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C3, C4, and CAM plants all have the same goal,
to make carbohydrates.
What happens to the triose-phosphates made in the
Calvin cycle?
1. Used to synthesize starch for storage in chloroplast.
2. Exported from chloroplast for sucrose synthesis in
the cytosol.
How is starch vs. sucrose synthesis regulated?
Why is it regulated?
Triose phosphates produced in the Calvin cycle can
be used for starch or sucrose synthesis.
Calvin
cycle
chloroplast
Triose-P
cytosol
Sucrose
Starch
Starch is synthesized in the chloroplast.
Fig. 8.15
Starch vs. sucrose synthesis is regulated by level of
cytosolic Pi as it affects triose-P export from chloroplast.
When [Pi] is high,
triose-P is exported
in exchange for Pi
& used to synthesize
sucrose.
If [Pi] is low, then
triose-P is retained
in chloroplast and
used to synthesize
starch.
Fig. 8.14
More on the ecological aspects
of photosynthesis
(Ch. 9)
Stomatal conductance
light, temperature, relative humidity, Y, [CO2]
carbon isotope discrimination
Light
Leaf movements
Sun and shade leaves - anatomical and photosynthetic properties.
Temperature
Leaf energy balance
C3 vs. C4
quantum yield differences
Atmospheric CO2
History of atmospheric CO2
Current trend of rising CO2
Implications for C3 & C4 photosynthesis
Review: Stomatal aperture regulates the conductance of the diffusion
pathway for CO2 entering the leaf and H2O leaving the leaf.
Fig. 4.10
What factors influence stomatal conductance?
Environmental cues
1. light
Effect on stomatal cond.
increases as light increases
2. relative humidity
increases as r.h. increases
3. temperature
increases as temp. increases
Internal cues
1. leaf water potential
2. internal [CO2]
3. hormonal control
(abscisic acid, ABA)
decreases as Y decreases
decreases as [CO2] increases
decreases as [ABA] increases
All these cues ultimately influence the turgor pressure of the guard
cell, which in turn causes the opening or closing of the stomatal pore.
light Light and leaf movements
Light affects photosynthesis and
leaf temperature
Solar tracking allows
leaves to increase
light absorption compared
to a fixed orientation.
Diaheliotropic leaves
Fig. 9.6
Some plants change
leaf angle to reduce
light absorption.
Paraheliotropic leaves
Why?
What happens to the light that strikes a leaf?
1. Absorbed
85 to 90% of the PAR, 60% of total energy
2. Reflected
0 to 8% of the PAR
3. Transmitted (passes through leaf)
0 to 8% of PAR
Fig. 9.2
Absorption is high for PAR and decreases greatly
at longer wavelengths.
Fig. 9.3
Light level attenuates (decreases) with depth in a plant
canopy because each layer of leaves absorbs light.
Fig. 9.7
Leaf anatomy responds
leaf
toSun
light
level.
Which is the “sun”
Leaf and which is the
“shade” leaf?
Shade leaf
Sun leaves
Shade leaves
Thicker,
more cell layers
Thinner, fewer
cell layers
More Rubisco
per unit chlorophyll
Less Rubisco
per chlorophyll
Less chlorophyll
per reaction center
More chl per
reaction center
Light acclimation (phenotypic plasticity) vs. light
adaptation.
Anatomical and biochemical differences between sun and
shade leaves determine
photosynthetic
properties.
Physiological
differences
of sun and
shade leaves
Fig. 9.9
Sun leaf vs. shade leaf
Sun leaf has:
Higher max. photo. rate
Higher light sat’n level
Higher light compensation
point
Fig. 9.10
Acclimation to
growth light
level - same
pattern as sun vs.
shade species
differences.
Light and leaf temperature.
Heat loads on leaves in the sun are large.
How do leaves prevent overheating?
Mechanisms of heat dissipation by leaves
Leaves can lose heat in
three main ways:
1. Emission of radiation
2. Conduction/convection
3. Evaporation
Each term can be included as
part of an “energy balance”
equation.
Leaf energy balance
At constant temperature:
Energy In = Energy Out
(Radiation absorbed + Conduction/Convection + Condensation)
= (Radiation emitted + Conduction/Convection
loss + Evaporation)
Leaf temperature and photosynthesis
Which is
C3 and C4?
C4
C3
Why does the quantum yield of C3 plants decrease with
increasing temperature? Why is the quantum yield of C4
plants insensitive to temperature?
Fig 9.23
Photosynthetic responses to CO2
History of atmospheric CO2
Fig. 9.16
Current trend of rising CO2
The Mauna Loa
CO2 record
Fig. 9.16
Photosynthetic response to CO2 of C3 & C4 plants
Photosynthetic response to temperature
Fig. 9.22
CO2-temperature
interaction in a C3
plant.
Why does the temp.
for maximum phot.
increase at elevated
CO2?
2. Second approach
Henry’s Law: concentration of a gas dissolved in water
is proportional to the gas partial pressure (or [gas] at
same total pressure) above the water.
Changing the gas partial pressure produces a
proportional change in dissolved concentration.
Examples
If dissolved concentration is 11.68µM at 345ppm CO2, then the
dissolved concentration is 2 x 11.68 if gas concentration is 2 X
345ppm.
At 250ppm CO2, dissolved CO2 is 11.68 x 250/345 = 8.46 µM.