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Keywords:

  • gas exchange;
  • oxygen;
  • relative humidity;
  • stable isotopes;
  • transpiration;
  • water

Summary

  •  The stable oxygen isotope ratio (δ18O) of plant material has been shown to contain essential information on water and carbon fluxes at the plant and ecosystem scales. However, the effective path length (Lm), a parameter introduced to leaf-water models still requires a comprehensive biological characterization to allow interpretation of δ18O values in plant material with confidence.
  •  Here, we tested the variability of Lm across and within three species that developed leaves in environments with different relative humidity. We also tested whether the Lm of fully developed leaves is affected by short-term fluctuations in relative humidity.
  •  We determined that significant differences in Lm exist among Phaseolus vulgaris, Rizinus communis and Helianthus annuus. Within a given species, however, Lm values did not differ significantly among individuals.
  •  These findings indicate that Lm is species specific and a relatively constant parameter and that Lm will not obscure the interpretation of δ18O values in plant material of a given species. We urge caution, however, because values for Lm are derived from fitting leaf-water models to measured values of δ18O, so care must be taken in assigning a ‘cause’ to values of Lm as they likely capture a combination of different biological leaf properties