oxygen and hydrogen in plants. outline: environmental factors fractionation associated with uptake...
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Oxygen and Hydrogen in Plants
Oxygen and Hydrogen in Plants
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Outline: Outline:
• Environmental factors
• Fractionation associated with uptake of water
• Metabolic Fractionation
• C3, CAM and C4 plants
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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
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Isotopes in PrecipitationIsotopes in Precipitation
Online Isotope in Precipitation Calculator (OIPC)http://wateriso.eas.purdue.edu/waterisotopes/
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Global Meteoric WaterlineGlobal Meteoric Waterline
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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).
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Xylem water = local sourceXylem water = local source
Ehleringer & Dawson, 1992
summer precipitation
groundwater
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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
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Diurnal change in leaf evaporationDiurnal change in leaf evaporation
Kahmen et al., 2008
leaf
wat
er
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Leaf evaporation depends upon humidityLeaf evaporation depends upon humidity
low RH(arid)
high RH(humid)
Santrucek et al, 2007
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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)
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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[ ]
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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Telling different types of plants apart-CAM, C3 and C4 differ
Telling different types of plants apart-CAM, C3 and C4 differ
Sternberg, 1989
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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
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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)
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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)
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