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Carbon Cycle, part 2
Ecophysiology of Leaves
• Dennis Baldocchi
– ESPM
– UC Berkeley
Courtesy of Rob Jackson, Duke
3/13/2013
ESPM 111 Ecosystem Ecology
Outline
• Photosynthetic Pathways and Cycles
• Environmental Physiology of Photosynthesis
–
–
–
–
–
Light
Temperature
CO2
Nutrients
Soil Moisture and Water Deficits
• Respiration
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Basic Photosynthesis Stoichiometry
6CO2 + 6H2O + (light)  (C6H12O6) + 6O2
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Physiological Attributes of Plants
• photosynthetic
pathway
– C3
– C4
– CAM
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Photosynthesis:
A balance between Supply and Demand
• Biochemical limitation: carboxylation rate
• Light limitation
• Enzyme limitation
• Physical limitation: delivery of CO2 to leaf
• Diffusion through Leaf Boundary Layer
• Diffusion through Stomatal Pores
• Potential Gradient between free Atmosphere and
substomatal Cavity
ESPM 111 Ecosystem Ecology
Critical Steps in C3 Photosynthesis
Calvin-Benson Cycle
• Light Reactions
– Chlorophyll, in chloroplasts, captures photons
– Light energy is used to produce chemical energy in the forms of
NADPH and ATP
– Oxygen is produced
• Dark Reactions
– A 3-C compound, PGA, is formed at the first Carboxylation step via
the reaction between CO2 and RUBP, a C5 compound, and its
subsequent cleaving (C6 => 2 C3)
– The enzyme RUBISCO catalyzes the reaction between CO2 and
RUBP
– RUBISCO has an affinity for both CO2 and O2, with the later leading to
photorespiration, a loss of CO2
– RUBISCO accounts for about 9 to 25% of the N in a leaf
– Chemical Energy (NADPH & ATP) is used to regenerate RUBP
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C3 Photosynthesis, The Calvin-Benson Cycle
CO2
O2
Photorespiration,
PCO
ATP
CO2
RUBP
Carboxylase/
oxygenase
(CH2O)n
ADP
PGA
NADP+ + H+
NADPH
RUBP
ADP
PCR Cycle
RUBP
Regeneration
Triose-P
ATP
ESPM 111, Ecosystem Ecology
Using Solar Energy for Life
Light (‘Hill’) Z Reactions:
PS II and Ps I
•
•
Visible solar energy (400 to 700 nm)
is absorbed by pigments
This energy is converted into high
energy compounds, ATP and NADPH,
by photosystems II and I (PS II and I)
– Photosystem II uses 680 nm energy
to generate ATP (non-cyclic electron
transport)
– PS I uses 700 nm solar energy to
generate NADPH (cyclic electron
transport).
– Excess energy is lost as heat or
fluorescence.
• 8 Photons per CO2 molecule fixed
700 nm
680 nm
Allen and Martin, 2007 Nature
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C4 leaf
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/C4leaf.gif
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Critical Steps in C4 Photosynthesis
• The enzyme PEP Carboxylase catalyzes a reaction
between CO2 and phosophenolpyruvate (PEP) to form a
C4 compound
• The C4 compound is transported into the specializes
cells, the bundle sheaths, and is decarboxylated
• CO2 is released into an environment and photosynthesis
is completed via the C3 cycle
• Photorespiration is low; RUBISCO favors CO2 in this
environment because the ratio between CO2:O2 is high
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C4 Photosynthesis
Mesophyll
PEP
Carboxylase
CO2
Mesophyll
Bundle Sheath
OAA
C4
acids
CO2
RUBP
Carboxy
lase
CO2
PGA
(CH2O)n
PCR
Cycle
C3 acids
PEP
RUBP
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C3 vs C4 Grass Distribution
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Edwards et al 2010 Science
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C3 vs C4: Isotopes and Ci/Ca
C4
C3
O’Leary, 1988 Bioscience
Tool to Study Advance and Retreat of C3 Grasslands During Ice Age
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Why are CA Hot Grasslands Compose of C3 grasses?
