Transcript Feb. 2, 2006 B4730/5730 Plant Physiological Ecology

Feb. 10th, 2011
B4730/5730
Plant Physiological Ecology
Gas Exchange
Terms and Definitions (and a little jargon)
• Standards: PDB limestone for C, Standard Mean
Ocean Water (SMOW) for H and O, atmospheric N2
for N
• If a sample has more of the heavy isotope than the
standard (or than another substance) it is “enriched”
or “heavier” and if it has less it is “depleted” or
“lighter”
• Examples:
d13C of PDB = 0‰
d13C of atmospheric CO2 = -8‰
d13C of C3 plants is around -25‰
• IF Pee Dee Belemnite (PDB) has 13C/12C = 0.011237,
what is the 13C/12C ratio of CO2 in air?
Kinetic and Diffusional Effects I
• Isotope effect () is defined as
– α=Rr/Rp where Rr is the 13C/12C of reactant
and product respectively which is the same as
the ratio of the rate constants
– isotope effects only occur at partially
reversible or alternative pathway reactions
• Deviation of α from unity is Δ
– Δ=(δa-δp)/(1+δp)
Kinetic and Diffusional Effects II
• Derivation of isotopic effects on
photosynthesis
– α=1+Δ=Ra/Rp=(1+a)((pa-pi)/pa)+(1+b)pi/pa=
or Δ=a+(b-a)pi/pa
where a is fractionation from diffusion (4%O); b is
carboxylation fractionation (27%O)—sometimes
include d for all other impacts (0-4%O)
-C4 plant replace b with b4+b3Φ where b4 is PEP
carboxylase fixation (-5.7%O), b3 carbonxylation
(30%O) and Φ is bundle sheath leakiness (variable
with anatomy and physiology)
-CAM plant replace b with b4 only
Kinetic and Diffusional Effects III
• Water isotopes vary from precipitation, soil
evaporation, root uptake (H but not O) and leaf
transpiration
• Δ18Oes=ε+ + εk + (Δ18Ov-εk)ea/ei
where Δ18Oes is enrichment of leaf water above
source, ε+ is vapor pressure depression by 18O
(temperature dependent), εk is fractionation
through conductance, Δ18Ov enrichment vapor
relative to source, ea and ei are mole fractions
of water vapor ambient and intercellular
Kinetic and Diffusional Effects IV
• Peclet effect is the ratio of convectional to
diffusional impacts of unenriched waterdepends on tortuosity of water path xylem to
cell wall surfaces
• Oxygen atoms in carbonyls are 27% more
enriched than aqueous solution
– 18O in sucrose (two triose phosphates) is 27% in full
isotopic equilibrium
– cellulose (other compounds from sucrose) not in full
isotopic equilibrium because O in carbonyls can
exchange with aqueous
The genus Aloe has both CAM and
C3 species (Vogel, 1980).
The Poaceae family has almost as
many C4 as C3 species (Vogel,
1980).