Transport in Plants II (cont.) Water Balance of Plants

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Transport in Plants II (cont.)Water Balance of
Plants

• It is wise to bring some water, when one goes
  out to look for water.
• Arab Proverb

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Big Picture
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Water Relations at 10 m
-0.8
0.2
Hint Think about the water relations of
mesophyll cells, and dont forget their function.
Xylem venation brings water close to every cell
in the leaf (and other living cells).
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Water Use
Water Loss

• Water is pulled from the xylem into the cell
  walls of the mesophyll cells, and evaporates.

Xylem venation brings water close to every cell
in the leaf.
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Transpiration
YP
Y
YS
Ym
Matric potential...
See Fig. 36.11
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Diffusion
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• DY across the plasma membrane and cell wall of
  the mesophyll cells produces tension that draws
  water through the xylem,
• DY is driven by the Dcwv (regulated by stomata),
• Note DYm is driven by the Dcwv.

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H2O versus CO2 Flux
Flux Conductance x Driving Force

• diffusion (concentration gradients),
• H2O representative concentrations
• Inside 1.27 mole m-3,
• Outside 0.47 mole m-3,
• D 0.8 mole m-3,
• CO2 representative concentrations
• Inside 0.0 mole m-3 (ideal),
• Outside 0.014 mole m-3,
• D 0.014 mole m-3,
• H2O gradient 57x greater than CO2.

• determined by pathway and diffusion
  coefficients,
• CO2 pathway is longer,
• through cytoplasm, through two chloroplast
  membranes,
• Conductance for H2O is 8-10x that for CO2.

450 - 600 H2O per every CO2
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Guard Cell Structure

• Features
• plasmodesmata between guard cells, but not
  between guard cells and other cells,
• Chloroplasts,
• typically the only epidermal cells with
  chloroplasts,
• Highly flexible walls,
• radially reinforced with cellulose microfibrils,
• Pore that opens and closes.

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Guard Cell Function
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• One method of regulationsucrose.

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Stomatal Function
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Guard Cell Control

• Light,
• blue light signal transduction,
• ATP synthesis,
• carbohydrates,
• CO2 concentration,
• Circadian rhythms,
• Hormones.

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Big Picture
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Cavitation...you can hear plants cry

• H2O in the transpiration stream is in a
  physically metastable state,
• experimental values for DY required to break a
  pure water column in a capillary tube exceed -30
  MPa,
• -3 MPa exceeds physiolgical requirements of even
  the tallest trees,
• As tension increases in the water, a higher
  probability of gas leaking into the system occurs
  (air seeding),
• Gases do not resist tensile forces, thus the gas
  bubble expands (cavitation).
• Gases also have reduced solubility in ice, thus
  freezing of the xylem sap also causes cavitation.
  

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Cavitationcures

• Bubbles do not spread far because the they do not
  spread through the pores in the pit membranes,
• Reduction of tension in times of limited
  transport might allow the bubble to go back into
  solution,
• Root pressure increases can cause a reduction in
  tension as well,
• Secondary cambium produces new xylem cells.

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Xylem Cells
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Transport in Plants IVPhloem

• There is no sugar cane that is sweet at both
  ends.
• Chinese proverb

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Transport

• molecular and ionic movement from one location
  to another,
• H2O,
• Sugars and other organics,
• Ions,
• Gases,
• Proteins, RNA, Hormones, etc.

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Phloem Transportoverview

• Long distance, bi-directional flow of sugars,
• Sugar alcohols,
• Organic acids,
• Amino acids,
• Hormones,
• Source (phloem loading),
• Sink (phloem unloading),
• Pressure is manipulated at source and sink in
  order to create bulk flow in phloem conducting
  cells.

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Phloem Cellsreview
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Sieve Tube Elements

• Living cells,
• Plasma membrane,
• No nucleus,
• No tonoplast, vacuole,
• Some cytoplasm,
• Sieve plate,
• no membrane between sieve tube members, i.e. does
  not divide!
• P-proteins, slime bodies, callous.

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P Protein and CalloseSlime and wound repair
In Cucurbita Phloem Protein 1 (PP1) Filament
protein, Phloem Protein 2 (PP2) Lectin (plant
defense proteins).
Wound Callose (b-1,3 Glucan) is synthesized on
the plasma membrane for long term solution.
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Phloem Locationreview
Root Stele
Leaf Midrib
Stem Vascular Bundle
Phloem is always in close proximity to xylem.
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Pressure- Flow-HypothesisMunch Hypothesis

• Source
• High concentration of sucrose, via
  photosynthesis,
• Dsucrose drives diffusion,
• Active H-ATPase,
• electrochemical gradient drives symporters,
• - Ys builds, water enters the cell, Yp builds.

• Sink
• Low concentration of sucrose,
• Dsucrose drives diffusion,
• Active H-ATPase,
• electrochemical gradient drives antiporters,
• - Ys drops, water exits the cell, Yp drops.

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Pressure-Flow-Hypothesis

• Yp
• Notice that the Ys at the source is more negative
  than at the sink!
• Why dont we expect water to flow toward the
  source?
• Water, along with solutes moves down the pressure
  gradient, not the water potential gradient.

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Water Cycling
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Control of Transportassimilation allocation and
partitioning

• Long distance, bi-directional flow of sugars,
• source and sink relationships are reversible, and
  under environmental and developmental control,
• Source
• sucrose synthesis balanced with starch synthesis,
• Sink
• Thought to be controlled by sink strength (sugar
  demand),
• Mechanisms for monitoring and switching unknown.

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Phloem Loadingsymplastic and apoplastic
symplastic via diffusion
apoplastic via secondary active transport
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Pressure- Flow-Hypothesispassive vs. active

• Driving Force
• Primarily Diffusion,
• very low Yp required, and phloem transport can
  occur at low temperatures,
• and in the presence of H-ATPase inhibitors.

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Concept Map
Phloem Transport