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