1. Why are non-photosynthetic tissues generelly 13C enriched compared with leaves in C3 plants?

2. 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

3. Carbon isotope composition of branch wood plotted against leaves

4. Simplified overview Carbon flow through a C3 plant

5. 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

6. 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

7. 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

8. Hypothesis 4: Fractionation during respiration Fractionation during dark respiration causes 13 C depletion of leaves and/or 13 C enrichment of heterotrophic tissues 13 C enrichment of leaf-respired CO 2 is assumed to be associated with decarboxylation of organic acids →may occur during CO 2 release by pyruvate dehydrogenase ( 13 C-depleted Acetyl-CoA)

9. Evidence opposed Dark respiration from woody tissues also tends to be 13 C -enriched commpared with organic material Difficult to draw a generalised conclusion about the impact of respiration on δ 13 C divergence

10. 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 CO 2 →fixation by this enzyme may result in the addition of 13 C -enriched organic material PEP carboxylase fixes HCO 3 − at a greater rate in heterotrophic tissues causing 13 C enrichment

11. Evidence opposed Some sink tissues have shown similar δ 13 C to carbon delivered to them in phloem sap, suggesting a very modest input from PEP carboxylase

12. Hypothesis 6: Developmental variation in photosynthetic discrimination against 13 C during leaf expansion Expanding leaves fix carbon more negative in δ 13 C than mature leaves (responsible for carbon export to heterotrophic tissue) Higher values of pi/pa in expanding leaves. But:Evidence also suggests a 13 C enriching mechanism in roots

13. Conclusion 13 C enrichment of heterotrophic tissues compared to leaves is a widespread phenomenon in C 3 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 13 C - enriched