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Introduction
Abiotic
Fractionation
Biotic
Fractionation
Case
Studies
References
Links
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Biotic Fractionation
Until the late seventies, isotopic ratios in plants were thought to simply
reflect the ratios in environmental water (Yakir 1992). In fact, all of
the organically bound hydrogen and most of the oxygen in a plant comes
from leaf water, and reflects its isotopic signature. However, fractionation
continues within
the plant, so that the observed delta values depend on the plant product
sampled (table 1) (Smith
and Jacobson 1976).
Oxygen and Hydrogen
Though abiotic fractionation events maintain an almost constant relationship
between 2H and 18O ratios, once the water molecule is split in plants,
the isotopes behave differently. There are two important differences.
First, oxygen inplants has three potential sources, H2O,
CO2 and O2. Hydrogen comes
from water. Secondly, because of its chemical properties, hydrogen undergoes
significant and variable fractionation in plants, whereas oxygen
values are comparatively similar across plant products.
Leaf Water
Because water enters roots passively, by diffusion, there is no fractionation
(Smith
and Ziegler 1990). The major physical fractionation in plants is of
leaf water. Evapotranspiration through the stomata enriches the leaf in
both 18O and D (see abiotic
fractionation) in a predictable way. Flanagan
et al (1991) provide the accepted leaf water model, which they adjusted
for boundary layer affects. Isotopic ratios vary throughout the leaf,
probably reflecting the difference between water in the veins (with values
almost identical to source water), and at the evaporative surfaces, where
it is enriched.
Fig 2. The difference between leaf, xylem, source, and atmospheric
water. Cascade head has higher humidity than the other sites, so
the leaf water is less enriched relative to source water.
From Roden
and Erhlinger 2000.
Fixation
The first biological fractionation is during photosynthesis.
Probably because the enzymes that mediate glucose have less affinity for
deuterium (Schmidt
et al 2001), it is significantly depleted during photosynthesis. The
magnitude of this effect is approximately -170‰ (Yakir
and DeNiro 1990). Oxygen does not fractionate so markedly. Photolysis
does not alter isotopic composition (Guy
et al. 1993), but there is a consistent +27‰ enrichment of cellulose
relative to source water (Yakir
and DeNiro 1990), and we do not yet know where that fractionation
occurs. Though the process of the enrichment has not been determined,
some suggest that CO2 equilibrates with H2O
before fixation, increasing the isotopic ratio (Yakir
1992).
Transport
Photosynthetic sugar then moves through the plant to be
stored or used as fuel in synthesizing compounds. During transport, it
is exposed to the medium xylem water. This water, direct from the roots,
has not undergone any fractionation. The deuterium in the biosynthates
is exchanges with this unfractionated, lighter water (Roden
et. al. 2000). Oxygen exchange with medium water during transport
may contribute to net 18O enrichment, though it is more tightly bound
than hydrogen, and less readily exchanged.
Biosynthesis
The products of photosynthesis are broken down to fuel
the synthesis of plant compounds, and the isotopic effect varies by compound.
For instance, 40% of organically bound hydrogen exchanges with the medium
water in forming cellulose, and overall enrichment is usually estimated
at +158 ‰ (Yakir
and DeNiro 1990), though published values range from +144-166‰,
depending on the substrate (Luo
and Sternberg 1992). Yakir and DeNiro (1990) hypothesized that light
hydrogen would be preferentially lost when enzymatic intermediates are
formed. Whatever the mechanism, this large enrichment in D counteracts
the large depletion during photosynthesis, leaving cellulose with an isotopic
signature close to that of the source water (Roden
et al.2000). Early research overlooked these strong and opposite fractionation
events because they go unrecorded in cellulose. In fatty acids, on the
other hand, D is depleted by two orders of magnitude relative to source
water (Sessions
et al. 1999). In the synthesis of fatty acids, D is not readily available
to exchange with source water, so it maintains the lowered ratio (Quemerias
et al 1995). The deuterium depletion of starches is roughly half that
of lipids (Luo
and Sternberg 1992). Oxygen enrichment, on the other hand, is consistently
around +27 ‰ regardless of the compound (but see also Luo
and Sternberg (1992) for values in starches). This suggests that,
though deuterium ratios are affected differently by enzymatic pathways
in different parts of the plant, there is only one oxygen isotope effect.
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