Combined Measurement of Carbon and Oxygen Isotope Ratios

A13C of plant tissue is known to be a photosynthesis-weighted integrator of carbon supply and demand (Farquhar et al., 1982). Variation in A13C may be driven by changes in gs, or changes in photosynthetic capacity (Vi), or changes in both, because A13C has been shown to be positively related to the ratio of intercellular to atmospheric CO2 concentration (ci/ca) (Farquhar et al., 1982) by:

where a is the 13C fractionation associated with diffusion through stomata and the boundary layer (4.4%o), and b is the effective biochemical fractionation by Rubisco during carbon fixation when c, is used, rather than the CO2 concentration at the site of carboxylation (about 27%o).

When variation in A13C is driven by changes in gs alone, a negative relationship between A13C and AlsO is predicted (equivalent to a positive correlation between <S13C and ┬┐lsO). If variation in A13C is driven by changes in Vi alone no relationship between A13C and AlsO is expected, because

AlsO is not affected by Vi. If variation in A13C is driven by increases in both gs and Vi, then the change in AlsO per unit change in A13C will be greater than if gs alone had increased (Barbour el al, 2002). As such, measurement of both A13C and AlsO will allow the gs and Vi effects on A13C to be teased apart (Farquhar et al, 1994; Yakir and Israeli, 1995; Scheidegger etal, 2000).

A review of published relationships reveals that a positive relationship between <513C and S180 (or the equivalent negative relationship between A13C and AlsO) has been found for a number of different experimental systems, including field-grown leaves (Sternberg et al, 1989; Barbour et al, 2000a), cellulose from field-grown trees (Saurer et al, 1997; Barbour et al, 2002) and cotton leaf tissue and its cellulose from plants grown in humidity-controlled glasshouses (Barbour and Farquhar, 2000). The AlsO:A13C relationship for an experimental system in which variation in A13C is driven by Vi alone has not been published to date. Of particular interest, Barbour et al (2002) interpreted variation in the slope of the tree ring cellulose 5lsO: <513C relationship between field sites as a function of average humidity at each site. As predicted by theory, a greater change in <518Oc per unit change in S13C of cellulose (<513Cc) was found when humidity was lower.

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