Abstract
ABOUT 99% of all carbon is the 12C isotope while 1% is 13C. The precise ratio of the isotopes will vary depending on the material analysed. In plants, fractionation of carbon is brought about primarily by carbon dioxide assimilation in photosynthesis and is due to preferential utilisation of 12C and exclusion of 13C. Curiously enough, it has been found recently1,2 that higher plants which fix carbon dioxide by way of the Calvin C3 cycle pathway differ in 13C/12C ratios from plants which fix carbon dioxide through the C4-dicarboxylic acid pathway. Temperate pasture species fix carbon by way of the Calvin pathway and have 13C/12C ratios of approximately −28‰ (ref. 3 and M.M.L., J.H.T., and R. J. Jones, unpublished), whereas tropical pasture grasses fix carbon through the C4-dicarboxylic acid pathway4 and have 13C/12C ratios of approximately −12‰ (ref. 2 and M.M.L., J.H.T., and R.J.J., unpublished). 13C/12C ratios are expressed relative to a carbonate standard5.
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