Presentation on theme: "C4 has two features that are advantages in warm, dry environments. 1.Suppression of photorespiration (more C gain) 2. Lower stomatal conductance (less."— Presentation transcript:
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
C4 plants Because of the CO 2 concentrating 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 2 levels.
CAM Crassulacean acid metabolism Another CO 2 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 CO 2 uptake into malate malate decarboxylates by PEPcaseand Calvin cycle is active
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 2 in air is about 99% 12 CO 2 and 1% 13 CO 2 12 CO 2 and 1% 13 CO 2 behave somewhat differently during photosynthetic CO 2 uptake 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 2 in their tissues than does the CO 2 in the atmosphere.
The relative abundance of 13 C and 12 C is any sample is expressed as a deviation, 13 C, from the 13 C / 12 C ratio of a standard reference material. 13 C/ 12 C sample 13 C/ 12 C standard Atmospheric 13 C / 12 C has a 13 C of -8‰, or parts per thousand from the standard reference. What are the 13 C values of plants? - 1 x 1000‰ 13 C =
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 2 in the air around their leaves? Why are C3 plants more depleted in 13 C than C4 plants?
Why do plants contain less 13 C than the CO 2 in the air around their leaves? CO 2 diffuses into leaves more slowly than 12 CO 2 2. Rubisco preferentially binds 12 CO 2. 13 C plant = 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‰.
13 C composition, ‰ C3 plants C4 plants From Cerling et al., ‘97 Why the wide range of 13 C values in C3 plants?
13 C composition, ‰ C3 plants C4 plants From Cerling et al., ‘97 less stomatal limitation more stomatal limitation The 13 C of C3 plants reflects how much stomatal conductance limits photosynthesis.
1.As stomatal conductance is reduced with water stress, C i decreases and more strongly limits carboxylation. CaCa Ci Ci Why does 13 C increase with water stress? 2. The carboxylating enzyme “Rubisco” discriminates less against 13 CO 2 when C i decreases. “Beggars can’t be choosers.”
Ca Ca Leaf interiorOutside air Ci Ci With a high stomatal conductance, C i remains relatively high (close to C a value), and Rubisco can “choose” to fix mostly 12 CO 2.
Leaf interiorOutside air CaCa CiCi As C i /C a decreases, Rubisco is less choosy With a lower stomatal conductance, C i is much lower, and Rubisco shows less preference for 12 CO 2.
13 C plant = 13 C atm (C i /C a ) Farquhar et al C i /C a 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
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.