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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).