Plant water regime Transport of water in xylem –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: J w = L rr  wrr = L rl  wrl = L s  ws = L ll  wll = L lr  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 / V w ) ln(e/e s )

Transport in xylem and phloem

Capacitances J w = J wl  J wc J w - total water transport, J wl - longitudinal water transport, J wc - 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 J w = (  r 4 /8  )  (-dP/dx) J w - volume flow per tube, r - cylinder radius,  - coefficient of viskosity, dP/dx - gradient of hydraulic pressure for xylem in stem: dP/dx   ws ;  r 4 /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  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 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 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 transport of HDO, HTO, H 2 18 O, 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