Transcript Oxygen and Hydrogen in Plants

Oxygen and Hydrogen in
Plants
Outline:
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Environmental factors
Fractionation associated with uptake of water
Metabolic Fractionation
C3, CAM and C4 plants
Environmental factors
• Regional
• Precipitation d18O and dD (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 Precipitation
Online Isotope in Precipitation Calculator (OIPC)
http://wateriso.eas.purdue.edu/waterisotopes/
Global Meteoric Waterline
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 source
summer precipitation
groundwater
Ehleringer & Dawson, 1992
Fractionation 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
leaf water
Diurnal change in leaf evaporation
Kahmen et al., 2008
Leaf evaporation depends upon humidity
low RH
(arid)
high RH
(humid)
Santrucek et al, 2007
Isotope 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 Model
• Different forms but same basic idea
• Modeling the equilibrium isotopic composition of water
within a leaf.
Rleaf  a
*
eq
1 h a R
*
k
soil
 hRatm
• Where:
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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.
• aeq = equilibrium isotopic fractionation factor
(@25°C,
H=1.076, O=1.092)
• ak = kinetic fractionation factor (H=1.016, O=1.032)

Water 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 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. Transpiration
• Water from evaporation and transpiration have different d18O and dD
• Transpired water = soil water
• Evaporated water = soil water + isotopic fractionation
• A Keeling plot of 1/[water vapor] vs. d of water vapor is a mixing line between
atmosphere and evapotranspiration
transpiration
atmosphere
evaporation
Tsujimura et al, 2007
Fractionation associated with metabolic
processes
• Photosynthesis (autotrophic)
• Post-photosynthetic tissue synthesis (heterotrophic)
• Oxygen and hydrogen differ
d18O in cellulose most affected by plant physiology while
dD most affected by biochemistry of plant
Photosynthetic 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 18Oenrichment of ~27‰ between leaf water and cellulose
Photosynthetic 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
• 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
• 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
O
No enrichment until leaves
Synthesized, metabolic oxygen is
consistently ~27‰ heavier than
O in leaf water
H
Synthesized hydrogen is depleted
in D relative to leaf water but
subsequently D from tissue water
exchanges with carbohydrate hydrogen.
Plant physiology and biochemical
pathways affect these processes
Yakir, 1992
Telling different types of plants
apart-CAM, C3 and C4 differ
Sternberg, 1989
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 always
differ in D- depends on type of
C4 photosynthesis
Sternberg, 1989
C4 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 Grasses
Back diffusion of 18O enriched leaf
water from stomata to vein water
Deviations in enrichment are
dependent on:
- distance from veins to
evaporative site
(Short interveinal distance = more
enrichment)
- Vein structure
(Helliker and Ehlringer, 2000)