Chapter 4-2
Transpiration – diffusion of water vapor
Transpiration – diffusion of water vapor
 Diffusion is primary means of any further
movement of the water out of the leaf. That is
water movement is controlled by the
concentration gradient of water vapor.
 Two factors determine transpiration :
Difference in water vapor concentration (Cwv)
Resistance of the pathway for vapor diffusion
(Diffusional resistance)
Leaf stomatal resistance (rs)
Boundary layer resistance (rb)
 E=
Cwv (leaf) – Cwv (air)
rs + rb
E : transpiration rate (mol/m2/s)
rs : resistance at the stomatal pore (s/m)
(leaf stomatal resistance)
rb : resistance due to the boundary layer
boundary layer : the layer of unstirred air at the leaf surface
(boundary layer resistance)
However, it is difficult to measure the Cwv (leaf). Sometimes
vapor pressure are used instead of concentrations, and the
difference is called “water vapor pressure deficit”.
Water vapor pressure deficit = Pwv (leaf) - Pwv (air)
Air boundary layer
 A thin film of still air on the surface of leaf and its
resistance to water vapor diffusion is proportional
to its thickness. The thickness of the boundary
layer is determined primarily by wind speed.
 When air surrounding the leaf is still, increases in
stomatal aperture have little effect on
transpiration rate. The thickness of the
boundary layer is the primary deterrent to water
vapor loss from the leaf.
 When wind velocity is high, the stomatal
resistance has the largest amount of control over
water loss.
Relative humidity (RH)
 RH = Cwv / Cwv (sat.)
 The Cwv (sat.) is strongly dependent on temp.
Water loss
 Water loss is regulated by :
Difference in water vapor concentration
The pathway resistance
 The concentration gradient for CO2 uptake is smaller than
the concentration gradient driving water loss.
 How can plant prevent water loss without simultaneously
excluding CO2 uptake?
 Temporal regulation –
 When water is abundant :
At night, no photosynthesis
Stomata close, preventing unnecessary loss of water.
Sunny morning, photosynthesis is demanding, supply
of water is abundant.
Stomata are wide open, decreasing the stomatal
resistance to CO2 diffusion.
 When soil water is less abundant
The stomata will open less or even remain closed on a
sunny morning.
Transpiration ratio
 A measure of the relationship between water loss and
carbon gain
 Transpiration ratio = moles of H2O transpired
moles of CO2 fixed
 The factors that cause large ratio of H2O efflux to CO2
influx :
 The concentration gradient driving water loss is about
50 times larger than driving CO2 influx.
 CO2 diffuse slower than water does.
MW of CO2 > MW of H2O
 CO2 has longer diffusion path
membrane -> cytoplasm -> chloroplast envelop ->
chloroplast
In dicot and non
grass monocot
Usually no
subsidiary cell
Kidney-shaped
guard cells
Stomata
Grass
stomatal
complex
pore
Oat
35
34
Ventral wall
(thicker ~ 5mm)
dorsal wall
(thinner)
Ion and organic molecules
Guard cell
s Decrease (s = -RTCs)
w decrease
Water moves into the guard cell
p increase
Stoma open
The mechanism of guard cell movement
The three principal raw materials for
photosynthesis
 H2O
 CO2
 Light energy
 Monocot leaf does not have palisade and
spongy mesophyll cells.
 Palisade cells generally have larger numbers of
chloroplasts than spongy mesophyll cells.
 The conc. of chlorophyll is also higher in
palisade cells than in spongy cell (1.5 ~ 2.5
times higher).
 Sieve effect : Since there is a significant
proportion of the cell volume that does not
contain chloroplast, a substantial amount of
light may pass through the first cell without
being absorbed.
 The major role of stomata is to allow entry of
CO2 into leaf for photosynthesis while
preventing excessive loss of water.
 However, plant can not uptake CO2 and
simultaneously preventing water loss.
 Stomatal complex (stomatal apparatus)
 Stoma
 Guard cells
 Subsidiary cells
Elliptic type
Kidney-shaped
Graminaceous type
Dumbbell-shaped
 The rate of CO2 diffusion is proportional to the
diameter of the pores, not the area. That is, as
the pore size decrease, the diffusion rate (mg
CO2/hr) of CO2 decrease proportionally.
 However, as the pore size decrease, the efficiency
of CO2 diffusion per unit area increases several
fold.
Are the stomata on the upper or lower
leaves?
Oat
35
34
Ventral wall
(thicker ~ 5mm)
dorsal wall
(thinner)
The mechanism of guard cell movement
 Driving force -> osmotic uptake of water ->
increase of hydrostatic pressure -> push the thin
dorsal walls outward into the neighboring
epidermal cells. -> cause cell to arch.
