Transpiration

1
Water limitations and stress xylem
dysfunction stomatal regulation a quick look at
drought responses
2
But first a special topic -- Hydraulic
Redistribution
3
Water in the xylem is in a metastable state of
tension, which makes it vulnerable to cavitation.
Cavitation occurs when an air bubble is
introduced into the xylem because the xylem is
under tension the bubble rapidly expands, and
initially forms a partial vacuum
4
An embolism occurs when an air bubble in the
xylem cell (from cavitation) equalizes with
atmospheric pressure in other words, the
partial vacuum is lost as air molecules diffuse
into the cavitated cell
5
Cavitation/embolism can result both from both
freezing and drought
6
Drought-induced cavitation is caused by air
seeding. When xylem water is under very high
tension, air bubbles may be pulled in through
pores of the xylem conduits.
7
The air-seeding hypothesis Zimmerman 1983
embolism
8
In gymnosperms the air bubble enters
through bordered pits
9
In gymnosperms, cavitated tracheids are isolated
from water filled tracheids by a special
anatomical structure called a torus
10
In angiosperms, air bubbles enter through simple
pits on vessels
11
Drought cavitation is related to the size and
geometry of the pits, not necessarily the size of
the tracheids or vessels.
12
Embolisms may also occur after freezing. Air
bubbles form when water inside the cells freezes.
Unless the bubbles are re-dissolve when the ice
thaws, embolisms will form.
13
Vulnerability to cavitation from freezing is
related to the size of the xylem element,
especially the diameter. Large bubbles are more
likely to form in large-diameter conduits.
So you can see that there is a direct tradeoff
between xylem vulnerability to cavitation from
freezing and xylem anatomy and conductivity.
14
Xylem embolisms reduce the hydraulic conductivity
of xylem, and therefore the hydraulic conductance
of whole stems is reduced
15
Recall that
Q K (Yw(soil) - Yw(tree top))
If K is reduced due to embolisms, what must
happen to Yw(tree top) in order to maintain a
constant Q?
If Yw(tree top) is reduced due to sustain Q, what
will be the impact on further cavitation?
16
Tyree and Sperry (1988) called this scenario
runaway cavitation -- (well come back to this
a bit later) Over short time scales, plants
avoid runaway cavitation by regulating stomatal
conductance
17
Stomatal conductance balances the atmospheric
demand for evaporation with the hydraulic
capacity to supply water
DEMAND VPD
Transpiration
?VPD LAI leaf conductance
SUPPLY Flow of liquid water (Yleaf Ysoil)
K
18
Stomatal opening and closure is caused by changes
in turgor of the guard cells. When guard cells
are fully turgid, the pore is open as the guard
cells lose turgor, the stomatal pore size is
reduced
19
Stomatal opening in the morning is caused by
influx of potassium ions (this is stimulated by
blue light) later in the day, K concentations
fall and sucrose accumulates.
20
Control over stomatal conductance is the most
important mechanism plants have for regulating
water balance on daily-to-weekly timescales.
Stomatal regulation also involves important
tradeoffs between carbon assimilation and water
loss. So it isnt surprising that there are
intricate controls over stomatal conductance.
21
Definitions
In anisohydric species, both leaf water potential
and stomatal conductance decrease with decreasing
soil water potential and vapor pressure deficit
(examples, sunflower, most crop plants, vines)
Isohydric species control stomata so that daytime
minimum leaf water potential never falls below a
species-specific minimum value, despite
differences in soil water potential (most
temperate woody plants are isohydric or near
isohydric) The following set of slides
illustrate typical isohydric behavior
22
Early morning demand is low
Stomatal conductance
transpiration
Leaf water potential
6
8
10
12
14
16
hour
Adapted from Bond and Kavanagh, 1999
23
mid morning demand increases, so does
transpiration
Stomatal conductance
transpiration
Leaf water potential
6
8
10
12
14
16
hour
Adapted from Bond and Kavanagh, 1999
24
Noon demand is high transpiration levels off
Stomatal conductance
transpiration
Leaf water potential
6
8
10
12
14
16
hour
Adapted from Bond and Kavanagh, 1999
25
mid afternoon demand increases transpiration
constant
Stomatal conductance
transpiration
Leaf water potential
6
8
10
12
14
16
hour
Adapted from Bond and Kavanagh, 1999
26
late afternoon demand subsides transpiration
constant
Stomatal conductance
transpiration
Leaf water potential
6
8
10
12
14
16
hour
Adapted from Bond and Kavanagh, 1999
27
A xylem vulnerability curve relates the loss in
conductivity from cavitation to xylem tension
28
Cold and drought tolerances of perennial species
are related to their xylem vulnerabilities to
embolism
Lambers, Chapin and Pons p. 179 also see
Pallardy p. 318
29
Vulnerability-curves of plants from different
habitats
loss hydraulic conductivity
Kolb and Sperry 1999
30
Many woody species control xylem water potential
(by opening or closing stomata) so that it is on
the edge of catastrophic loss in conductivity
due to cavitation (see red alder, at right).
But some species (e.g. temperate conifers) appear
to be somewhat more conservative, controlling
xylem water potential so that it is above the
danger zone of cavitation.
White arrows denote typical mid-day minimum water
potentials.
31
There are inevitable tradeoffs between xylem
safety and growth rates
32
Plants do not overbuild their xylem
Xylem pressure at death (MPa)
11
Minimum observed xylem pressure (MPa)
Pockman and Sperry 2000
33
Wheeler et al 2005
For angiosperms greater drought tolerance means
smaller pit area/vessel area lower hydraulic
conductance
Pit surface area per vessel area
More drought tolerant
Less drought tolerant
P50 (-MPa)
34
Also true in conifers.
Domec et al 2006
35
So far weve focused on regulation of water
balance over short time periods. How do woody
plants acclimate to drought over longer time
periods? Here are a few examples. Also see
Pallardy chapter 12 for more.
36
Consequences of low water potential to living
cells remember, if the water potential of xylem
is low, the water potential of the leaves
attached to the xylem must be equally as low (or
else what would happen?)
37
Remember, Yw Yp Ys
If the total water potential of tissue
surrounding living cells falls below the osmotic
potential in living cells, the cells will undergo
plasmolysis. Even if the cells dont plasmolyze,
reduced turgor will reduce the potential for
growth.
A plasmolyzed cell from onion. This is an
irreversible process.
38
Many plants acclimate to drought conditions by
osmotic adjustment Ys of cell sap is reduced
through accumulation of organic and inorganic
solutes in the cytoplasm and vacuole.
The solutes in the cytosol usually have low
physiological activity. Examples sorbitol (a
sugar alcohol) or proline (an amino acid).
39
Deeper roots
40
allometric adjustments leaf area/sapwood area
ratio and leaf area/root area ratio
DEMAND VPD
Whole tree transpiration
?VPD Leaf area leaf conductance
SUPPLY Flow of liquid water (Yleaf Ysoil)
KL Leaf area
41
An important part of both adaptation and
acclimation to water stress is variation in the
ratio of leaf area to sapwood area
42
From Lambers, Chapin and Pons p. 180