The multifaceted relationship between leaf water 18O enrichment and transpiration rate

Leaf water becomes enriched in the heavy isotope 18 O through the process of transpiration. The leaf water 18 O signal is recorded in plant organic material (Barbour 2007 ), in atmospheric CO 2 via the activity of carbonic anhydrase (Farquhar et al . 1993 ), and in atmospheric O 2 through photosynthetic O 2 evolution (Guy, Fogel & Berry 1993 ). These signals can be harnessed for a range of applications in global change research and plant science, including reconstruction of climate, estimation of primary productivity, and analysis of environmental and genetic effects on stomatal conductance. Mechanistic models of leaf water 18 O enrichment are required for these applications. A model to predict the 18 O enrichment of an evaporating water surface was initially developed by Craig & Gordon ( 1965 ) and refined for application to leaves by Dongmann et al . ( 1974 ). Building upon this, Farquhar & Lloyd ( 1993 ) proposed a model to relate the average, or bulk, leaf water 18 O enrichment (Δ 18 O L ) to that at the evaporative sites in the leaf (Δ 18 O e ), whereby the diffusion of 18 O enriched water away from the evaporative sites is opposed by the convection of unenriched vein water towards the evaporative sites (Fig.  1 ). They referred to this as a Péclet effect. As a result of the Péclet effect, the Δ 18 O L is predicted to decrease as stomata open and transpiration increases for a given set of environmental conditions. This means that for plants growing alongside one another, the 18 O enrichment of organic material can potentially provide an integrated measure of variability in stomatal conductance, which would be especially useful in ecophysiology and plant breeding