Oxygen and Hydrogen in Plants

Outline: Environmental factors Fractionation associated with uptake of water Metabolic Fractionation C3, CAM and C4 plants

Environmental factors Regional Precipitation 18 O 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 Precipitation Online Isotope in Precipitation Calculator (OIPC)

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 Ehleringer & Dawson, 1992 summer precipitation groundwater

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 18 O and 2 H Leaf water is enriched in heavier 18 O and 2 H Process is exacerbated in arid regions and reduced in humid regions Also affected by wind speed

Diurnal change in leaf evaporation Kahmen et al., 2008 leaf water

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. Where: R leaf = isotopic value of water in leaf R soil = isotopic value of water in soil R soil = isotopic value of water vapor in the air h * = relative humidity (0 h 1) normalized to leaf temp. eq = equilibrium isotopic fractionation factor H=1.076, O=1.092) k = 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 18 O and 2 H 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 18 O 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 Photosynthesis (autotrophic) Post-photosynthetic tissue synthesis (heterotrophic) Oxygen and hydrogen differ O in cellulose most affected by plant physiology while D most affected by biochemistry of plant

Photosynthetic effects on oxygen Potential sources for oxygen O 2 gas, CO 2 and water Cellulose and carbohydrate 18 O/ 16 O correlate mainly with tissue water Unclear where 18 O-enrichment occurs between synthesis of carbohydrates (photosynthesis) and synthesis of cellulose (metabolism). Regardless of species, there is a consistent overall 18 O- enrichment of ~27 between leaf water and cellulose

Photosynthetic effects on hydrogen Unlike oxygen, H sources only from water Nonetheless, complicated 1 H is preferentially incorporated into sugars 1 H 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 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 but subsequently D from tissue water exchanges with carbohydrate hydrogen. Plant physiology and biochemical pathways affect these processes H O

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 18 O along the length of the leaf Chain of Pools Gat-Bowser model (Helliker and Ehlringer, 2000)

More C4 Grasses Deviations in enrichment are dependent on: - distance from veins to evaporative site (Short interveinal distance = more enrichment) - Vein structure Back diffusion of 18 O enriched leaf water from stomata to vein water (Helliker and Ehlringer, 2000)