 In dumbbell-shape guard cells, the bulbous ends of
the cells push against each other as they swell,
driving the central handles apart in parallel and
widening the pore.
 In 1960, it became evident that K+ levels are high in
open guard cells and very low in closed guard cells.
 Upon opening, large amounts of K+ move from the
subsidiary and epidermal cells into the guard cells.
Ion and organic molecules
Guard cell
s Decrease (s = -RTCs)
w decrease
Water moves into the guard cell
p increase
Stoma open
Mechanism
 Proton extrusion
Apply fusicoccin -> stimulate H+ extrusion ->
stimulate stomatal opening
Vanadate (VO3-, 釩酸鹽) -> inhibit proton
pump -> inhibit stomatal opening
 Establish a pH gradient across membrane
 Influx of K+ by passive uptake through K+
channels
 In most plants, malate is used as counterions to
balance K+ uptake.
 In plants whose guard cells lack chloroplast or
starch, Cl- is also used as counterions.
 The evidence for malate as a counterion
Malate conc. is high in opened stomata.
PEP carboxylase which catalyzed the
formation of malate is high in opened
stomata.
The starch conc. is decreasing in opened
stomata that correlates with the increasing
amount of malate.
ADP + Pi
Osmotic potential decrease
Water potential decrease
Water enter into vacuole
Increase turgor
Stomata open
Mechanism for stomatal close
 Uptake of Ca+2 into the cytosol
 Depolarize the membranes
 Anion channel opened and Cl- and malate released from
the vacuole.
 K+ channel opened and K+ released from vacuole and
subsequently into subsidiary cells.
ADP + Pi
Ca+2
Ca+2
Osmotic potential increase
Water potential increase
Water comes out from vacuole
Ca+2
decrease turgor
Stomata close
Control of stomatal movement





CO2 level
Light
Water stress
Temperature
Circadian rhythms
Effect of CO2 on stomatal movement
 CO2 level
 CO2 conc. decrease => stomata open => to uptake more
CO2
 CO2 conc. increase => stomata close
 The response of the stomata is to the intracellular conc.
of CO2 in the guard cells.
When CO2 level decrease or photosynthesis is
needed, guard cells will take up water and swell to
open the pores, in order to take more CO2.
When CO2 level increase or the water stress override
the photosynthesis, the guard cells will close.
Effect of light on stomatal movement
 Light
 Light (blue and red light) => stomata open (to admit
CO2 for photosynthesis)
 Stomata closed by exposure to high CO2 can be
induced to open slowly if placed in the light.
 CO2 conc. increase -> stomata close -> under light ->
stomata open
 Stomata normally open at dawn.
Dual-beam experiment
The pulse of blue light stimulates a
significant increase in stomatal
conductance (increase stomatal
opening).
Effect of light on stomatal movement
 Indirect effect
High fluence rate => CO2 conc. in intercellular
space is decreasing due to photosynthesis in
mesophyll cells.
Light affects stomatal movement through
regulation of intercellular CO2 levels.
Closure of the guard cells in the dark can be
attributed to the accumulation of respiratory
CO2 inside the leaf.
 Direct effect
Effect of light on stomatal movement
 Indirect effect
 Direct effect
 Blue light has direct effect on stomatal opening.
 At low fluence rate, blue light causes stomatal opening,
but not red light.
 At high fluence rate, stomatal opening under blue light is
consistently higher than under red light.
 Blue light -> blue/UV-A crytochrome -> proton extrusion
-> photosynthesis -> ATP production -> stomatal opening
 Why ?
Light on signal
sunflecks
Effect of water on stomatal movement
 Hydropassive closure
 Water loss from the guard cells exceeds the rate of
movement into the guard cells. => decrease in turgidity of
the guard cells => stomata close
 Hydroactive closure
 ABA (abscisic acid, 離層酸) referred to as an
antitranspirant.
 Plants sense water deficit => ABA is released into the
apoplast and then transferred to the guard cells => stomata
close => ABA conc. is increased => more stomata close
 ABA originally accumulate in the chloroplast -> water
stress -> pH in chloroplast decrease and the pH in
cytoplasm and apoplast increase -> ABA release into
apoplast.
Effect of temperature on stomatal movement
 Temp increase -> stimulate respiration and impair
photosynthesis -> CO2 conc. increase -> stomata
close
 Midday closure
temp increase
-> photosynthesis reduced
water deficit occur -> photosynthesis reduced
Effect of circadian rhythms on stomatal
movement
 An endogenous circadian rhythm control
stomatal opening, but it is still unclear how
the rhythm control stomatal movement.
END