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Summary Traditionally, leaves were thought to be supplied withCO2for photosynthesis by the atmosphere and respiration. Recent studies, however, have shown that the xylem also transports a significant amount of inorganic carbon into leaves through the bulk flow of water. However, little is known about the dynamics and proportion of xylem‐transportedCO2that is assimilated, vs simply lost to transpiration.Cut leaves ofPopulus deltoidesandBrassica napuswere placed in eitherKCl or one of three [NaH13CO3] solutions dissolved in water to simultaneously measure the assimilation and the efflux of xylem‐transportedCO2exiting the leaf across light andCO2response curves in real‐time using a tunable diode laser absorption spectroscope.The rates of assimilation and efflux of xylem‐transportedCO2increased with increasing xylem [13CO2*] and transpiration. Under saturating irradiance, rates of assimilation using xylem‐transportedCO2accounted forc.2.5% of the total assimilation in both species in the highest [13CO2*].The majority of xylem‐transportedCO2is assimilated, and efflux is small compared to respiration. Assimilation of xylem‐transportedCO2comprises a small portion of total photosynthesis, but may be more important whenCO2is limiting.
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Quantitative critique of leaf‐based paleo‐CO 2 proxies: Consequences for their reliability and applicability
A variety of proxies have been developed to reconstruct paleo‐CO2from fossil leaves. These proxies rely on some combination of stomatal morphology, leafδ13C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter‐comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under‐predict CO2, especially in warm (>15°C) and moist (>50%humidity) environments. This identification of a structural under‐prediction may help to explain the common observation that the simpler proxies often produce estimates of paleo‐CO2that are lower than those from the more complex proxies and other, non‐leaf‐based CO2proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leaf‐based model as possible.
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- Award ID(s):
- 1636005
- PAR ID:
- 10454513
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Geological Journal
- Volume:
- 56
- Issue:
- 2
- ISSN:
- 0072-1050
- Page Range / eLocation ID:
- p. 886-902
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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