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 N 2 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:  13 C of PDB = 0‰  13 C of atmospheric CO 2 = -8‰  13 C of C 3 plants is around -25‰ IF Pee Dee Belemnite (PDB) has 13 C/ 12 C = , what is the 13 C/ 12 C ratio of CO 2 in air?

Kinetic and Diffusional Effects I Isotope effect () is defined as –α=R r /R p where R r is the 13 C/ 12 C 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=(δ a -δ p )/(1+δ p )

Kinetic and Diffusional Effects II Derivation of isotopic effects on photosynthesis –α=1+Δ=R a /R p =(1+a)((p a -p i )/p a )+(1+b)p i /p a = or Δ=a+(b-a)p i /p a 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 Δ 18 O es =ε + + ε k + (Δ 18 O v -ε k )e a /e i where Δ 18 O es is enrichment of leaf water above source, ε + is vapor pressure depression by 18 O (temperature dependent), ε k is fractionation through conductance, Δ 18 O v enrichment vapor relative to source, e a and e i 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 water- depends on tortuosity of water path xylem to cell wall surfaces Oxygen atoms in carbonyls are 27% more enriched than aqueous solution – 18 O 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).