Download Why are non-photosynthetic tissues generelly 13C enriched

Why are non-photosynthetic
tissues generelly 13C enriched
compared with leaves in C3
plants?
Introduction
• Heterotrophic tissues in C3
plants tend to be enriched in
13C compared with leaves
• 6 hypothesis aimed at
explaining this isotopic
pattern in C3 plants
Carbon isotope composition of branch wood
plotted against leaves
Simplified overview
Carbon flow through a C3 plant
Hypothesis 1: Variation in biochemical
composition
• Metabolits have different
isotopic signatures
• Leaves and heterotrophic
tissues differ in their
biochemical compostion
(different pathways)
 Different composition could
lead to changes in isotopic
distribution in tissues
But: Single constituents e.g.: cellulose showed δ13C differences between
plant organs
Hypothesis 2: Seasonal separation of
growth
• Synthesis of heterotrophic tissues and leaves takes place at
different times during growing season
Spring:
production of vegetative tissues
enough Water
-> more discrimination
Sommer:
prodution of seeds, fruits
less water
-> less discrimination
Differences in photosynthetic discrimination against 13C
• But: In experiments with simultaneous leaf, stem and root
growth still some δ13C variations were observed
Hypothesis 3: Day versus night
translocation
• Night-time export associated with transitory starch breakdown,
producing sucrose enriched in 13
• Day-time export associated with sucrose biosynthese from triose
phosphates, producing sucrose depleted in 13C
But: Species that do not show contrasting diel patterns in growth
between leaves and heterotrophic tissues still have δ13C
differences between organs
Hypothesis 4: Fractionation during
respiration
• Fractionation during dark respiration causes 13C
depletion of leaves and/or 13C enrichment of
heterotrophic tissues
•
13C
enrichment of leaf-respired CO2 is assumed
to be associated with decarboxylation of organic
acids
→may occur during CO2 release by pyruvate
dehydrogenase (13C-depleted Acetyl-CoA)
Evidence opposed
• Dark respiration from woody tissues also
tends to be 13C -enriched commpared with
organic material
• Difficult to draw a generalised conclusion
about the impact of respiration on δ 13C
divergence
Hypothesis 5: Carbon fixation by PEP
carboxylase in heterotrophic tissues
• Heterotrophic tissues have a proportionally
larger production and retention of PEP
carboxylase-derived organic molecules than
leaves
• PEP shows a discrimination to CO2
→fixation by this enzyme may result in the addition
of 13C -enriched organic material
• PEP carboxylase fixes HCO3− at a greater rate
in heterotrophic tissues causing 13C enrichment
Evidence opposed
• Some sink tissues have shown similar δ
13C to carbon delivered to them in phloem
sap, suggesting a very modest input from
PEP carboxylase
Hypothesis 6: Developmental variation in
photosynthetic discrimination against 13C
during leaf expansion
• Expanding leaves fix carbon more
negative in δ 13C than mature leaves
(responsible for carbon export to
heterotrophic tissue)
• Higher values of pi/pa in expanding
leaves.
But: Evidence also suggests a 13C
enriching mechanism in roots
Conclusion
•
13C
enrichment of heterotrophic tissues compared to
leaves is a widespread phenomenon in C3 plants
• The extent can vary depending on tissue and
species
• May not exist a single explanation for this pattern
→Each of these six hypothesis can contribute to the
tendency for heterotrophic tissues to be 13C enriched