The oxygen isotopic composition of molecules other than sucrose and cellulose has not often been measured (but see Schmidt et al., 2001). However, it is known that whole leaf tissue is significantly less enriched than cellulose (Barbour and Farquhar, 2000; Barbour et al, 2000a). Of the secondary metabolites, the <5lsO of lignin has been studied in more detail.
Lignin forms the second most abundant component of many plant tissues, notably wood, and is known to be isotopically rather different to cellulose (Gray and Thompson, 1977). Lignin is formed from three precursors (monolignols), which differ in the degree of methoxylation of the aromatic ring. The oxygen atoms in the methoxyl groups are added by cytochrome P-450-linked monooxygenases, which cleave molecular oxygen and add one oxygen atom to the aromatic ring, the other being reduced to water. At least initially, these oxygen atoms should retain the isotopic composition of molecular oxygen, minus any fractionation that occurs during the reaction (Barbour et al., 2001). Some exchange of these oxygen atoms with sink cell water during subsequent biochemical steps seems likely (Schmidt et al., 2001), and indeed Barbour etal. (2001) found that lignin SlsO was positively correlated with modeled mean annual <5lsO of rain for wood samples from Pinus species from around the world.
Chloroplastic water, enriched by evaporation, provides the substrate for photosynthetically produced O2 (Guy et al., 1987). On regional and global scales this enriched O2 represents the terrestrial contribution to the 23.5%o enrichment of atmospheric molecular oxygen above mean ocean water (i.e., the Dole effect; Bender et al, 1985). While SlsO of O2 in the atmosphere is fairly constant around the world, 518Or varies so that AlsO of O2 will be variable.
When source-water-related variation in 5lsO of lignin was removed by considering AlsO, 37% of variation in AlsO of lignin was explained by variation in AlsO of molecular oxygen in the atmosphere. Using a fitting procedure to a model of the isotopic history of oxygen in lignin, Barbour et al. (2001) suggested that about 60% of the oxygen in lignin from monooxygenase reactions (and hence from O2) had exchanged with local water during subsequent reactions, most likely during monolignol polymerization.
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