Frankly, I don't think it makes much difference either, but I don't claim to
be able to explain it. I just have my doubts about assumptions that
drought-tolerant plants (including C4's and CAM's) are able to produce more
biomass with less water than non-drought-tolerant plants (say, C3's). I'm
asking, not telling. But I think it's a fair question.
I SUSPECT that drought-tolerance or drought-evasion involves physiological
processes that limit growth, and that if one added up the water and the
biomass there would be a pretty close correspondence across species and even
metabolic strategies; that is, that the differences in productivity are not
likely to be impressively great, or greatly significant. That's the reason
for the question; I want all y'all who are smarter or who have studied this
matter more extensively or intelligently than I to correct my suspicions
with solid evidence. Hence my post.
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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