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The influence of species and growing conditions on the18-O enrichment of leaf water and its impact on‘effective path length’Ansgar Kahmen1,2, Kevin Simonin1, Kevin Tu1, Gregory R. Goldsmith1and Todd E. Dawson11Center for Stable Isotope Biogeochemistry, Department of Integrative Biology, University of California, Berkeley, CA, USA;2Institute of Plant Sciences,ETH Zurich, SwitzerlandAuthor for correspondence:Ansgar KahmenTel: 01 510 6431749Email: [email protected]Received:17 April 2009Accepted:23 June 2009New Phytologist(2009)doi: 10.1111/j.1469-8137.2009.03008.xKey words:gas exchange, oxygen, relativehumidity, stable isotopes, transpiration,water.SummaryThestable oxygen isotope ratio (d18O) of plant material has been shown tocontain essential information on water and carbon fluxes at the plant and ecosys-tem scales. However, the effective path length (Lm), a parameter introduced toleaf-water models still requires a comprehensive biological characterization toallow interpretation ofd18O values in plant material with confidence.Here,we tested the variability ofLmacross and within three species that devel-oped leaves in environments with different relative humidity. We also testedwhether theLmof fully developed leaves is affected by short-term fluctuations inrelative humidity.Wedetermined that significant differences inLmexist amongPhaseolus vulga-ris,Rizinus communisandHelianthus annuus. Within a given species, however,Lmvalues did not differ significantly among individuals.Thesefindings indicate thatLmis species specific and a relatively constantparameter and thatLmwill not obscure the interpretation ofd18O values in plantmaterial of a given species. We urge caution, however, because values forLmarederived from fitting leaf-water models to measured values ofd18O, so care mustbe taken in assigning a ‘cause’ to values ofLmas they likely capture a combinationof different biological leaf propertiesIntroductionThe stable oxygen and hydrogen isotope ratios (d18O anddD, respectively) of plant materials have been shown tocontain essential information for understanding plant andecosystem water and carbon fluxes (Dawsonet al., 2002;Barbour, 2007). Applications ofd18O anddD data rangefrom the reconstruction of paleoclimates and neoclimatesusing tree rings (Schiegl, 1974 Epstein & Yapp, 1977) orplant-derived organic compounds in lake sediments (Sachseet al., 2004, 2006), with the heavy isotope (i.e.18O) to theanalyses of carbon and water fluxes at the global (Farquhar& Lloyd, 1993) and ecosystem scales (Yakir & Wang,1996; Moreiraet al., 1997; Williamset al., 2004). In addi-tion,d18O anddD data collected at the leaf level can pro-videcriticalinformationonplantecophysiologicalresponsestoenvironmentalvariability(Barbouret al.,2000a; Cernusaket al., 2007, 2008; Ripulloneet al., 2008)and anthropogenic factors such as air pollution (Gramset al., 2007; Jaggi & Fuhrer, 2007; Bassinet al., 2009).

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