CA annual Grasslands experience cool wet winter growing season
Adapted from Ehleringer et al. 1997
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C3 vs C4 Photosynthesis:
Light Use Efficiency, CO2 and Temperature
Ehleringer et al, Oecologia
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Comparative EcoPhysiology
C3
C4
CAM:
day
CAM:
night
?
internal [CO2]
0.7 Ca
0.4 Ca
0.5 Ca
Krantz ananatomy
NO
YES
Succulent
carboxylase
RuBP
PEP
RuBp
PEP
photorespiration
YES
NO
Fixation product
PGA
C4 acids
PGA
C4 acids
CO2 compensation pt
30 -80 ppm
< 10 ppm
50 ppm
< 5 ppm
optimum temperature
15-30 C
25-40C
35 C
Max Ps (mmol m-2 s-1)
14-40
18-55
6
Response at full sunlight
saturated
Linear
Saturated
yes
No
-30 to -10
-30 to -10
Enhancement in low O2
yes
no
Quantum req.
15-22
19
Quantum yield
0.052
0.064
Carbon isotope fractionation, per
mill
-26 (-23 to -33)
-14
Adapted from Jones, 1992
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Resistance -Analog
A 
(C a  C c )

rle a f  rm e s o p h y ll  rc h lo r o p la s t
g b (C a  C s )  g s (C s  C i )  g m (C i  C c )
Flexas et al 2008, Plant, Cell, Environ
ESPM 111 Ecosystem Ecology
Leaf Photosynthesis
A = V c - 0.5 V o - Rd
Leaf photosynthesis (A) is a function of:
Rate of carboxylation (Vc), which is the minimum of the rates
associate with regeneration of RuBP by electron transport and the
saturation of RUBISCO by CO2,
Rate of oxygenation (Vo, photorespiration) which is governed by
the competition between CO2 and O2 for RUBISCO
Rate of dark respiration (Rd)
(all have units of mol m-2 s-1).
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A-Ci Curve: C3 Leaf
From Chapin et al
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Light Response Curve, C3 Leaf
From Chapin et al
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Wc : demand limited by RUBISCO saturation
Wj : demand limited by RuBP regeneration
by electron transport
Carboxylation rate (mol m-2 s-1)
60
50
40
30
20
supply
~stomatal conductance
(gs)
10
0
0
200
400
600
800
1000
[CO2] (ppm)
Ci
ESPM 111 Ecosystem Ecology
Maximum rate of Carboxylation, Vcmax, scales
with maximum rate of electron transport, Jmax
After Wullschleger. 1993
350
300
250
200
Jmax
150
100
50
0
0
20
40
60
80
100
120
140
160
Vcmax
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Vcmax scales with maximum photosynthesis at
saturating light and ambient CO2, Pml
Wohlfarht 1999
mountain grassland species
100
Vcmax=2.36+1.66 Pmx
r2=0.87
Vcmax (mol m-2 s-1)
80
60
40
20
0
0
10
20
30
40
50
60
Pml (mol m-2 s-1)
ESPM 111 Ecosystem Ecology
Seasonality of Model Parameters:
e.g. Photosynthetic Capacity
Quercus alba (Wilson et al)
Quercus douglasii (Xu and Baldocchi)
140
Live Fast, Die Young
In Stressed Environments
120
Vcmax
100
80
60
40
20
0
100
150
200
250
300
350
DOY
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Photosynthesis and Temperature
Jack pine (NSA)
Data of Hank Margolis
Qp = >1000 mol m-2 s-1
4
-2 -1
A (mol m s )
3
2
1
0
-10
0
10
20
30
40
Leaf Temperature (C)
Berry and Bjorkman 1980, Ann Rev Plant Physiol
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Stress Physiology
•
•
Adaptation is a response to long-term changes which result in inheritable
genetic alterations. These alterations are stable and will remain in the
population over generations.
