that there's
> a
> low limit to their acid-storing capabilities. So they lose less water
> in
> exchange for performing less photosynthesis each day, but are still
> creating the same biomass with less water? A saguaro is bigger than a
> sagebrush, but it took longer for it to get that way? I'm guessing
> that
> this will not be the technique they are teaching at the CSU symposium.
>
> If I've got any of this wrong, some one please let me know.
>
> Surely there must be ways to raise a plant's water use efficiency aside
> from changing the photosynthetic process. I mean, I just spent my
morning
> picking out which variety of Buffalo Grass to replant my Kentucky
> Bluegrass
> lawn with. How about making the plant hairier? Give it a smaller leaf
> size and orient the leaves directly upwards. Make the leaves waxy
> with stomatas that don't open fully. Give it stem pleats (such as in
> cacti) that create shade. But it's my understanding that many of these
> adaptations also limit CO2 intake and therefore biomass production. I
> don't know if these adaptations will actually let you breathe in more
> CO2
> for the amount of water lost in transpiration. Anyone?
>
> Maybe I'm completely off base but it seems confusing to me to suggest
that
> selection hasn't allowed plants to create the same biomass with less
> water. Thank you for this conversation -- writing this email really
> made
> me think.
>
> Merran
>
>
> -----
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