Lecture notes on stomatal conductance.
Agron 516: Crop physiology.
Dr. Mark Westgate.
Diurnal variation of
stomatal conductance has direct consequences
for leaf and canopy gas exchange
Measure diurnal pattern of
transpiration and photosynthesis
by a corn canopy
Adopted from Christy, A.L., et al. 1986
Idealized diurnal pattern of
stomatal conductance at three
levels of water availability
Representative values of leaf conductances, RH, and water vapor
affecting movement of water out of a leaf through a stomatal pore.
Ψw at 25°C
-1.38 MPa
-7.04 MPa
Fig. 8.6 P.S. Nobel
-95.2 MPa
1. Stomatal conductance typically controls transpiration rate
2. Transpiration rate = stomatal conductance * ΔΨwv (air - leaf)
Nobel, Fig 8.6
At 25°C, Ψwv = (137.3 MPa) * ln (RH/100)
Formalizing these concepts…
Flux = conductance X force
Transpiration rate = leaf conductance X ΔCwv (leaf - air)
r )
Jwv = g total wv * (Cias wv - Caiwv
but…
g stwv << g iaswv and g bl wv
therefore… _ Jwv for transpiration ~ _ g stwv
General components of diffusive resistance within the
leaf to CO2 exchange with the atmosphere
4. Mesophyll/Chloroplast (CO2-liq)
3. Intercellular air spaces
2. Stomata
1. Boundary layer
Taiz & Zeiger, Fig. 19.7
Stomata also control the rate of CO2 flux
into/out of leaves
Table 8.4 P.S. Nobel
Again, formalizing these concepts…
Flux = conductance X force
Photosynthesis rate = leaf conductance X ΔCCO2 (air-leaf)*
JCO = gtotal CO * (C airCO – Cchl CO )
2
2
2
2
Noting that flux through the gaseous and liquid parts of the path are equal…
g gas CO2 * (C airCO2 – Cias CO2) = gliq CO2 * (C ias CO2 – CchlCO2)
And that stomata have the lowest conductance along the path…
gstCO
Therefore,
2
<< gblCO and giasCO
2
2
JCO ~ g stCO
2
2
*gradient opposite for water vapor
Calculation of internal [CO2] and
water use efficiency
For water vapor exchange:
Jwv = g totalwv * (Ciaswv - Cairwv ) = g totalwv (eiaswv – eairwv)
Patm
Where
Cwv = mole fraction (eg. ppm)
ewv = partial pressure (eg. Pa) = Cwv * Patm
For CO2 gas exchange:
JCO = g gasCO * (CiasCO - CairCO ) = g gasCO (aairCO – aiasCO )
2
2
2
2
2
2
Patm
Where aCO
2
= CCO * Patm
2
2
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 JCO
2
equation….
JCO = g
2
total
wv
* (aairCO – aiasCO )
2
2
1.6 * Patm
Then solve for aiasCO …
2
aiasCO
2
= aairCO – 1.6 * Patm * JCO
2
2
g totalwv
To calculate Water Use Efficiency (WUE):
WUE = JCO /Jwv ~ Biomass/Crop Water Use ~ Grain yield/Seasonal Transpiration
2
JCO
2
Jwv
g gasCO2 * (aairCO2 – aiasCO2)/ Patm
=
g totalwv * (eiaswv – eairwv)/ Patm
Recall that:
Substituting:
JCO
Jwv
2
gCO2 = gwv
1.6
=
(aairCO – aiasCO )
2
2
1.6 * (eiaswv – eairwv)
For C3 plants this ratio is about 1:1000, for C4 plants the ratio is about 1:400
What determines when/how stomatal open??
Open and closed stomata of Vicia faba.
Taiz & Zeiger, Fig. 18.10
Guard Cell Turgor
(Ca+, K+, sucrose, malate, etc.)
