Gas Exchange in Plants

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Gas Exchange in Plants
Chptr 10, pages 191 193 Chptr 36, pages 759 -
762
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Diffusion in media

• Movement along concentration gradients
• Gases diffuse faster in atmosphere than water
• Rate of diffusion dependent on square root of the
  mass of molecule H2O diffuses 1.56 times CO2

3
Crossing barriers

• Surfaces must be moist
• Concentration gradient must be present
• Short pathway important for rapid exchange

4
Tradeoffs

• Selectivity does not occur
• Enhancing the exchange rate of one gas enhances
  exchange of another gas unless change in
  concentration gradients occur

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Exchange pathways

• Cuticle
• stomata

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Leaf epidermis with stomata
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Stomate structure
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Distribution of Stomata

• Amphistomatous - both surfaces
• Epistomatous - upper surface
• Hypostomatous - lower surface
• No stomata

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Size of stomata

• Width 3 12 ?
• Length 10 40 ?
• Density 1,000 60,000 cm2
• Total opening per leaf surface consistently 1-2

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Boundary layer structure and relationship to
leaf size
gtot (gleaf x gbl)/(gleaf gbl)
gbl 1/rbl
rbl blmm/Dj
blmm 4 x ?(Lm/Vms-1)
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Gas Flux in Leaves

• FluxH2O (H2Oin - H2Oout)/rtot
• FluxCO2 (CO2in - CO2out)/rtot
• rtot rbl rs
• Thus fluxgas (gasin - gasout)/(rbl rs)
• When stomata just open, rs is large relative to
  rbl and increase in flux is linear.
• When somata near wide open, rs is small relative
  to rbl (constant) and flux levels off.

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Change in flux with increasing stomatal aperture
flux
0
incr
1/rs
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Effect of leaf size on gas exchange
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Acacia koa leaves
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Effect of leaf width on boundary layer and
comparison of conductance in juvenile leaves
versus phyllodes
blmm 4 x ?(Lm/Vms-1)
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Photosynthesis in phyllodes versus juvenile
leaves and effect of light intensity on
photosynthesis
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Water relations in guard cells runs counter to
water relations in adjacent cells
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Water potential dynamics of stomata
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Fig. 36.13
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Factors affecting stomatal operation

• Light
• CO2
• Humidity
• Temperature
• Abscisic acid

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Gas exchange efficiencies

• Water use efficiency (WUE)
  - CO2 gained (A)/water lost (E)
• Quantum Efficiency (QE)
  - CO2 gained (A)/photon absorbed (Q)

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Measuring WUE

• Spot measurements of CO2 uptake and water loss
• Long term using C13/C12 ratios in plant material
• RuBP discriminates against C13 in favor of C12.
• When Ci is high (open stomata, low WUE), C13/C12
  is lower in sugars and products.
• When Ci is low (more closed stomata, high WUE),
  C13/C12 is higher in sugars and products.

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Modifying WUE

• WUE CO2 gained/H2O lost
• Since the resistances for both are similar, WUE
  can be approximated by (CO2in -
  CO2out)/(H2Oin - H2Oout)
• To increase WUE, either CO2 gradient must be
  increased or H2O gradient decreased.
• The gradient for water loss is dictated by the
  environment, since plant is close to saturation.
• Atmospheric carbon CO2out is constant, thus
  plant can only increase WUE by increasing its
  affinity for CO2 and decreasing internal CO2in
  thus increasing driving force for uptake.

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Calvin-Benson cycle - C3
RuBP requires gt50ppm CO2
RuBP can combine with O2 leading to
photo- respiration and depression of Ps at high
light intensities.
Fig. 10.17
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Hatch and Slack pathwayC4
PEP requires lt10 ppm CO2 and does not combine
with O2.
O2 present in low amounts in bundle sheath
cells.
Fig. 10.18
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Internal Anatomy of C3 and C4 leaves
C3 leaf
C4 leaf
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Crassulacean Acid MetabolismCAM
Fig. 10.19
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Changes in internal conditions forCAM over 24
hrs.
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Comparison of Photosynthetic pathways
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The End