Oxygen and Hydrogen in Plants

Oxygen and Hydrogen in Plants

Outline: Outline:

• Environmental factors

• Fractionation associated with uptake of water

• Metabolic Fractionation

• C3, CAM and C4 plants

Environmental factorsEnvironmental factors

• Regional• Precipitation 18O and D (latitude, altitude and

continental effects)• Relative humidity, type and amount of precipitation, air

vapor pressure, seasonality and temperature

• Local• Water sources (isotopic composition and contribution

of ground, rain and surface waters), wind and evaporation

Isotopes in PrecipitationIsotopes in Precipitation

Online Isotope in Precipitation Calculator (OIPC)http://wateriso.eas.purdue.edu/waterisotopes/

Global Meteoric WaterlineGlobal Meteoric Waterline

Fractionation associated with uptake of water

Fractionation associated with uptake of water

• Plants have access to two main, isotopically distinct types of water:• Groundwater from saturated soil zone• Recent precipitation

• No fractionation of water from soil into roots, trunk or stems of plants

• Significant fractionation occurs in plant leaves due to evapotranspiration

• Water in different plant tissues mix (affected by moisture stress).

Xylem water = local sourceXylem water = local source

Ehleringer & Dawson, 1992

summer precipitation

groundwater

Fractionation of water in leavesFractionation of water in leaves

• Two processes:• Fractionation during phase change (liquid-vapor)• Diffusion of vapor into under saturated air

• Lighter isotopes are concentrated in the vapor relative to liquid & lighter isotopes diffuse faster• Evaporating vapor is depleted in heavier 18O and 2H• Leaf water is enriched in heavier 18O and 2H

• Process is exacerbated in arid regions and reduced in humid regions

• Also affected by wind speed

Diurnal change in leaf evaporationDiurnal change in leaf evaporation

Kahmen et al., 2008

leaf

wat

er

Leaf evaporation depends upon humidityLeaf evaporation depends upon humidity

low RH(arid)

high RH(humid)

Santrucek et al, 2007

Isotope Fractionation During EvaporationIsotope Fractionation During Evaporation

• Equilibrium fractionation• rates of evaporation and condensation

are equal

• Kinetic fractionation• forward and backward reactions not

equal (e.g. diffusion)

Evaporation is a two step process:– equilibrium fractionation between liquid water surface and saturated

boundary layer (depends on temperature)

– kinetic fractionation from diffusion into undersaturated atmosphere (depends upon water vapor gradient from leaf to atmosphere)

Craig-Gordon ModelCraig-Gordon Model

• Different forms but same basic idea • Modeling the equilibrium isotopic composition of water

within a leaf.

• Where: • Rleaf = isotopic value of water in leaf• Rsoil = isotopic value of water in soil• Rsoil = isotopic value of water vapor in the air• h* = relative humidity (0 h 1) normalized to leaf temp.• eq = equilibrium isotopic fractionation factor

(@25°C, H=1.076, O=1.092)

• k = kinetic fractionation factor (H=1.016, O=1.032)

€

Rleaf =α eq* 1− h*

( )α kRsoil + hRatm[ ]

Water within leavesWater within leaves

• Several pools of water contribute to isotopic composition of leaves:• Apoplastic water (mobile water) ~85% total

• Vein water• Evaporating water

• Symplastic/ semi crystalline water not involved in transpiration ~15% total

• These pools can mix

Leafwater RecapLeafwater Recap• Transpired water = soil water composition (by mass

balance)• Leafwater enriched in 18O and 2H at lower humidity• Temperature effects:

• equilibrium fractionation

• vapor pressure deficit

• Plant physiology matters too:• stomatal conductance (links carbon and water in plants)

• leaf veination

Evap. vs. TranspirationEvap. vs. Transpiration• Water from evaporation and transpiration have different 18O and D

• Transpired water = soil water• Evaporated water = soil water + isotopic fractionation

• A Keeling plot of 1/[water vapor] vs. of water vapor is a mixing line between atmosphere and evapotranspiration

Tsujimura et al, 2007

transpiration

evaporation

atmosphere

Fractionation associated with metabolic processes

Fractionation associated with metabolic processes

• Photosynthesis (autotrophic)

• Post-photosynthetic tissue synthesis (heterotrophic)

• Oxygen and hydrogen differO in cellulose most affected by plant physiology while D most affected by biochemistry of plant

Photosynthetic effects on oxygenPhotosynthetic effects on oxygen

• Potential sources for oxygen • O2 gas, CO2 and water

• Cellulose and carbohydrate 18O/16O correlate mainly with tissue water

• Unclear where 18O-enrichment occurs between synthesis of carbohydrates (photosynthesis) and synthesis of cellulose (metabolism). • Regardless of species, there is a consistent overall 18O-

enrichment of ~27‰ between leaf water and cellulose

Photosynthetic effects on hydrogenPhotosynthetic effects on hydrogen

• Unlike oxygen, H sources only from water

• Nonetheless, complicated

• 1H is preferentially incorporated into sugars

• 1H used to synthesize initial sugars but readily exchanges D-enriched leaf water.

• Amount of exchange dependent on temp. and distance transported

Heterotrophic metabolic effects on oxygen

Heterotrophic metabolic effects on oxygen

• Sugars transported throughout plant to create new tissues.• Carbonyl oxygen in sugars can exchange with oxygen in

water. • Consistent- regardless of species

• Cellulose tends to be 27‰ +/-3 ‰ higher than water in leaves.

• Fractionation related to 3-carbon sugar carbonyl hydration

supported by synthesis of cellulose from glycerol

Sternberg, 1989

Heterotrophic metabolic effects on hydrogen

Heterotrophic metabolic effects on hydrogen

• Complicated and variable• Hydrogen in sugars transported into other tissues exchanges

with H in water. Bigger effect than for oxygen. • Depending on distance transported, ~50% exchange is

possible!• Proportion of H exchanged depends on type of substrate

(lipids, starch, sugar) used to synthesize cellulose. • Variation can be reduced by analyzing only cellulose nitrate

extracted from tissues.

Recap: Fractionation in plants

Recap: Fractionation in plants

Yakir, 1992

No enrichment until leaves

Synthesized, metabolic oxygen is consistently ~27‰ heavier than O in leaf water

Synthesized hydrogen is depleted in D relative to leaf water butsubsequently D from tissue waterexchanges with carbohydrate hydrogen.

Plant physiology and biochemical pathways affect these processes

H

O

Telling different types of plants apart-CAM, C3 and C4 differ

Telling different types of plants apart-CAM, C3 and C4 differ

Sternberg, 1989

Where do C3, C4 and CAM differ?Where do C3, C4 and CAM differ?

Unclear:Probably during carbohydrate metabolism

-Cellulose Nitrate values differ-No difference in lipids

C3 and C4 do not alwaysdiffer in D- depends on type of C4 photosynthesis

Sternberg, 1989

C4 GrassesC4 Grasses

C4 plants differ from Craig-Gordon model predictions Cycling of oxygen progressively

enriches 18O along the length of the leaf“Chain of Pools” Gat-Bowser model

(Helliker and Ehlringer, 2000)

More C4 GrassesMore C4 Grasses

Deviations in enrichment are dependent on:- distance from veins to evaporative site (Short interveinal distance = more enrichment)

- Vein structure

Back diffusion of 18O enriched leaf

water from stomata to vein water

(Helliker and Ehlringer, 2000)