Schematic representation of events leading to opening
and closing of stomatal pores
Fig. 8.2 P.S. Nobel
Stomatal aperature is regulated by many ‘internal’ and
‘external’ factors:
External:
epidermal cell turgor
apoplast pH
N-nutrition
xylem ABA
cytokinins
PAR
Blue light
Vapor pressure deficit
CO2
Internal:
Ion channel activity
ATPases
secondary messengers
Nitric Oxide
zeaxanthin
CHO metabolism
Diurnal change in stomatal aperature in
broad bean in relation to K+ and sucrose
content. Taiz & Zeiger, Fig 18.17
Stomata close when leaf Ψw decreases…
Brodribb and Holbrook, 2003
Leaf hydraulic conductance can decrease rapidly at low leaf Ψw
Ψp = 0
What causes
the decrease in
hydraulic
conductance?
Brodribb and Holbrook, 2003
Leaf hydraulic conductance (Kleaf) is determined by the rate of leaf re-hydration
Kleaf = C * ln(Ψo/Ψf) / t
How is the change in leaf hydraulic conductance
related to stomatal closure?
Tritcum aestivum
1. The water in the xylem is
under tension (i.e. Ψpxyl < 0)
2. As leaf Ψw decreases, the
tension on the water in the
xylem vessels increases
(i.e. Ψpxyl < < 0)
0.1 mm
Eucalyptus crenulata
3. Large tensions cause embolisms
to develop in the xylem vessels
4. Water flow to affected sections
of the leaf decrease
Then what happens??
0.3 mm
Stomatal closure is closely associated
with an increase in xylem ABA content
Sunflower leaves
● natural soil drying
○ ABA fed to cut leaf
Zhang and Davies, 1989a
xylem [ABA] increased in
water stressed leaves
No H2O
ABA: abscisic acid
ABA appears to be involved in stomata closure even
before there is a change in leaf water status
No
H2O
Maize leaves
Stomata
start to
close before
leaf water
status
changes
WW
WS
Zhang and Davies, 1989b
But the water status of roots in the upper part of
the soil profile does change, and these roots produce
Stomata start
ABA
No
H2O
to close on day 6
WW
WS
Root ABA
content
increases
dramatically as
the soil dries
Zhang and Davies, 1989b
How can the leaves remain at high Ψw when the roots are drying?
No
H2O
Roots in the
upper soil
profile dry
first, and
produce ABA
Roots lower in
the profile
remain
hydrated, and
supply the
shoot with
water
Maize roots
Open symbols: WS
Closed symbols: WW
20 -100 cm
0 - 20 cm
Zhang and Davies, 1989b
ABA produced in dehyrdating roots in upper soil layers
and transported to the shoot may enable plants to
‘anticipate’ continued soil drying by inducing stomatal
closure (feed forward response).
How does the ABA get to the leaves?
How does the ABA get to the guard
cells?
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 guard
cells.
Taiz & Zeiger, Fig. 23.3
Cytoplasmic Ca2+ mediates ABA-induced turgor loss in guard cells
J. Schroeder, et al. 2001
[high
]
[low]
(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.
PLC: phospholipase C, InsP3: inositol tri-phosphate, cADPR: cyclic ADP ribose
Cytosolic Ca2+ levels oscillate!
Disrupting the oscillation pattern prevents stomatal closure.
Hypothesis: Ca2+ oscillations encode information required for
processing closure signals
ABA-induced Ca2+ oscillations in Arabidopsis guard cells expressing
fluorescent dye (yellow cameleon 2.1)
J. Schroeder, et al. 2001
Proposed regulators mediating guard cell response to external ABA
NB: integrated involvement of transporters, pumps, sugar metabolism
J. Schroeder, et al. 2001
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 and
engineering drought hardiness in plants. Nature 410: 327-330.
•
Zhang, J., and W.J. Davies. 1989. Abscisic acid produced in dehydrating roots may enable the
plant 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 to
prolonged soil drying and the involvement of zylem sap ABA I the regulation of stomatal
behaviour of sunflower plants. New Phytol. 113: 167-174.
•
Brodribb, T.J., and N.M. Holbrook. 2003. Stomatal closure during leaf dehydration, correlation
with 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.