Water transport

• Water evaporates from intercellular spaces in the leaves

• Negative hydraulic pressure (water potential)

• Cohesive forces among water molecules —> integrity of water column

• Pores on the learf surface to regulate gas exchange

• Close at night and under drought

• Initial water transport driven by Osmosis

• apoplastic (through cell wall) and symplastic (through symroute cytoplasm)

• Tracheids in Softwoods (pits)

• Vessels in HW (perforation plates)

• Waterflow as a physical process

• Follows path of least resistance (from higher to lower water potential)

• Theoretical tensile strength of water: -130MPa

Advantages:

• If soil is very dry, water can be pulled out from the roots (follows water potential)

• Saves energy

Disadvantage:

• If stomata are closed —> no water transport

• If there is little transpiration (verdunstung) —> no water transport —> but Xylem sap contains solutes (nutrients & sugar) required for growth

• Abprupt transition from metastable liquid to gas phase

• Gas bubble formed —> expand in water column under tension —> gases can not resist tensile forces

• Staining methods (färbemethode)

• Xray microthomography

• Freezing induced: bubbles expand when ice melts and xylem water is under tension from transpiration

• Drought induced: air is sucked through a pit into functional water filled conduit

• Percentage loss of conductance for full range of xylem potential

• P50: pressure at which 50% of conductance is lost due to cavitations

• Centrifuge method (Cavitron)

  • Stem segment span in centrifuge

  • Centrifugal forces are converted to water potential

  • Percentage of loss is determined for different water potentials

• Alternative methods: micro CT etc.

• Curves differ among species

• Not possible if Xylem is under tension

• Depolymerization of starch to sugar in neighboring parenchyma cells to increase osmotic potential

• Some species are able to refill embolism under positive xylem pressure at night (unlikely to occur)

Tyloses form as parenchyma cells from adjacent rays protrude into embolized vessels

—> comparmentalize embolized vessels from still functional vessels

• Hagen posseuilles law: Durchflussmenge an Wasser ist abhängig vom radius hoch 4

• More wood cross section is needed to contain many small vessels having a combined conductivity of a few large vessels

• Providing a safety net

• Gurantee a min level of water transport

• Extreme drought cavitates most earlywood vessels

• All earlywood vessel are cavitate in winter or even before

• water transport for newly forming ring

• 
  

• Tracheids are subjected to enormous tension during drought stress

• Tracheids at the risk of implosion

• Biomechanical resistance to implosion: t/bhoch2

• t: cell wall thickness

• b: lume breite

• HW: wide vessel conduct water more efficiently; narrow vessels are more resistant to dysfunction

• SW: link between tracheid size and hydraulic failure is more complex; tracheid size and cell wall reinfocement are negatovely linked indicating a trade-off between hydraulice eff. and safety

  
  

• Xylem network offers alternative pathways when specific conduit element is embolized

• Connectivity of Xylem also increase the risk of cavitation spreading from one conduit to another

• Allocating space for water conducting cells reduces space available for cells providing mechanical strength and support and storage