Lecture 1: Xylem, the vulnerable pipeline

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Lecture 1 Xylem, the vulnerable pipeline

• Teaching Aims to introduce the structure and
  driving forces for transport of water in the
  xylem, and appreciate that plants operate a
  delicate balance between maximising water
  transport and the risk of cavitation and loss of
  hydraulic conductance
• Learning outcomes to understand that the
  function of xylem is a trade-off between
  structural support and water transport
  criticisms of the cohesion-tension theory are not
  supported by recent evidence suggesting that
  living cell processes contribute to functioning
  and repair of cavitated vessels

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• 1.1 Soil-Plant-Atmosphere continuum
• 1.2 Water Relations of cells and tissues osmotic
  adjustment
• 1.3 Xylem structure and function
• 1.4 Cohesion- tension and rates of sap flow
• 1.5 Cavitation and embolism ecological
  trade-offs
• 1.6 Repair of cavitation
• Key reference Tyree MT and Sperry JS (1989)
  Vulnerability of xylem to cavitaion and embolism
  Ann. Rev. Pl. Physiol. Mol. Biol. 40, 19-38
• Useful background text Lambers, Chapin and Pons
  (1998)
• Chapter 3 Plant Physiological Ecology, Springer

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• 1.1 Soil-Plant-Atmosphere continuum

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• 1.1 Soil-Plant-Atmosphere continuum
• Water transport along the SPAC consists of a
  liquid water moving along a gradient with the
  following driving forces
• Into the roots- it is the water potential between
  soils and cells of the roots, with transport
  across a semi-permeable membrane
• From roots to leaves- it is a gradient of
  negative hydrostatic pressure in the xylem
• From leaves to the atmosphere- it is the vapour
  pressure gradient, which ultimately drives the
  whole process
• Components and conventions of water potential
  terminology
• ?w ?s ?p Water potential solute potential
  (-ve) turgor potential (ve)
• ?w P - p Water potential Turgor pressure
  (ve) minus osmotic pressure (ve) try
  equations with ?w -1.0 MPa , ?s -1.2
  MPa or p 1.2 Mpa what are ?p and P equal to?

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• 1.1 Soil-Plant-Atmosphere continuum
• Water flux, J, (mm3 s-1) reflects the rate of
  water movement between two points in the SPAC as
  a function of the gradient
• Soil water availability depends on clay and
  organic content, with the hydrostatic pressure
  often described as a matric potential (-ve) this
  becomes more negative as water content decreases
  from field capacity towards permanent wilting
  point, as soil water becomes increasingly
  difficult for the plant to extract from soil
  pores
• A positive root pressure can be generated by
  the transport of ions into the xylem, as
  described by Prof Leigh
• Water movement through the xylem requires less
  pressure that movement through living cells
• a typical sap flow rate in the apoplastic xylem
  of a tree will require a gradient of 0.02 MPa
  m-1 for symplastic flow across the membranes of
  plant cells, we would need a gradient of 2.108
  MPa m-1 so the gradient would need to be 10
  billion times greater- showing the efficiency of
  water flux in the xylem
• The leaf to air vapour pressure difference is the
  driving force for transpiration

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• Data for Hammadia scoparia, a C4 plant
• Plants re-equilibrate over night with soil water
  .
• Pre-dawn ? tracks soil ?
• Midday leaf ? is not in equilibrium as recharge
  by xylem lags behind evaporation
• Note that from June to August ?w is more negative
  than ?s by day.
• what are the implications for leaf turgor?

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• 1.2 Water Relations of cells and tissues osmotic
  adjustment

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• 1.2 Water Relations of cells and tissues osmotic
  adjustment
• For a drying soil, ? progressively declines with
  time
• Each day, ? leaf declines then recovers and
  re-equilibrates at night it, too, progressively
  declines each day
• If ?s (or p in this diagram) does not adjust,
  turgor is zero by day 5
• If the cell sap solute concentration increases,
  leading to a decrease in ?s (or increase in p),
  then turgor loss point is not reached until day
  7
• Osmotic adjustment can be brought about by the
  accumulation of ions and charged organic solutes
  in the vacuole, but the cytoplasm need s to
  accumulate compatible solutes
• These include proline, glycine betaine and sugar
  alcohols (polyols)
• 1.3 Xylem structure and function
• Tracheids are elongated, spindle shaped cells,
  which overlap and transfer water by circular pits
  in lateral walls, often as pit pairs
• Vessel elements are fond only in angiosperms and
  the Gnetales- shorter, wider with a perforated
  end plate stacked end to end vessel at end
  of vessel, transfer via pit pairs
• Fibres evolved from tracheids
• Stiffening prevents collapse under high tensions
  created by water transport

