C4 has two features that are advantages in warm, dryenvironments.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

C3C4

C4 plantsBecause 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 plantsThe [CO2] decreases from the leaf internal air spaces to the chloroplast, and photosynthesis is not saturated at current CO2 levels.

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CAMCrassulacean acid metabolism

Another CO2 concentratingmechanism that results in even

greater water savings.

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SaguaroCereus 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 hittersObligate vs. facultative CAM

Night - stomates open Day - stomates are closedCO2 uptake into malate malate decarboxylatesby PEPcase and Calvin cycle is active

Stable carbon isotopes

12C is about 99% of all C13C is about 1% of all C (extra neutron), not radioactive, a “stable” isotope

The CO2 in air is about 99% 12CO2 and 1% 13CO2

12CO2 and 1% 13CO2 behave somewhat differently duringphotosynthetic CO2 uptake.

1. 13CO2 diffuses a bit more slowly2. 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, 13C, from the 13C /12C ratio of a standard reference material.

13C/12C sample 13C/12C standard

Atmospheric 13C /12C has a 13C of -8‰, or parts per thousand from the standard reference.

What are the 13C values of plants?

- 1 x 1000‰13C =

13C composition, ‰

C3 plants C4 plants

From Cerling et al., ‘97

Plants are depleted in 13C relative to atmospheric CO2.

Atmosphere ≈ -8‰

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 12CO2

2. Rubisco preferentially binds 12CO2.

13Cplant = 13Catm - (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‰.

13C composition, ‰

C3 plants C4 plants

From Cerling et al., ‘97

Why the wide range of 13C values in C3 plants?

13C composition, ‰

C3 plants C4 plants

From Cerling et al., ‘97

lessstomatal limitation

morestomatal limitation

The13C of C3 plants reflects how much stomatalconductance limits photosynthesis.

1. As stomatal conductance is reduced with water stress,

Ci decreases and more strongly limits carboxylation.Ca

Ci

Why does 13C increase with water stress?

2. The carboxylating enzyme “Rubisco” discriminates less against 13CO2 when Ci decreases.

“Beggars can’t be choosers.”

Ca

Leaf interior Outside air

Ci

With a high stomatal conductance, Ci remains relatively high (close to Ca value), and Rubisco can “choose” to fix mostly 12CO2.

Leaf interior Outside air

CaCi

As Ci/Ca decreases, Rubiscois less choosy

With a lower stomatal conductance, Ci is much lower, and Rubisco shows less preference for 12CO2.

13Cplant = 13Catm - 4.4 - 22.6(Ci/Ca)

Farquhar et al. 1982

Ci/Ca

13Cplant

Examplesextreme water stress, Ci/Ca might be 0.3well watered plant, Ci/Ca might be 0.8

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.