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| Titel |
Intramolecular isotope distributions reveal lower than expected increases in photosynthesis over the past 200 years |
| VerfasserIn |
Ina Ehlers, Angela Augusti, Iris Köhler, Pieter Zuidema, Iain Robertson, Mats Nilsson, Jurgen Schleucher |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250110241
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| Publikation (Nr.) |
 EGU/EGU2015-10219.pdf |
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| Zusammenfassung |
| The ability of the biosphere to act as CO2 sink through photosynthesis strongly influences
future atmospheric CO2 concentrations and crop productivity. However, plant responses to
increasing atmospheric CO2 are poorly understood, in particular on time scales of decades
that are most relevant for the global carbon cycle. Most plants in the global terrestrial
vegetation and most crops use the C3 photosynthetic pathway. Photorespiration
is a side reaction of C3 photosynthesis that reduces CO2 assimilation in all C3
plants. By studying intramolecular isotope distributions (isotopomer abundances)
in century-long archives of plant material, we reconstruct how the atmospheric
CO2 increase since industrialization has changed the ratio of photorespiration to
photosynthesis.
For 12 tree species from five continents, we observe that the CO2 increase has reduced the
photorespiration / photosynthesis ratio. However, the observed reduction is on average 50 %
smaller than expected from CO2 manipulation experiments. This apparent discrepancy is
explained by results from a factorial CO2 / temperature manipulation experiment, which
shows that isotopomers reflect the integrated effect of CO2 and temperature on
the photorespiration / photosynthesis ratio. Thus, the 50 % smaller suppression of
photorespiration in trees is explained by increases in leaf temperature of 2 Ë C, due to global
warming and a possible contribution of reduced transpirational cooling due to stomatal
closure.
Previous studies of long-term effects of increasing CO2 on trees have measured 13C
fractionation of leaf gas exchange (δ13C) in tree-ring series. In several studies a discrepancy
was observed: strong historic increases in photosynthesis are estimated, but increases in
biomass are not observed. The temperature influence revealed by our isotopomer
data resolves this discrepancy; the lower estimate of CO2 fertilization has major
implications for the future role of forests as CO2 sink and for vegetation-climate
interactions.
Isotopomer abundances reflect metabolic regulation, because enzyme isotope effects alter
the isotope abundance in individual intramolecular positions. Thus, isotopomers of long-lived
metabolites of historic plant material are the first tool to connect plant ecophysiology with
paleo research. Another strength is that ratios of isotopomers are independent of
source isotopic signatures (δ13C of CO2 and δD of water). Thus, isotopomer ratios
and source isotopic signatures are orthogonal signals of plant processes and of
environmental changes, respectively. Glucose has seven deuterium- and six 13C
isotopomers, each influenced by specific fractionation mechanisms, therefore several
climatic and/or physiological signals may be retrieved from just one metabolite. |
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