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Water transport through plants - continued
epidermis cortex endodermis pericycle xylem
< 0.1 mm
Radial path of water across root:
tiny distance but biologically very important
Radial path of water across root:
regarded as the largest resistance to the movement of H2O anywhere in the plant
the rate-limiting barrier to flow through the plant
imposes a hydraulic resistance 10,000 times greater than that measured in xylem of stems or petioles
Why is root resistance important?
Why is root resistance important?
Because leaf will be strongly dependent upon it.
More specifically, because must become larger to pull H2O up to the leaves.
leaf must decrease
cell enlargement, growth, photosynthesis inhibited
Environmental stresses and root resistance:
droughtsalinitylow (chilling) temperaturehigh temperatureO2 starvationhigh CO2
each increases root resistance (i.e., decreases root hydraulic conductivity)
a water “supply” problem
The conducting system:
see Fig.4.6 Taiz & Zeiger (2010)
Xylem:
specialized to conduct H2O
low frictional resistance
dead (empty) when functional
no membranes, protoplasm
lignified cell walls
targeted for early death (PCD)
tracheids connected by pit pairs
B = bordered pit
Courtesy W.C. Brown Center, SUNY
Bordered pit of eastern hemlock (Tsuga canadensis). Courtesy of W.A. Cote.
R.J. Thomas, NCSU
Bordered pit pair between pine tracheids
aperture approx 20 nm (bubbles can’t pass)
P=-15 P=-15Two adjacent tracheids, connected by a bordered pit
Non-embolized
P=-15 P= +1
injury air rushes in from intercellular spaces
or
high tension ruptures columns embolism (cavitation) can be detected acoustically (~ 500 Hz)
P=-15 P= +1 P in lumen of embolized cell between vacuum (0) and atmospheric (+1), but neighboring cell still under –P
hugh P exists across pit membrane
What happens next?
P=-15 P= +1 Torus moves to side of least pressure
aperture is plugged, shutting off flow
embolized cells become isolated and damage is confined
Impact of cavitation minimized by:
1. gas spread stopped by pits
2. interconnections possible detours possible
3. if transpiration is low, the vessels may re-fill (some species)
If transpiration is low, the vessels may re-fill
parenchyma cells
embolized xylem element
residual H2O
• ions pumped into residual H2O• additional H2O follows (osmosis)
Efficiency versus “safety” of the hydraulic system
recall Pousieulle: P
X
r2
8.Jv =
large diameter vessels increase efficiency but greater tendency to cavitate
maximum diameter in nature ~ 500 m
Vessel diameter (m)
0 100 200 300 400 500
Mid
-day
sap
vel
oci
ty (
m h
r-1 )
0
10
20
30
40
50
conifers
oaks, ash, hickory, locust
birch, beech, maples
Typically, no cell in the spongy mesophyll layer is more than 1 or 2 cells away from a vascular bundle.
Stomata
When stomata are open, virtually all H2O passing through the plant escapes through here.
Goal: Understand the mechanism controlling the aperture between two guard cells
H2O
CO2
turgor-operated valve with extreme sensitivity to:
Environmental factors: light (dominant factor) temperature water status, humidity CO2
Internal physiological factors: [starch] hormones (ABA, maybe CK) pH Ca2+
H2O
CO2
Stomata optimize the balance between H2O loss and CO2 gain.
Figure 4.14 Taiz and Zeiger (2010) p. 101
Eliptical (most common) Graminaceous (in parallel rows)
Stomatal complex of a monocot leaf
Subsidiary (epidermal) cell
Guard cell• cytosol and vacuole• thickened cell wall
Figure 4.12 Taiz and Zeiger
Onion epidermis: outside surface
view from the stomatal cavity
Guard cells
Epidermal cell
Subsidiary cell
Figure 4.14 Taiz and Zeiger (2002) p. 61
substomatal cavity
pore
outside air
vacuole
nucleus
Tools:
Model species: broad bean (Vicia faba)dayflower (Commelina communis)Arabidopsis thaliana
Epidermal peel technique:
detached epidermis floated on KCl solution illuminate observe opening mesophyll cells not required for function allows separation of environmental effects
1. Stomatal aperture closely tracks PAR
2. Guard cells are sensitive to blue light stomata open
2a. Blue light causes guard cells to acidify their suspension medium
Proton-pump blockers (e.g., vanadate) prevent the acidification
blue light
Protoplasts in dark Protoplasts swell in blue light
2b. and vanadate prevents blue- light-stimulated swelling of guard cells
Conclude: acidification due to the activation of a proton-pumping ATPase in the guard cell plasma membrane
Partial summary:
blue light
activates a proton-pumping H+- ATPase in the GC plasma membrane
extrudes protons, acidifying the media
Stomatal opening / closing
Pore opens when two opposing GCs take up water from neighboring epidermal cells (subsidiary cells)
H2O influx caused by solute accumulation in guard cells
**
water
1. What is the solute?
2. Where does it come from?
The solute is principally K+ (known since 1968)
moves from surrounding cells into the guard cells (vacuoles)
increases of more than 0.5 M K+ observed sufficient to decrease by 20 bars !!
0.1 M K+ 0.55 M K+
Rapid and large K+ transport between
accessory cells guard cells
light causes buildup of K+ in GCs
when transferred to dark K+ leaks back out
The vast change in [K+] must be counter balanced – otherwise electrical charges within cells would become unbalanced
Electrical neutrality maintained by fluxes of counterions:
Cl - (some species)
malate 2- (most species)
COO-
|H – C – OH | CH2
|
COO-
Question #2. Where do the K+ ions come from? And what is the mechanism of their movement?
Chloroplasts in the GCs
they are photosynthetically active (unlike subsidiary cells)
can readily supply the ATP to operate proton pumps
can supply sucrose (a 2nd osmoticum)
Sequence of events for stomatal opening:
1. H+ (proton) extrusion via H+-ATPase in GC plasma membrane
creates a difference in electrical potential across the membrane (~ 50 mV) and
creates a pH gradient of 0.5 – 1.0 unit
Sequence of events for stomatal opening:
2. This favors passive influx of K+
100 mMclosed
400-800 mMopen
3. Electrical charge must be counterbalanced
counterions: Cl- or malate 2-
4. decreases
Sequence of events for stomatal opening:
5. H2O drawn in (osmosis)
6. Hydrostatic pressure (turgor) increases
7. Pore opens GC walls highly elastic (can or volume
by 40-100%)
differential wall thickness
radial arrangement of cellulose microfibrils
vacuole
cytoplasm
H+H+
K+K+
Cl- Cl-
malate2-
blue-light-activated proton pump extrudes H+ ions ATP
ADP
chloride and malate maintain electrical neutrality
PAR
VAC
CYT
malate2-
starch hydrolysis
PEP
OAA
Where does the malate come from?
(3C)
(4C)
(4C)
CO2
Lights off:
1. proton pump immediately stops extruding H+
2. K+, sucrose leak out
3. of subsidiary cells decreases, H20 enters
4. [malate2-] and starch in GC
either: malate respired to CO2 in mitochondria or: malate converted to sugars starch
H+ out
K+ in
Cl or malate in
decreases
H2O in
turgor increases
pore opens
closed open
closed open
H+ out
out K+ in
out Cl or malate in
increases decreases
out H2O in
decreases turgor increases
closes pore opens