C4 has two features that are advantages in warm, dry environments. 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 C3 Photo- synthesis Stomatal conductance

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.

Crassulacean acid metabolism Another CO2 concentrating CAM Crassulacean acid metabolism Another CO2 concentrating mechanism that results in even greater water savings. 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 Day - stomates are closed CO2 uptake into malate malate decarboxylates by PEPcase and Calvin cycle is active PP08121.jpg

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% 12CO2 and 1% 13CO2 12CO2 and 1% 13CO2 behave somewhat differently during 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. 13C/12C sample 13C/12C standard Atmospheric 13C /12C has a d13C of -8‰, or parts per thousand from the standard reference. What are the d13C values of plants? d13C = - 1 x 1000‰

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?

d13Cplant = d13Catm - (diffusion effect + enzyme effect) 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. 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? 13C composition, ‰ From Cerling et al., ‘97

The d13C of C3 plants reflects how much stomatal conductance limits photosynthesis. C3 plants C4 plants less stomatal limitation more stomatal limitation 13C composition, ‰ From Cerling et al., ‘97

Why does d13C increase with water stress? 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 Ca Leaf interior Outside air

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

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. High elevation low elevation