Lecture notes on stomatal conductance.

Agron 516: Crop physiology.

Dr. Mark Westgate.

Diurnal variation of stomatal conductance has direct consequencesfor leaf and canopy gas exchange

Idealized diurnal pattern ofstomatal conductance at three

levels of water availability

Measure diurnal pattern oftranspiration and photosynthesis

by a corn canopy

Adopted from Christy, A.L., et al. 1986

Ψw at 25°C

-1.38 MPa

-7.04 MPa

-95.2 MPa

Representative values of leaf conductances, RH, and water vaporaffecting movement of water out of a leaf through a stomatal pore.

At 25°C, Ψwv = (137.3 MPa) * ln (RH/100)

1. Stomatal conductance typically controls transpiration rate

2. Transpiration rate = stomatal conductance * ΔΨwv (air - leaf)

Fig. 8.6 P.S. Nobel

Nobel, Fig 8.6

Flux = conductance X forceTranspiration rate = leaf conductance X ΔCwv (leaf - air)

Jwv = g totalwv * (Cias

wv - Caiwv)

but… g stwv

<< g iaswv

and g blwv

Formalizing these concepts…

therefore… _ Jwv for transpiration ~ _ g stwv

r

4. Mesophyll/Chloroplast (CO2-liq)

3. Intercellular air spaces

2. Stomata

1. Boundary layer

General components of diffusive resistance within theleaf to CO2 exchange with the atmosphere

Taiz & Zeiger, Fig. 19.7

Stomata also control the rate of CO2 fluxinto/out of leaves

Table 8.4 P.S. Nobel

Flux = conductance X forcePhotosynthesis rate = leaf conductance X ΔCCO2 (air-leaf)*

Again, formalizing these concepts…

*gradient opposite for water vapor

gstCO2

<< gblCO2

and giasCO2

JCO2= gtotal

CO2* (Cair

CO2– Cchl

CO2)

g gasCO2

* (CairCO2

– CiasCO2

) = gliqCO2

* (CiasCO2

– CchlCO2

)

Noting that flux through the gaseous and liquid parts of the path are equal…

And that stomata have the lowest conductance along the path…

Therefore, JCO2

~ g stCO2

Calculation of internal [CO2] andwater use efficiency

Jwv = g totalwv * (Cias

wv - Cairwv ) = g total

wv (eiaswv – eair

wv)

Patm

Where Cwv = mole fraction (eg. ppm) ewv = partial pressure (eg. Pa) = Cwv * Patm

JCO2 = g gas

CO2 * (Cias

CO2 - Cair

CO2 ) = g gas

CO2 (aair

CO2 – aias

CO2)

For water vapor exchange:

For CO2 gas exchange:

Patm

Where aCO2 = CCO2

* Patm

To calculate internal [CO2]:

Water vapor and CO2 diffuse along the same path…but CO2 is 60% heavier than water

Therefore…. gCO2 = gwv

1.6

Substituting into the JCO2equation….

JCO2 = g total

wv * (aairCO2

– aiasCO2

)

1.6 * Patm

Then solve for aiasCO2

…

aiasCO2

= aairCO2

– 1.6 * Patm * JCO2

g totalwv

To calculate Water Use Efficiency (WUE):

WUE = JCO2/Jwv ~ Biomass/Crop Water Use ~ Grain yield/Seasonal Transpiration

JCO2g gas

CO2 * (aairCO2

– aiasCO2

)/ Patm

g totalwv * (eias

wv – eairwv)/ Patm Jwv

=

gCO2 = gwv

1.6Recall that:

Substituting: JCO2 (aair

CO2 – aias

CO2)

1.6 * (eiaswv – eair

wv) Jwv

=

For C3 plants this ratio is about 1:1000, for C4 plants the ratio is about 1:400

Open and closed stomata of Vicia faba.Taiz & Zeiger, Fig. 18.10

What determines when/how stomatal open??

Guard Cell Turgor (Ca+, K+, sucrose, malate, etc.)

Schematic representation of events leading to openingand closing of stomatal pores

Fig. 8.2 P.S. Nobel

Diurnal change in stomatal aperature inbroad bean in relation to K+ and sucrosecontent. Taiz & Zeiger, Fig 18.17

External:epidermal cell turgorapoplast pHN-nutritionxylem ABAcytokininsPARBlue lightVapor pressure deficitCO2

Internal:Ion channel activityATPasessecondary messengersNitric OxidezeaxanthinCHO metabolism

Stomatal aperature is regulated by many ‘internal’ and‘external’ factors:

Brodribb and Holbrook, 2003

Stomata close when leaf Ψw decreases…

Brodribb and Holbrook, 2003

Kleaf = C * ln(Ψo/Ψf) / tLeaf hydraulic conductance (Kleaf) is determined by the rate of leaf re-hydration

Leaf hydraulic conductance can decrease rapidly at low leaf Ψw

Ψp = 0

What causesthe decrease in

hydraulicconductance?

