To Wayne and others:
Sorry about the C4/CAM confusion. It has been many years since I
have thought about them and I forgot some important distinctions (but it
did seem to make the thread come to life).
Wayne, in answer to your question regarding this question I put
forth:
"...Is this any less plausible than other manipulations that have been
carried out in the past that have resulted in increased yields?"
You will notice that it makes no reference to water use. I was addressing
in a general way the fact that artificial selection WORKS. Wayne, your
questions seemed to imply a null hypothesis that selection or genetic
engineering CANNOT increase plants' productivity under low-water
conditions. It seems to me ALL physiological processes are subject to
alteration, which in turn can influence the efficiency and productivity.
Has anyone ever found a genetic trait for which there was no genetic
variability or that did not respond at all to selection?
Can anyone seriously believe that all plants are equally efficient at
surviving, photosynthesizing, and producing biomass with low water
availability? If we can hypothesize two plants that differ in this regard,
we can imagine that there are manipulations to make the one more like the
other. It seems to me that the plausible starting assumption is that the
yields of crop plants under drought conditions CAN be increased. If
serious attempts show that it cannot be done, that will be some sort of a
revolution in our understanding of physiology and and evolution.
Martin M. Meiss
2011/12/21 Ganter, Philip <[email protected]>
> 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
> >
> >
> > -----
> > No virus found in this message.
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> >
>
