C4 has two features that are advantages in warm, dry
environments.
1. Suppression of photorespiration (more C gain)
2. Lower stomatal conductance (less water loss)
• C4 plants can achieve high photosynthetic rates at
lower stomatal conductance than C3 plants. How?
C4
Photosynthesis
Stomatal conductance
C3
C4 plants
Because of the CO2 concentrating mechanism,
the [CO2] at Rubisco is much higher than in the leaf
internal air spaces. A saturating level of [CO2] at
Rubisco can be achieved at low stomatal aperture
and current atmospheric [CO2]
C3 plants
The [CO2] decreases from the leaf internal air spaces
to the chloroplast, and photosynthesis is not saturated
at current CO2 levels.
CAM
Crassulacean acid metabolism
Another CO2 concentrating
mechanism that results in even
greater water savings.
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Saguaro
Cereus giganteus
Many succulents of dry environments open stomates
at night and close then during the day!
In these plants, the acidity of the leaf cells increases
(pH decreases) throughout the night, and then pH
increases throughout the light period.
Some CAM plants are switch hitters
Obligate vs. facultative CAM
Night - stomates open
CO2 uptake into malate
by PEPcase
Day - stomates are closed
malate decarboxylates
and Calvin cycle is active
Stable carbon isotopes
12C
is about 99% of all C
13C is about 1% of all C (extra neutron), not radioactive,
a “stable” isotope
The CO2 in air is about 99%
12CO
13CO
and
1%
2
2
12CO
13CO behave somewhat differently during
and
1%
2
2
photosynthetic CO2 uptake.
1. 13CO2 diffuses a bit more slowly
2. Rubisco fixes 12CO2 a bit more efficiently
The result of these differences is that plants have slightly
less 13CO2 in their tissues than does the CO2 in the atmosphere.
The relative abundance of 13C and 12C is any sample is
expressed as a deviation, d13C, from the 13C /12C ratio of
a standard reference material.
d13C
=
13C/12C
sample
-1
13C/12C standard
x 1000‰
Atmospheric 13C /12C has a d13C of -8‰,
or parts per thousand from the standard reference.
What are the d13C values of plants?
Plants are depleted in 13C relative to atmospheric CO2.
C3 plants
C4 plants
Atmosphere ≈ -8‰
13C
composition, ‰
From Cerling et al., ‘97
Why do plants contain less 13C than the CO2 in the air around
their leaves? Why are C3 plants more depleted in 13C than C4 plants?
Why do plants contain less 13C than the CO2 in the air
around their leaves?
1. 13CO2 diffuses into leaves more slowly than
2. Rubisco preferentially binds 12CO2.
12CO
2
d13Cplant = d13Catm - (diffusion effect + enzyme effect)
Diffusion effect is small, 4.4‰
Rubisco effect is large, up to 30‰
Why are C3 plants more depleted in
13C
than C4 plants?
C3 plants use Rubisco for initial carboxylation, while C4 plants use
PEPcase, which shows little preference for 12C. PEPcase
“discrimination” is only 2 to 6‰.
Why the wide range of d13C values in C3 plants?
C3 plants
13C
C4 plants
composition, ‰
From Cerling et al., ‘97
The d13C of C3 plants reflects how much stomatal
conductance limits photosynthesis.
C3 plants
less
stomatal
limitation
C4 plants
more
stomatal
limitation
13C
composition, ‰
From Cerling et al., ‘97
Why does d13C increase with water stress?
1. As stomatal conductance is reduced with water stress,
Ci decreases and more strongly limits carboxylation.
Ca
Ci
2. The carboxylating enzyme “Rubisco” discriminates less
against 13CO2 when Ci decreases.
“Beggars can’t be choosers.”
With a high stomatal conductance,
Ci remains relatively high (close to Ca
value), and Rubisco can “choose” to
fix mostly 12CO2.
Ci
Leaf interior
Ca
Outside air
With a lower stomatal conductance,
Ci is much lower, and Rubisco shows
less preference for 12CO2.
Ci
Ca
As Ci/Ca decreases, Rubisco
is less choosy
Leaf interior
Outside air
d13Cplant = d13Catm - 4.4 - 22.6(Ci/Ca)
Farquhar et al. 1982
d13Cplant
Examples
extreme water stress, Ci/Ca might be 0.3
well watered plant, Ci/Ca might be 0.8
Ci/Ca
Note: we only briefly discussed this in class, but you should understand
that patterns depicted.
C isotope composition of dominant C3 & C4 plants and mineral soil carbon
along the C. Hart Merriam elevational gradient on the S. F. Peaks.
High elevation
low elevation