C4 and C3 photosynthetic pathway in plants

Biology

1. 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?
Stomatal conductance
Photo-
synthesis
C3
C4

2. C4 plants
Because of the CO
2concentrating mechanism,
the [CO
2] at Rubisco is much higher than in the leaf
internal air spaces. A saturating level of [CO
2] at
Rubisco can be achieved at low stomatal aperture
and current atmospheric [CO
2]
C3 plants
The [CO
2] decreases from the leaf internal air spaces
to the chloroplast, and photosynthesis is not saturated
at current CO
2levels.

3. CAM
Crassulacean acid metabolism
Another CO
2concentrating
mechanism that results in even
greater water savings.
Saguaro
Cereus giganteus

4. 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

5. Night -stomates open Day -stomates are closed
CO
2uptake into malate malate decarboxylates
by PEPcase and Calvin cycle is active

6. Stable carbon isotopes
12
C is about 99% of all C
13
C is about 1% of all C (extra neutron), not radioactive,
a “stable” isotope
The CO
2in air is about 99%
12
CO
2and 1%
13
CO
2
12
CO
2and 1%
13
CO
2 behave somewhat differently during
photosynthetic CO
2uptake.
1.
13
CO
2 diffuses a bit more slowly
2. Rubisco fixes
12
CO
2 a bit more efficiently
The result of these differences is that plants have slightly
less
13
CO
2in their tissues than does the CO
2in the atmosphere.

7. The relative abundance of
13
Cand
12
Cis any sample is
expressed as a deviation, d
13
C, from the
13
C/
12
Cratio of
a standard reference material.
13
C/
12
C sample
13
C/
12
C standard
Atmospheric
13
C/
12
Chas a d
13
Cof -8‰,
or parts per thousand from the standard reference.
What are the d
13
C values of plants?
-1 x 1000‰d
13
C =

8. 13
C composition, ‰
C3 plants C4 plants
From Cerling et al., ‘97
Plants are depleted in
13
C relative to atmospheric CO
2.
Atmosphere ≈ -8‰
Why do plants contain less
13
C than the CO
2in the air around
their leaves? Why are C3 plants more depleted in
13
C than C4 plants?

9. Why do plants contain less
13
C than the CO
2in the air
around their leaves?
1.
13
CO
2diffuses into leaves more slowly than
12
CO
2
2. Rubisco preferentially binds
12
CO
2.
d
13
C
plant= d
13
C
atm-(diffusion effect + enzyme effect)
Diffusion effect is small, 4.4‰
Rubisco effect is large, up to 30‰
Why are C3 plants more depleted in
13
C than C4 plants?
C3 plants use Rubisco for initial carboxylation, while C4 plants use
PEPcase, which shows little preference for
12
C. PEPcase
“discrimination” is only 2 to 6‰.

10. 13
C composition, ‰
C3 plants C4 plants
From Cerling et al., ‘97
Why the wide range of d
13
C values in C3 plants?

11. 13
C composition, ‰
C3 plants C4 plants
From Cerling et al., ‘97
less
stomatal
limitation
more
stomatal
limitation
Thed
13
C of C3 plants reflects how much stomatal
conductance limits photosynthesis.

12. 1.As stomatal conductance is reduced with water stress,
C
idecreases and more strongly limits carboxylation.
C
a
C
i
Why does d
13
C increase with water stress?
2. The carboxylating enzyme “Rubisco” discriminates less
against
13
CO
2when C
idecreases.
“Beggars can’t be choosers.”

13. C
a
Leaf interior Outside air
C
i
With a high stomatal conductance,
C
iremains relatively high (close to C
a
value), and Rubisco can “choose” to
fix mostly
12
CO
2.

14. Leaf interior Outside air
C
aC
i
As C
i/C
adecreases, Rubisco
is less choosy
With a lower stomatal conductance,
C
iis much lower, and Rubisco shows
less preference for
12
CO
2.

15. d
13
C
plant= d
13
C
atm-4.4 -22.6(C
i/C
a)
Farquhar et al. 1982
C
i/C
a
d
13
C
plant
Examples
extreme water stress, C
i/C
a might be 0.3
well watered plant, C
i/C
a might be 0.8

16. High elevation low elevation
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.