Figure 4.10 Water pathway through the leaf
Figure 4.12 Electron micrographs of stomata; (B) Stomatal complexes of Carex
Figure 4.13 Electron micrograph showing a pair of guard cells from the dicot tobacco
4.16 Radial alignment of the cellulose microfibrils in guard and epidermal cells
1
Figure 4.10 Water pathway through the leaf
Water movement into and out of guard cells is controlled
by ion transport processes which causes changes to cell s and p
Pumping K+ into guard cells increases p - stomatal opening
Pumping K+ out of guard cells decreases p - stomatal closing
Ohm’s Law Analogy for Transpiration
Transpiration – Water Loss from Leaves
Current (amps) 
Transpiration is a flux of water vapour from a leaf
Potential Difference (volts)
Resistance (ohms)
Diffusion of water vapour out of leaf is controlled by:

(i) a driving force – which is the gradient in water vapour
concentration (or partial pressure) between the leaf and the air
(ii) resistance to water loss which is controlled primarily by
stomatal opening
Transpiration Rate = Driving Force / Resistance
Conductance = 1/Resistance
Transpiration Rate = Driving Force x Conductance
Abbreviations used in Transpiration and Conductance Calculations
Symbol Definition
Usual Units
E
gw
P
ei
ea
mmol m-2 s-1
mmol m-2 s-1
kPa
kPa
kPa
Transpiration rate
Conductance to water vapour
Atmospheric pressure
Intercellular water vapour partial pressure
Ambient water vapour partial pressure
Ohm’s Law Analogy for Transpiration
E  gw
Further subscripts can be applied to the symbol for conductance (gw) to
indicate which section of the diffusion pathway is being referenced.
For example, if you are referring to conductance over the total pathway
from the intercellular air spaces to the atmosphere, the total conductance
term is indicated as:
gw-total
If you are referring only to stomatal conductance:
gw-stomata
If you are referring only to boundary layer (BL)
conductance:
gw-BL
(ei  ea )
P
Transpiration (E) = conductance (gw) x driving force (VPD)

2
Atmosphere
Epidermis
Internal Air Space
Leaf Cells
Xylem
Liquid
Water
ea
ei
Humidity Concepts
1.
2.
3.
4.
Saturation Vapour Pressure
Relative Humidity
Actual or Absolute Humidity
Vapour Pressure Difference (VPD)
Water Vapor
Water Vapor
Relative Humidity
<< 100 %
Liquid
Water
Relative Humidity
100 %
4.12 Concentration of water vapor in saturated air as a function of air temperature
Humidity Concepts
1. Saturation Vapour Pressure
The absolute amount of water vapour the air can hold when
saturated varies as an exponential function of air temperature
Saturation Vapour Pressure (kPa) = 0.611 e ((17.502 T)/(T + 240.97))
Saturation Vapour Pressure (kPa)
where e is the base of the natural logarithm and
T is air temperature in °C
Saturation Vapour Pressure Varies with
Temperature
14
12
10
8
6
4
2
0
0
10
20
30
40
Air Temperature (°C)
50
Humidity Concepts
Humidity Concepts
2. Relative Humidity
3. Absolute Humidity
The ratio of the absolute amount of water vapour in the air divided
by how much the air could hold if it was saturated
Relative Humidity (RH) =
Actual or Absolute Humidity (kPa)
Saturation Vapour Pressure (kPa)
RH may be expressed as a proportion (as done here) or as a percentage
The absolute amount of water vapour in the air
It is calculated from measurements of air temperature and air RH
Air temperature is used to determine the saturation vapour pressure
Absolute Humidity (kPa) = Saturation Vapour Pressure (kPa) x RH
3
Transpiration – Water Loss from Leaves
Humidity Concepts
4. Vapour Pressure Difference (VPD)
The difference in absolute vapour pressure between the leaf
intercellular air spaces (ei) and the well-mixed air outside of
a leaf (ea)
VPD (kPa) = ei (kPa) – ea (kPa)
• Solar radiation energy input causes the
water-to-vapour phase change
• Driving force is a vapor pressure gradient (VPD)
– Depends on air temperature and the water vapor content
in the bulk well-mixed air and also on leaf temperature
• Stomata are usually the major resistance
– Stomatal conductance varies and is dependent on:
•
•
•
•
Soil moisture
Vapor pressure of air
Light intensity
CO2 concentration of air
Stomatal Conductance (g)
Conductance (gw) is calculated by re-arranging
the Ohm’s Law analogy equation
E  gw
gw 

(ei  ea )
P
EP
(ei  ea )
Humidity Response
Light Response
PPFD =
Photosynthetically active
Photon Flux Density
Soil Moisture Response

• Boundary layers can also be a resistance to
water loss from leaves
A boundary layer develops as air flows across a leaf
Boundary layer conductance depends on
• Leaf size and leaf shape
• Wind speed
This boundary layer retards the transfer of CO2
and H2O from the leaf to the surrounding air
4
Figure 4.11 Dependence of transpiration flux on the stomatal aperture of zebra plant
Plant Response to Water Stress
• water shortage - drought
• water excess – flooding
Textbook Reading:
Chapters 3 & 4 All
Chapter 26: pp. 756-760
pp. 765-772
Plant Response to Water Stress: Drought
MPa
-0.03 -0.2
-0.81 -1.60
Soil Water Potential
Figure 26.4 Effects of water stress on photosynthesis and leaf expansion of sunflower
Figure 26.1 Dependence of leaf expansion on leaf turgor
5
Osmotic Adjustment
Figure 26.10 Solute adjustments during osmotic stress
• it is possible to decrease w by decreasing s
- this allows high p which is required for growth
• this can be actively done in living cells by
accumulating compatible solutes in the cytoplasm
• compatible solutes: sugars, sugar alcohols, proline
w=-0.9 MPa
• ions can be compartmented into the vacuole to
prevent the charged molecules from interfering
with enzyme activity in the cell cytoplasm
Pressure potential (p)
Figure 26.11 Four groups of molecules frequently serve as compatible solutes
Drought Induced Reduction in Stomatal Conductance:
Mechanisms and Signals
Initial ideas about water stress-induced reduction in
stomatal conductance assumed that reductions in leaf
water potential and pressure potential (p or turgor)
were the mechanism that controlled changes in guard cell
turgor and reduced stomatal opening.
This is what occurs in primitive (non-seed plants).
However, in more advanced seed plants (conifers and
angiosperms) stomatal closure is part of a complex
signaling system involving the plant hormone,
Abscisic Acid (ABA).
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