Terrestrial vegetation currently absorbs approximately a third of anthropogenic CO 2 emissions, mitigating the rise of atmospheric CO 2 . However, terrestrial net primary production is highly sensitive to atmospheric CO 2 levels and associated climatic changes. In C 3 plants, which dominate terrestrial vegetation, net photosynthesis depends on the ratio between photorespiration and gross photosynthesis. This metabolic flux ratio depends strongly on CO 2 levels, but changes in this ratio over the past CO 2 rise have not been analyzed experimentally. Combining CO 2 manipulation experiments and deuterium NMR, we first establish that the intramolecular deuterium distribution (deuterium isotopomers) of photosynthetic C 3 glucose contains a signal of the photorespiration/photosynthesis ratio. By tracing this isotopomer signal in herbarium samples of natural C 3 vascular plant species, crops, and a Sphagnum moss species, we detect a consistent reduction in the photorespiration/photosynthesis ratio in response to the ∼100-ppm CO 2 increase between ∼1900 and 2013. No difference was detected in the isotopomer trends between beet sugar samples covering the 20th century and CO 2 manipulation experiments, suggesting that photosynthetic metabolism in sugar beet has not acclimated to increasing CO 2 over >100 y. This provides observational evidence that the reduction of the photorespiration/photosynthesis ratio was ca. 25%. The Sphagnum results are consistent with the observed positive correlations between peat accumulation rates and photosynthetic rates over the Northern Hemisphere. Our results establish that isotopomers of plant archives contain metabolic information covering centuries. Our data provide direct quantitative information on the "CO 2 fertilization" effect over decades, thus addressing a major uncertainty in Earth system models.A tmospheric CO 2 levels have increased from ∼200 ppm during the last ice age to currently 400 ppm, and they may, according to pessimistic scenarios, exceed 1,000 ppm in the year 2100 (1). Understanding plant responses to increasing CO 2 is currently hampered by two fundamental limitations: First, it is unknown how well manipulation experiments represent responses to the gradual CO 2 increase over decades and centuries. In Free-Air CO 2 Enrichment (FACE) experiments, which most closely mimic natural conditions, increases in [CO 2 ] generally increase plant growth, but this "CO 2 fertilization" effect often declines after a few years of enrichment (2). Such transient responses may be related to the step increases in [CO 2 ] used in the experiments, their limited duration (2), or factors other than CO 2 becoming limiting (3). Second, in response to the [CO 2 ] increase since industrialization, genetic (4) and phenotypic plant responses (5-7) have been observed. Although century-scale changes have been detected in carbon isotopes (δ 13 C) and attributed to [CO 2 ], these responses are tied to differences in intercellular substrate concentrations that reflect several metabolic fluxes and dif...
a b s t r a c tThere is a drive toward the mandated lowering and reporting of selected toxicants in tobacco smoke. Several studies have quantified the mainstream cigarette emissions of toxicants, providing benchmark levels. Few, however, have examined how measured toxicant levels within a single product vary over time due to natural variation in the tobacco, manufacturing and measurement. In a single centre analysis, key toxicants were measured in the tobacco blend and smoke of 3R4F reference cigarette and three commercial products, each sampled monthly for 10 months. For most analytes, monthly variation was low (coefficient of variation <15%); but higher (P20%) for some compounds present at low (ppb) levels. Reporting toxicant emissions as a ratio to nicotine increased the monthly variation of the 9 analytes proposed for mandated lowering, by 1-2 percentage points. Variation in toxicant levels was generally 1.5-1.7-fold higher in commercial cigarettes compared with 3R4F over the 10-month period, but increased up to 3.5-fold for analytes measured at ppb level. The potential error (2CV) associated with single-point-intime sampling averaged $20%. Together, these data demonstrate that measurement of emissions from commercial cigarettes is associated with considerable variation for low-level toxicants. This variation would increase if the analyses were conducted in more than one laboratory.
Summary• The abundance of the hydrogen isotope deuterium (D) in tree rings is an attractive record of climate; however, use of this record has proved difficult so far, presumably because climatic and physiological influences on D abundance are difficult to distinguish.• Using D labelling, we created a D gradient in trees. Leaf soluble sugars of relatively low D abundance entered cellulose synthesis in stems containing strongly D-labelled water. We used nuclear magnetic resonance (NMR) spectroscopy to quantify D in the C-H groups of leaf glucose and of tree-ring cellulose.• Ratios of D abundances of individual C-H groups of leaf glucose depended only weakly on leaf D labelling, indicating that the D abundance pattern was determined by physiological influences. The D abundance pattern of tree-ring cellulose revealed C-H groups that exchanged strongly (C(2)-H) or weakly (C(6)-H 2 ) with water during cellulose synthesis.• We propose that strongly exchanging C-H groups of tree-ring cellulose adopt a climate signal stemming from the D abundance of source water. C-H groups that exchange weakly retain their D abundance established in leaf glucose, which reflects physiological influences. Combining both types of groups may allow simultaneous reconstruction of climate and physiology from tree rings.
Stable isotopes in tree rings are important tools for reconstruction of past climate. Deuterium (D) is of particular interest since it may contain climate signals and report on tree physiology. Measurements of the D/H ratio of tree-ring cellulose have proven difficult to interpret, presumably because the D/H ratio of the whole molecule blends the abundances of the seven D isotopomers of cellulose. Here we present a method to measure the abundance of the D isotopomers of tree-ring cellulose by nuclear magnetic resonance spectroscopy (NMR). The method transforms tree-ring cellulose into a glucose derivative that gives highly resolved, quantifiable deuterium NMR spectra. General guidelines for measurement of D isotopomers by NMR are described. The transformation was optimized for yield and did not alter the original D isotopomer abundances, thus, conserving the original signals recorded in wood cellulose. In the tree-ring samples tested, the abundances of D isotopomers varied by approximately +/-10% (2% standard error). This large variability can only be caused by biochemistry processes and shows that more information is present in D isotopomer abundances, compared to the D/H ratio. Therefore, measurements of the D isotopomer distribution of tree rings may be used to obtain information on long-term adaptations to environmental changes and past climate change.
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