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

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

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

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

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

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

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

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

Boundary layer Thickness (da) is dependent on wind speed, leaf shape, roughness of his surface, presence of trichomes ( 0.01 - 1 mm) da = 4 L/v L - leaf length in the direction of air flow, v - wind speed

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

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 1.7 - 28.6 % 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.

Leaf epidermis structure

Cuticle matrix

Arrangement of cuticular waxes on leaf surface

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

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 20 - 2 000 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

Stomatal heterogeneity on leaf area of Commelina communis

Stomatal patchiness in Nicotiana tabacum

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

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