1. Plant water regime Transpiration Leaf energy balance Fick laws
Boundary layer
Cuticle
Stomata

2. Leaf energy balance Energy sources: Energy balance:
shortwave sun radiation
longwave radiation emitted by sky and surrounding
Solar radiation might be reflected, absorbed or transmitted (according to wave length)
Energy balance:
SRnet + LRnet + C + E + M = 0
SRnet, LRnet - absorbed short-wave or long-wave radiation, C - heat transport (sensible heat flow), E - heat used for vaporization (latent heat flow), M - energy consumption or production by metabolism
Bowen ratio = C/E

3. Effect of plant on energy balance Absorbance, reflectance and transmittance is affected by leaf area, orientation in space, quality of leaf surface, leaf anatomy Latent heat flow by transpiration

4. Penman-Montheith equation
s Rnet + c (es-e)/ra
E = 
s +  (ra+rs)/ra
E - latent heat flow
s - change of water vapour pressure with temperature,
Rnet - absorbed radiation,
 - density,
c - specific heat,
es - e – difference between saturated and actual water vapour pressure,
 - psychrometric constant,
ra - boundary layer resistance,
rs - stomatal resistance

5. Fick laws 1st Fick law: Jv = -D (dc / dx) 2nd Fick law:
dc / dt = - dJv / dx = D (d2c / dx2)
Jv - transport rate, D - diffusion coefficient, dc/dx - gradient of concentration,
t - time
sub-stomatal cavities, stomata, boundary layer
Fick laws

6. Transpiration rate E = gl c gl = gs + gc 1/gl = 1/ga + 1/gs + 1/gi
E - transpiration rate mmol m-2 s-1 or g m-2 s-1
gl - leaf conductance (gl = -D) mol m-2 s-1 or m s-1
c - difference in water vapour concentration between ambient air and air in sub-stomatal cavities (dc/dx) mmol mol-1
gl = gs + gc
1/gl = 1/ga + 1/gs + 1/gi
gs - stomatal conductance, gc - cuticular conductance,
ga - boundary layer conductance, gi - conductance in intercellular spaces
g = 1/r, r - resistance

7. Transpiration stomatal (Es), cuticular (Ecu) and peristomatal (Ep)

8. Water vapour concentration in substomatal cavity and boundary layer
cl = cs ew(Vw/RT)
cl - water vapour concentration in sub-stomatal cavity,
cs - saturated concentration,
w - leaf water potential
Vw - molar volume of water
R - gas constant
T - temperature

9. Boundary layer
Thickness (da) is dependent on wind speed, leaf shape, roughness of his surface, presence of trichomes
( mm)
da = 4 L/v
L - leaf length in the direction of air flow, v - wind speed

10. Relationship between transpiration rate and stomatal conductance is affected by boundary layer thickness (wind speed)

11. Cuticle Cuticle - adaptation to drought
Structure and composition: upper layer of the cell wall is impregnated by cutin and waxes (endocuticular or epicuticular waxes)
Cuticular conductance for water is % of stomatal conductance
Cuticular conductance for CO2 is only 6 % of that for water
Cuticular conductance has higher importance when stomata are partially or completely closed
Cuticular conductance is dependent on plant species, age and conditions. It is very low in species adapted to dry conditions, in contrast, it is high in plants in vitro. Usually it increases during leaf ontogeny and decreases during dehydration.

12. Leaf epidermis structure

13. Cuticle matrix

14. Arrangement of cuticular waxes on leaf surface

15. Effect of air humidity on permeability of cuticle in different plant species

16. Stomata
Area of apeture of all stomata is about 1 % of leaf area, nevertheless, water vapour efflux corresponds to that from free water
Two types: kidney-shaped and dumbbell shaped
Development during leaf ontogeny
Stomatal density mm-2, different size
Amfistomatic and hypostomatic leaves, adaxial/abaxial ratio, e.g.:
wheat 33/14, maize 48/52, oat - 25/23, sunflower 85/156,
tomato 12/130, apple tree 0/235
Sun/shade leaves : beech 113/416, hornbeam 170/365
Heterogeneity on one leaf area
Stomatal patchiness

18. Stomatal heterogeneity on leaf area of Commelina communis

19. Stomatal patchiness in Nicotiana tabacum

20. Methods 1) Gravimetric methods
2) Transpiration curves (water loss by detached leaves) enable to differentiate stomatal and cuticular transpiration
3) Determination of transpiration rate from the changes in air humidity in leaf chamber
4) Calculation of transpiration rate from measurements of water flow in xylem
5) Calculation of evapotranspiration from energy balance

22. Methods 1) Cuticular conductance
a) transport HTO or fluorescent dyes in isolated cuticle
b) measurement of water efflux from epidermis without stomata or with closed stomata
2) Microscopic methods for determination of stomatal density, shape, size and aperture
a) in situ
b) microrelief (replica) methods
3) Determination of stomatal conductance
a) diffusion porometers
b) mass flow porometers
c) calculation from transpiration rate