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Perforation plates and pits in oak vessels

• 1.3 Xylem structure and function

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• 1.4 Cohesion- tension and rates of sap flow
• Cohesion-tension- proposed early in the last
  century, how trees up to 100m tall raise perhaps
  50 to 100 litres per day to the canopy
• A perfect vacuum pump can only raise water 10m,
  but a capillary 20mm in diameter supports a
  column 0.75m by capillary action.
• Water in stem in under tension- negative
  hydrostatic pressure- due to capillarity of xylem
  and cohesion of water molecules (remember the
  operation of the pressure bomb)
• Can trees really support such tensions- Zimmerman
  says no, with evidence from the xylem pressure
  probe (but everyone else thinks there is a leak
  or the probe breaks the column) Canny also
  disagrees.
• Latest evidence creates tensions in stem segments
  using a centrifuge, and checking the balance
  pressure with a pressure bomb- good agreement
• Water column can in theory support a pressure
  difference of 30 Mpa for a tree 100 m x 0.02
  MPa m-1, 2 MPa required
• In practice, the weight of the water column adds
  1 MPa pressure at the bottom of the tree
  THEORETICAL TOTAL 3 MPa

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• The hydraulic conductance is proportional to the
  fourth power of the vessel diameter
• Many small diameter vessels are therefore
  considerably less efficient than a few large
  diameter vessels
• Vessel length often correlates more with pore
  width than vessel diameter
• Seasonal variations in vessel length and diameter
  give year rings in ring-porous trees, but are
  randomly produced in diffuse porous
• The tradeoff is with structural support the stem
  of a liana has the same hydraulic conductance as
  a tree with a tenfold greater sap area

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• 1.5 Cavitation and embolism
• Pits provide the greatest resistance in the
  pathway
• Most are simple, but a more complicated structure
  with a centrally thickened torus is found in
  conifers
• Under high tensions, a bubble of air can enter
  through a pit membrane
• Water then evaporates explosively into the
  bubble, registered as an acoustic event-
  cavitation
• Surface tension in the pit membranes prevent the
  transmission of the gas water meniscus- so
  cavitation is contained

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• 1.5 Cavitation and embolism ecological
  trade-offs
• Embolisms restrict conduction of water, and can
  limit growth of herbaceous plants and trees
• In Zea mays, daily water flow can be reduced by
  50 by embolisms in Acer saccharum (sugar maple)
  flow in the main trunk was 31 at the end of
  summer and 60 by the end of winter, with many
  small twigs completely blocked

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• In beech, seasonal cycle of cavitation in summer
  and winter
• Maximum conductivity occurs in spring to coincide
  with budburst
• Cavitation can be caused by drought and
  freeze-thaw cycles
• Conifers from cold climates- more likely to be
  transpiring when freezing occurs
• hence small tracheids are less vulnerable but
  restrict sap flux
• Ring porous cannot refill overwintering xylem
  therefore leafing-out is later to avoid frosts
• Pathogens induce embolisms

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• Vulnerability to cavitation is also related to
  desiccation tolerance
• Plot loss of hydraulic conductance as a function
  of ? in the xylem
• Differences in xylem anatomy reflects a trade-off
  between large xylem diameter (maximising
  conductance) and strength (minimises cavitation)
  remember the vines
• For conifers Juniper is less vulnerable than
  Abies (balsam fir)
• For hardwoods Acer (Maple) is more vulnerable
  than mangrove
• The risk of cavitation differentiates mesic and
  drought adapted species, and investment in
  transport matches seasonal water supply

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• Cavitation tradeoff between conductance and
  safety
• safe xylem is less efficient at conducting
  water
• Safety margins differ between species
• in mesic habitats, plants operate close to
  tensions causing 100 cavitation (0.4 to 0.6
  MPa)
• in arid habitats, where water deficits may last
  for months, (eg Larrea, creosote bush) 100
  cavitiaon occurs at 16MPa, whereas minimum water
  potentials are closer to 9MPa
• Pit membrane diameter is the key to reducing
  extent of cavitation, more than vessel diameter
• Length of vessel also critical, so short and
  narrow conduits close to nodes or junctions in
  stems prevent runaway embolism and can be used
  as safety zones to isolate stems and twigs
• dieback is a common occurrence in semi-arid
  shrubs, due to cavitation
• Gradation in height is also associated with
  increasing water deficits in semi-arid regions

