Variation of 13C within plants at Prairie Ridge, NC
Various samples of differentiated plant tissue were collected and run for each
plant sample obtained (roots, stems, leaves, etc.) The samples were analyzed for δ13C
ratio and carbon to nitrogen ratio. The results were grouped the δ13C values into
broadleaf and grass varieties (Table 4.1). For each plant sample we examined either a
stem, leaf, root, decayed matter, or any combination of the four. The class applied an
instrument correction to correct for the instruments difference to actual values. This
allowed us to obtain the actual values and entered into Table 4.1. On average C3 plants
(Fescue, Horsenettle, and Spanish needle) were around 30 0/00 depleted while the one
plant that fell into the C4 class (Bermuda grass) averaged about 13 0/00 depleted relative
to the standard. All major plant tissues were enriched in δ13C relative to the leaves with
the roots being the most enriched.
δ13C values for the roots in the C3 plants showed
much greater enrichment than the Bermuda grass. The C3 plant roots were around 3 0/00
enriched while the Bermuda grass was only enriched by 0.7 0/00 with one potential outlier
that showed depletion by about 15 0/00.
C3 and C4 plants differ mainly in their metabolic pathways and associated
enzymes and processes. All of the samples in this study were collected in an extremely
close proximity (one square meter); therefore it seems unlikely that the large differences
in isotopic values and especially differentiation of isotopic values in specific organ tissue
can be attributed to a difference in atmospheric CO2 based on the sample site. It has been
shown that δ13C ratios vary between the C3 and C4 plants we collected, but the variance
between the leaves and the roots does not match this pattern so some other force must be
at work. One possible way could be through the transformation of carbon compounds.
Differences in how and when carbon is divided and transferred in the different metabolic
systems developed in the two plant classes can account for such a difference (Hobbie &
Werner, 2004). The process of transforming leaf sugars to lignin and some lipids causes
fractionation, which leads to these molecules becoming depleted in 13C. Due to this the
left over sugars to become more and more enriched until finally being transported into the
root tissue (Park & Epstein 1961). Since sugars are the product of photosynthesis in the
leaves and travel down the xylem into the roots where they are utilized; it follows that
leaves are the most depleted part of the plant and roots are the most enriched. (Fig 5.1).
From Table 4.1 we can see both classes showing evidence for this enrichment of
C towards the root, but one much more than the other. So why are C3 root δ13C values
13
so much lower than C4 plants? If simple sugar derivatives do cause a change in δ13C
values then perhaps they could be measured in each class of plant to determine a
difference. Collister et al. (1994) examined products of plant metabolism of the simple
sugars generated during photosynthesis. Alkanes and lipids created during metabolism
showed 4-6 0/00 depleted relative to bulk ratio in C3 plants while in C4 plants they
showed greater depletion ranging from 8-10 0/00. As these compounds are continually
synthesized from the original leaf sugar during transport down the xylem, the lignin and
lipid compounds will always end up depleted, thus enriching the basic sugar more and
more until when it is finally utilized within the root tissue it is much more enriched than
the rest of the plant. Given that the alkanes appeared more depleted in C4 plants than C3
plants it seems odd that there was no a greater difference in C4 roots than we observed.
The answer for this could be that in C4 plants, lipid concentration was found to be about
half that in C3 plants (Chikaraishi & Naraoka, 2001). Thus with less lipid production the
difference from leaf δ13C values will not be as great in C4 plants as it is in C3 plants.
Conclusions
Between a combined approach of tissue specific sampling and literature review it
has been shown that both C3 and C4 metabolism plants have differing isotopic values in
different tissue systems. C3 plants averaging a bulk δ13C value of about 30 0/00 showed a
great deal of enrichment in the roots ~ 12 0/00 relative to the bulk composition likely due
to depletion during sugar transformation (Collister et. al 1994). C4 plants showed much
less enrichment than C3 plants only about 0.5 0/00. This could be due to a lower lipid
concentration in C4 plants (Chikaraishi & Naraoka, 2001). With lipids and alkanes
becoming depleted in δ13C during sugar transformation a plant with very few lipids will
not show as strong an enrichment of δ13C, just as our Bermuda grasses samples did. In
the future a more detailed analysis must be done with larger sample populations to
achieve statistical significance and determine the presence of outliers if any exist.
Figures
Fig. 5.1 Fraction as leaf sugars move from leaf to root. (Hobbie & Werner 2004)
Table 4.1 (Skidmore & Harris 2006)
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