Eucalyptus crenulata

Tritcum aestivum

How is the change in leaf hydraulic conductancerelated to stomatal closure?

0.1 mm

0.3 mm

1. The water in the xylem isunder tension (i.e. Ψpxyl < 0)

2. As leaf Ψw decreases, thetension on the water in thexylem vessels increases(i.e. Ψpxyl < < 0)

3. Large tensions cause embolismsto develop in the xylem vessels

4. Water flow to affected sectionsof the leaf decrease

Then what happens??

Stomatal closure is closely associatedwith an increase in xylem ABA content

Zhang and Davies, 1989a

xylem [ABA] increased inwater stressed leaves

No H2O

Sunflower leaves

● natural soil drying○ ABA fed to cut leaf

ABA: abscisic acid

ABA appears to be involved in stomata closure evenbefore there is a change in leaf water status

Maize leavesNoH2O

Stomatastart to

close beforeleaf water

statuschanges

Zhang and Davies, 1989b

WW

WS

NoH2O

But the water status of roots in the upper part ofthe soil profile does change, and these roots produce

ABAStomata startto close on day 6

Root ABAcontent

increasesdramatically asthe soil dries

Zhang and Davies, 1989b

WW

WS

Roots in theupper soilprofile dryfirst, and

produce ABARoots lower in

the profileremain

hydrated, andsupply theshoot with

water

0 - 20 cm

20 -100 cm

Open symbols: WSClosed symbols: WW

Maize roots

NoH2O

Zhang and Davies, 1989b

How can the leaves remain at high Ψw when the roots are drying?

ABA produced in dehyrdating roots in upper soil layersand transported to the shoot may enable plants to

‘anticipate’ continued soil drying by inducing stomatalclosure (feed forward response).

How does the ABA get to the leaves?

How does the ABA get to the guardcells?

What does the ABA do when it gets there?

Re-distribution of ABA from the xylem during water stress

Increase in xylem pH (apoplast pH in general) converts some ABAH to ABA-,which decreases uptake by the mesophyll cells, so more ABA reaches the guardcells.

Taiz & Zeiger, Fig. 23.3

Cytoplasmic Ca2+ mediates ABA-induced turgor loss in guard cellsJ. Schroeder, et al. 2001

(1, 9, 10) ABA increases Ca2+ in cytoplasm by increasing uptake, and release from vacuole(2,3,6) Ca2+ activates anion channels and K+

out channels, and inhibits H+ATPase pump(4,5) Ca2+ promotes K+ release from vacuole, increases cytoplasm pH to promote K+

out channels(8,9,10,11) Ca 2+ as second messenger: PLC, InsP3, cADPR, CICR systems. Sustained stomatal closure requires sucrose removal and conversion of malate to starch.

[high]

[low]

PLC: phospholipase C, InsP3: inositol tri-phosphate, cADPR: cyclic ADP ribose

ABA-induced Ca2+ oscillations in Arabidopsis guard cells expressingfluorescent dye (yellow cameleon 2.1)

J. Schroeder, et al. 2001

Cytosolic Ca2+ levels oscillate!Disrupting the oscillation pattern prevents stomatal closure.Hypothesis: Ca2+ oscillations encode information required for processing closure signals

J. Schroeder, et al. 2001

Proposed regulators mediating guard cell response to external ABA

NB: integrated involvement of transporters, pumps, sugar metabolism

ROS: reactive oxygen species, PP2C: protein phosphatase, --PK: protein kinases FTase: farnesyl transferase, PLD: phospholipase D, ABC: ATP-binding cassette

References• Nobel, P.S. 1991. Physicochemical and environmental plant physiology. Academic Press, Inc.

San Diego.

• Schroeder, J.I., J.M. Kwak, and G.J. Allen. 2001. Guard cell abscicis acid signalling andengineering drought hardiness in plants. Nature 410: 327-330.

• Zhang, J., and W.J. Davies. 1989. Abscisic acid produced in dehydrating roots may enable theplant to measure the water status of the soil. Plant, Cell, Environ. 12: 73-81.

• Zhang, J., and W.J. Davies. 1989. Sequential response of whole plant water relations toprolonged soil drying and the involvement of zylem sap ABA I the regulation of stomatalbehaviour of sunflower plants. New Phytol. 113: 167-174.

• Brodribb, T.J., and N.M. Holbrook. 2003. Stomatal closure during leaf dehydration, correlationwith other leaf physiological traits. Plant Physiol. 132: 2166-2173.

• Christy, A.L., D.R. Williamson, and A.S. Wideman. 1986. p 11-20. In: J.C. Shannon et al (eds.)Regulation of carbon and nitrogen reduction and utilization in maize. Amer. Soc. Plant Physiol.Rockville, MD.

• Taiz, L. and E. Zeiger. 1998. Plant Physiology, second edition. Sinauer Associates, Inc.Sunderland, MA.