Presentation on theme: "C4 has two features that are advantages in warm, dry"— Presentation transcript:
1 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 atlower stomatal conductance than C3 plants. How?C4C3Photo-synthesisStomatal conductance
2 C4 plantsBecause of the CO2 concentrating mechanism,the [CO2] at Rubisco is much higher than in the leafinternal air spaces. A saturating level of [CO2] atRubisco can be achieved at low stomatal apertureand current atmospheric [CO2]C3 plantsThe [CO2] decreases from the leaf internal air spacesto the chloroplast, and photosynthesis is not saturatedat current CO2 levels.
3 Crassulacean acid metabolism Another CO2 concentrating CAMCrassulacean acid metabolismAnother CO2 concentratingmechanism that results in evengreater water savings.SaguaroCereus 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 pHincreases throughout the light period.Some CAM plants are switch hittersObligate vs. facultative CAM
5 Night - stomates open Day - stomates are closed CO2 uptake into malate malate decarboxylatesby PEPcase and Calvin cycle is activePP08121.jpg
6 Stable carbon isotopes 12C is about 99% of all C 13C is about 1% of all C (extra neutron), not radioactive,a “stable” isotopeThe CO2 in air is about 99% 12CO2 and 1% 13CO212CO2 and 1% 13CO2 behave somewhat differently duringphotosynthetic CO2 uptake.1. 13CO2 diffuses a bit more slowly2. Rubisco fixes 12CO2 a bit more efficientlyThe result of these differences is that plants have slightlyless 13CO2 in their tissues than does the CO2 in the atmosphere.
7 The relative abundance of 13C and 12C is any sample is expressed as a deviation, d13C, from the 13C /12C ratio ofa standard reference material.13C/12C sample13C/12C standardAtmospheric 13C /12C has a d13C of -8‰,or parts per thousand from the standard reference.What are the d13C values of plants?d13C =x 1000‰
8 Plants are depleted in 13C relative to atmospheric CO2. C3 plantsC4 plantsAtmosphere ≈ -8‰13C composition, ‰From Cerling et al., ‘97Why do plants contain less 13C than the CO2 in the air aroundtheir leaves? Why are C3 plants more depleted in 13C than C4 plants?
9 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 12CO22. 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‰.
10 Why the wide range of d13C values in C3 plants? 13C composition, ‰From Cerling et al., ‘97
11 The d13C of C3 plants reflects how much stomatal conductance limits photosynthesis.C3 plantsC4 plantslessstomatallimitationmorestomatallimitation13C composition, ‰From Cerling et al., ‘97
12 Why does d13C increase with water stress? As stomatal conductance is reduced with water stress,Ci decreases and more strongly limits carboxylation.CaCi2. The carboxylating enzyme “Rubisco” discriminates less against 13CO2 when Ci decreases.“Beggars can’t be choosers.”
13 With a high stomatal conductance, Ci remains relatively high (close to Ca value), and Rubisco can “choose” to fix mostly 12CO2.CiCaLeaf interiorOutside air
14 Ca Ci With a lower stomatal conductance, Ci is much lower, and Rubisco shows less preference for 12CO2.CaCiAs Ci/Ca decreases, Rubiscois less choosyLeaf interiorOutside air
15 d13Cplant = d13Catm - 4.4 - 22.6(Ci/Ca) Farquhar et al. 1982d13CplantExamplesextreme water stress, Ci/Ca might be 0.3well watered plant, Ci/Ca might be 0.8Ci/Ca
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.High elevation low elevation