The influence of species and growing conditions on the 18-O enrichment of leaf water and its impact on 'effective path length'

P> The stable oxygen isotope ratio (delta 18O) 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 (L-m), a parameter introduced to leaf-water models still requires a comprehensive biological characterization to allow interpretation of delta 18O values in plant material with confidence. Here, we tested the variability of L-m across and within three species that developed leaves in environments with different relative humidity. We also tested whether the L-m of fully developed leaves is affected by short-term fluctuations in relative humidity. We determined that significant differences in L-m exist among Phaseolus vulgaris, Rizinus communis and Helianthus annuus. Within a given species, however, L-m values did not differ significantly among individuals. These findings indicate that L-m is species specific and a relatively constant parameter and that L-m will not obscure the interpretation of delta 18O values in plant material of a given species. We urge caution, however, because values for L-m are derived from fitting leaf-water models to measured values of delta 18O, so care must be taken in assigning a 'cause' to values of L-m as they likely capture a combination of different biological leaf properties.