Farquhar, Graham D., Cernusak, Lucas A., and Barnes, Belinda (2007) Heavy water fractionation during transpiration. Plant Physiology, 143 (1). pp. 11-18.

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Abstract

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 18 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/[gwV], 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 V is the leaf-to-air water vapor concentration difference. Carbon isotope composition of leaves can be measured to determine the carbon isotopic discrimination, Δ13C, during photosynthesis. In turn, Δ13C 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 (δ18O) of leaf material could be used to pick up differences in V (Farquhar et al., 1994).

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