Plant water regime
• Transport of water in xylem
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Transport soil-plant-atmosphere
Cohesion theory
Cavitations
Methods
Longitudinal water transport
• Transport of water in every parts of the whole pathway is dependent on
the respective gradient of water potential and conductance:
• Jw = Lrr wrr = Lrl wrl = Ls ws = Lll wll = Llr wlr
• It is not possible connected transport of liquid water and transport of
water vapour in one pathway as the former is mass flow with gradient
of water potential as driving force and the later is diffusion with
gradient of air humidity as driving force
• Relationship between w and air humidity is not linear:
• w = (RT / Vw) ln(e/es)
Transport in xylem and phloem
Capacitances
• Jw = Jwl  Jwc
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Jw - total water transport,
Jwl - longitudinal water transport,
Jwc - transport of water from/to
capacitances
• Diurnal changes in transpiration
and absorption
• Volume changes in tree trunks,
stems, fruits;
dependent on plant species, age
and environmental conditions
Radial pattern of sap flow in different parts of tree trunks with different wood anatomy
Sap flow in veins corresponds to transport in capillaries
• Poiseuille law
• Jw = (r4/8)  (-dP/dx)
• Jw - volume flow per tube, r - cylinder radius,  - coefficient of
viskosity, dP/dx - gradient of hydraulic pressure
• for xylem in stem: dP/dx  ws; r4/8  L ws
• High conductance (e.g. maize vein radius for protoxylem 10 m,
primary metaxylem 20 m, late metaxylem 100 m)
• Transport in veins is usually not limiting
• Viskosity is temperature dependent
Cohesion theory
Hydrogen bonds
 surface tension ( = 0.07 N m-1 při 20 C);
 cohesion
Adhesion: capillary rise (adhesioncohesion) or
depression (adhesioncohesion)
h = (2cos)/rg
r - radius,  - density, g - gravitation
Large veins are joined to system of
microcapillaries with radius 0.1 - 0.01 m
Water columns must be continuous
Due to transpiration water in xylem is under
tension (negative pressure, less than
atmospheric).
dp/dx  0.01 MPa m-1
p is very variable according to transpiration or
absorption
During growth the water columns are prolonged
Interruptions of water columns
• Cavitations (embolism) occur at high transpiration or during
freezing and thawing
• homogenous - vaporization of water at extreme tensions
• heterogenous - bubbles of air from intercellular speaces
• larger vein radius - larger conductance but larger danger of
cavitation
• cavitations are also dependent on properties of vein walls
• Xylem water potential at cavitation (cav) is species specific
and dependent on environmental conditions
Cavitation induced loss of conductivity in different species
Cavitation induced loss of conductivity under different conditions
Occurrence of cavitations in different plant parts and
their persistence
• cav is different in different plant parts
• usually higher in peripheral parts such as petioles or roots than in
central parts (stems or trunks)
• dependent on environmental conditions – usually decreases under
drought or salinity
• Cavitations might be:
• irreversible – cavitated veins are out of operation and they are replaced
by new ones – wood annual rings
• reversible after long time – importance of root pressure
• reversible after short time – refilling of water from neighbour tissue
(parenchyma, phloem)
Diurnal course of hydraulic conductance and leaf water potential in
Caryocar brasiliense and Schefflera macrocarpa
Hydraulic signals
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Mechanism is still not quite clear
Stomatal conductance and in consequence transpiration rate is decreased when
vein embolism occurs. Thus the tension does not further increase and
cavitations in other veins are not created.
The relationships between stomatal conductance and hydraulic conductance or
transpiration rate and hydraulic conductance are observed frequently
Hydraulic conductance limits maximum transpiration rate
Capacity for water transport limits maximum size of whole plant and
individual organs
Methods
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1) Gravimetric methods
a) determination of water absoption and transpiration in laboratory conditions
b) determination of water consumption in field conditions - lysimeters (under
sufficient water supply they are also used for determination of
evapotranspiration of artificial or natural stands)
2) Measurement of water uptake by potometers
a) for whole root system
b) for individual parts
3) Measurement of root pressure
a) determination of amount bleeding sap after shoot excision
b) by pressure probe
Gravimetric method
Contemporary determination of
water absorption and
transpiration rates in laboratory
conditions
Potometers
Pressure probe adapted for determination of root pressure
Measurements of water transport in xylem
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transport of HDO, HTO, H218O, dyes
transport of heat
nuclear magnetic resonance, e.g. NMR imaging
determination of flow rate under pressure (e.g. high pressure flow
meter)
adaptation of pressure probe
calculation of hydraulic conductance from transpiration rate and w
gradient
determination of cavitations by acustic methods
determination of cavitations by microskopic methods
Heat transport by transpiration stream