[We obviously need more plant physiological ecologists on this list.
I'm not one, and I'm going off the top of my head, so please forgive
any errors I might make.]
Water-use efficiency is not synonymous with drought tolerance - it's a
measure of the amount of biomass a plant can make while "spending" a
unit quantity of water. It's actually quite easy to alter WUE because
the vast majority of the water isn't spent making biomass, it's lost
when the plant opens its stomata. As carbon dioxide concentrations
increase, WUE increases - plants can fix more biomass per unit time
that their stomata are open. If water is more limiting than carbon,
chances are that the plant will spend less time with their stomata
open, and thus be better able to tolerate drought. So increased WUE
may be a mechanism of drought tolerance, but it does not have to be.
There are (as you mentioned) many other ways that a plant can increase
its WUE. The distribution of stomata can do this (more on the
underside of the leaf, fewer on the upper surface), the density of
hairs on a leaf, the way the plant alters leaf orientation in response
to water loss. Sunken stomata are another important way to conserve
water. The age of leaves can also affect WUE, since older stomata are
"leakier" when their closed.
Now obviously none of this is "free" to the plant - most take
resources to make, and for the most part, limiting the rate of water
loss will also limit the rate of CO2 uptake. But these relationships
are not linear, and relative humidity varies more than CO2
concentration. So, depending on the environment, a plant can break
even or gain a substantial advantage through traits like these. When
you're talking about growing crop plants, you're talking about
minimising the cost of many of these trade-offs through human
intervention. Many of these sorts of traits are variable within
species, and some of them are induced responses to drought - so
there's a fairly large range of variables that a plant breeder can
work with (assuming that these traits exist in the crop that the
breeder is working with - which is, of course, not a given.
But it's important to remember that drought tolerance is far more than
WUE. It's also a matter of the ability to a plant to forage for soil
water, and to extract that water from drying soils. Plants in dry
environments tend to allocate a larger proportion of their belowground
processes. This may take the form of deep roots that are able to tap
deep soil moisture, or it may be allocating sugars to mycorrhizae or
proteins to the rhizosphere. Again, if water is not limiting, there's
no advantage to this, but if it is, the payoff is likely to exceed the
investment in sugars. Water potential is another important factor. The
ability of plants to extract water from the soil is governed (at least
in part) by the osmotic gradient between the root hairs and the soil
water. [Yes, I know, this is a gross simplification.] Plants that can
generate lower water potentials can extract water from drier soils
(famously on the order of -9MPa for creosote). Obviously the ability
to tolerate such low water potentials must come at some cost, but
these are trade-offs that a plant can work to its advantage (in
certain environments). Again, these are (presumably) things that one
could alter in a plant-breeding programme.
[C4 and CAM are a whole different set of issues, but this message is
already too long.]
Quoting Merran <[email protected]>:
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