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• 1.6 Repair of cavitation
• The cure force the air back into solution
• Root Pressure at night, active ion influx
  continues though transpiration is reduced,
  generating 0.1-0.2MPa (also guttation)
• In Betula (birch) and Vitis (grape) large root
  pressures develop in spring- leading to frothing
  and sap dripping from pruned vines!
• Where root pressure is insufficient, in many
  woody species, there was some evidence that
  cavitation could be repaired on a daily basis,
  leading Canny to suggest a role or parenchyma in
  pressurisng tissues
• Missy Holbrook and M Zwienicki (1999 Plant
  Physiology, 120, 7-10 see also paper that
  follows by Tyree et al) provide a solution
  activity of living cells adjacent to the xylem is
  required
• Girdling or MgCl2 in transpiration stream reduces
  recovery from embolism
• Is there a role for a mechanism similar to that
  during root exudation? But xylem sap osmotic
  pressure in repairing elements not enough.

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Integral membrane proteins (MIPS and TIPS)- water
selective channels in plants- aquaporins

• The key to the process is hydraulic isolation-
  which contrains the embolism, and allows local
  positive pressures to force gas back into
  solution
• Xylem wall must be relatively impermeable, with
  droplets remaining attached to the wall by the
  contact angle generated
• Influx of water compresses the gas phase (black
  arrows) with air forced into solution (yellow
  arrows)
• Bordered pit geometry (inset), as an inverted
  funnel, prevents water from entering the pit
  channel until the lumen is entirely filled-
  thereby stabilising the restoration of hydraulic
  continuity and minimising the volume of
  undissolved gas

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From Holbrook and Zwienicki, 1999
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• This mechanism allows the tension in adjacent
  vessels to be transmitted to the refilling
  conduit as soon as the advancing water contacts
  the pit membrane
• The previous photograph, taken from Tyree et al
  1999, Plant Physiol 120,11-21, shows the
  refilling process occurring in bay (Laurus
  nobilis)- using cryoscanning EM. (A) shows
  cavitated vessels (B) shows one cavitated vessel
  and one with small air bubbles (C) shows sap
  adhering to the cell wall and (D) shows water
  droplets entering the vessel
• Reversibility of embolisms makes the balance
  between damage and repair a much more dynamic
  process
• But what is the evidence for such an active
  process? Once again, Zimmerman tries to use it to
  disprove the overall cohesion tension theory in
  resurrection plants
• But Missy strikes back Zwienecki et al 2001
  Science 1059-1062- Hydrogel control of hydraulic
  resistance in plants..

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• Hydraulic conductance is increased by adding salt
  solutions to xylem sap
• Microchannels in the pit membranes are altered by
  the swelling and contraction of pectins
  (hydrogels)- allowing the xylem to alter internal
  rates of flow and highly relevant to root shoot
  signalling in response to drought, which we will
  meet later .

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• Conclusions
• Strictly speaking, the driving forces for
  movement of water through the SPAC vary between
  soil-root, xylem and leaf- air
• Understand the different potential/pressure
  terminologies used to describe solute and turgor
  interactions with water potential- well need it
  for guard cells!
• The xylem, thought to be an inert, apoplastic
  pathway is the most effective way to transport
  water long distances with pit membranes and
  diameter of the conducting elements the most
  important resistance in the pathway
• Plant water potential must track soil water
  supply, although osmotic adjustment can help to
  maintain gradients of water uptake uder drought
  and saline conditions
• There is a trade-off between vessel diameter,
  trunk strength and susceptiblity to cavitation
• Embolisms are acoustically detectable as the
  water column, seeded by air, snaps water flow
  can proceed around the blockage via pits, but
  daily and seasonal restrictions in water flow are
  significant

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• Cavitation can be caused by drought and
  freeze-thaw cycles, with some plants capable of
  restoring the connection in spring using root
  pressure
• The risk of cavitation marked driver of lifeform
  and ecological adaptations to drought- with a
  reduction in height, denser, smaller xylem
  conduits and the strategy of segmentation
  (shedding stems) common in semi-arid shrubs
• Safety margins depend on how reliably a plant can
  refill or restore conductivity on a daily or
  seasonal basis
• Facilitated movement of water through aquaporins
  may help to restore water content of cavitated
  vessels, with the isolation helping to force air
  back into solution and prevent runaway caviation
• The xylem pathway is clearly not as inert as once
  thought!!!