Update on Transpiration and Isotopes Heavy Water Fractionation during Transpiration

A small proportion of water molecules contain the heavier isotopes of hydrogen and oxygen. There is a tendency for these heavier molecules of water to accumulate in leaves during transpiration. This has several interesting repercussions, including effects on the isotopic composition of organic matter, and of atmospheric water vapor, carbon dioxide, and oxygen. In turn, these effects aid temporal reconstruction of climate and spatial and temporal reconstruction of primary production in various ways. A recent, novel report by Miller et al. (2006) showed that tree-ring O measurements carried a record of hurricane activity. The motivation for our laboratory to study water isotopes was to enhance studies of transpiration efficiency (TE; the leaf contribution to water-use efficiency at the plant, crop, or ecosystem level). The instantaneous TE is A/[gwn], where A is the rate of photosynthetic uptake of CO2, gw is the conductance to diffusion of water vapor to the atmosphere from the sites of evaporation within the leaf (made up of the stomatal conductance, gs, and boundary layer conductance, gb, in series), and n is the leaf-to-air water vapor concentration difference. Carbon isotope composition of leaves can be measured to determine the carbon isotopic discrimination, DC, during photosynthesis. In turn, DC relates to A/gt, the ratio of CO2 assimilation rate to the total conductance to diffusion of CO2 from the atmosphere to the sites of carboxylation. The two measures are obviously related, and it was suggested that the oxygen isotope composition (dO) of leaf material could be used to pick up differences in n (Farquhar et al., 1994). Further, because gw affects d O and A does not, it was hoped that measurements of DC and dO in organic matter could be combined to separate photosynthetic capacity effects on TE from gw effects (Yakir and Israeli, 1995; Saurer et al., 1997; Farquhar et al., 1998). The heavy isotopes of water, expressed in organic matter, have promise as a means of identifying genetic variation in gw. In an ecological and population biology context, the carbon, hydrogen, and oxygen isotope measurements could provide a practical surrogate for measures of the marginal water cost of carbon gain, @E/@A, with E the rate of transpiration (Cowan and Farquhar, 1977), as one might expect different stomatal and photosynthesis strategies to be associated with environments with differing rainfall statistics (Cowan, 1982). In this Update on heavy water fractionation, we focus on how transpiration ‘‘leads’’ to enrichment and ask whether enrichment is a measure of transpiration. We then sketch some applications.