Acclimation is a response induced by an environmental change which
causes a phenotypic alteration over a single generation time without any
compositional change in the genetic complement
–
–
•
Stomata close to limit water loss
Temperature optimum shifts with warmer temperatures
Tolerance
– Blue oaks are able to photosynthesize despite super severe water deficits (below
-5.0 Mpa)
•
Avoidance
– CA grasses adopt annual life strategy. Survive dry summer in the form of
dessicated seeds
– Blue oaks develop deep roots that tap into the ground water
•
Huner et al 1993 Photosyn Res
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Ps and Temperature Acclimation
Berry and Bjorkman 1980, Ann Rev Plant Physiol
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GPP: Acclimation with Temperature
for Canopy CO2 uptake (C)
Temperature Optimum
35
30
25
20
15
b[0] 3.192
b[1] 0.923
r²
0.830
10
5
5
10
15
20
25
30
M e a n S u m m e r T em p e ra tu re (C )
Analysis of E. Falge
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Photosynthesis and Leaf Nitrogen
Field and Mooney, 1986
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Reich et al 1997 PNAS
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Stomatal conductance adjusts to match photosynthetic capacity
(or vice versa)
Schulze et al 1994 Ann Rev Ecol
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Stomatal conductance and water deficits
Schulze 1986 Ann Rev Plant Physiol
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Vcmax scales with water deficits
Quercus douglasii
140
120
b[0]: 111.3
b[1]: 22.69
b[2]: 1.734
r ² 0.7532
Vcmax
100
80
60
40
20
0
-7
-6
-5
-4
-3
-2
-1
0
Pre-dawn Water Potential (MPa)
Baldocchi and Xu, 2007 Adv Water Res
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Canopy Photosynthesis
•
•
•
•
•
•
Sunlight, and Environmental Conditions
Leaf Area Index
Leaf Clumping
Leaf Angle Distribution
Diffuse vs Direct Light
Leaf Thickness and Photosynthetic
Capacity
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W HEAT
0
-5
-1 0
-1 5
Fc (mg m-2 s-1)
Light and
Photosynthesis:
Leaves, Canopies
and Clumping
-2 0
-2 5
-3 0
-3 5
-4 0
(a )
-4 5
-5 0
0
500
1000
1500
2000
a b s o rb e d P A R (µ m o l m -2 s -1 )
D 2 0 8 O a k le a f , f o r e s t f lo o r
o
T le a f : 2 5 C
C O 2 : 360 ppm
12
d a ta
m odel
-40
6
4
2
0
0
200
400
600
Q
800
par
1000
( m o l m
-2
1200
1400
1600
1800
-1
-2
-1
A (mol m s )
8
-30
-2
Fc (mol m s )
10
-20
-10
0
measured
model: clumped leaves
10
s -1 )
0
500
1000
1500
-2
2000
-1
PPFD (mol m s )
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Dry Matter Production scales with Sunlight
Monteith 1981 Phil Trans Roy Soc
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CO2 uptake-Light Response Curve: Role of
C3 and C4 Crops
0 .2 5
W HEAT
Fc/LAI (mg m-2 leaf area s-1)
CORN
0 .0 0
-0 .2 5
-0 .5 0
-0 .7 5
0
250
500
750
1000
1250
1500
1750
2000
Q a (µ m o l m -2 s -1 )
Linear Function and High r2 (~0.90)
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Baldocchi, 1994, AgForMet
Canopy Light Response Curves: Effect of Diffuse Light
Temperate Broad-leaved Forest
Spring 1995 (days 130 to 170)
10
5
Sunny days
diffuse/total <= 0.3
0
Cloudy days
diffuse/total >= 0.7
-2
-1
NEE (mol m s )
-5
-10
-15
-20
-25
-30
-35
-40
0
500
1000
1500
-2
2000
-1
PPFD (mol m s )
Baldocchi, 1997, PCE
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Summary Points
•
Leaf photosynthesis involves a set of light and dark reactions.
•
The light reactions involve the harvesting of sunlight by chlorophyll and the transfer of electrons to
the produce of biochemical energy compounds adenosine triphosphate (ATP) and nicoadenomine di-phosate (NADPH) via the process called photophosphorylation.
•
The energy contained in molecules ATP and NADPH is subsequently used in the dark reactions
to drive the photosynthetic carbon reduction cycle, or Calvin Cycle.
•
There are three photosynthetic pathways, C3 (the most common), C4 and CAM.
•
Photorespiration, due to the competitive affinity for CO2 and O2 by the enzyme RUBISCO, is an
attribute of C3 photosynthesis.
–
–
This leads to its lower efficiency than C4 photosynthesis
C4 plants circumvents this inefficiency by using a different enzyme to carboxylze CO2 (PEP carboxylase)
and by minimizing the diffusion of O2 deep in the leaf with its unique leaf anatomy (bundle sheaths).
•
Rates of photosynthesis achieved by a leaf is a balance between the supply and the demand for
carbon dioxide. The supply is related to the CO2 concentration in the air and its diffusion through
the leaf boundary layer and stomata. The demand is controlled by the Michaelis-Menton kinetic
reactions that are a function of CO2 at the chloroplast.
•
Rates of photosynthesis are a function of light, temperature, CO2, moisture, leaf nutrition, and
phytotoxics.
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