Two Categories of 13C/12C Ratios for Higher Plants

"C/"C ratios have been determined for plant tissue from 104 species representing 60 families. Higher plants fall into two categories, those with low 5PDB "3C values (-24 to -34%o) and those with high "3C values (-6 to -19%0). Algae have 8 "C values of -12 to -23%o. Photosynthetic fractionation leading to such values is discussed. Carbon isotope fractionation is associated with photosynthesis. This fractionation results in lowering the "C/"C ratio by about 20 per mille for land plants and 10 per mille for marine plants relative to atmospheric CO,. A model has been proposed by Park and Epstein (12) to delineate the processes associated with this fractionation. To understand more fully the carbon isotope record and its implications for plant physiology, a more extensive investigation of the "C/"C ratio in plants was undertaken. One hundred and four species representing 60 families have been investigated and the "C/"C ratio for these samples shows a much wider variation than previously reported. These results bear on more recent ideas regarding the biochemical mechanisms or pathways of carbon fixation as well as showing the relevance of "C/"C studies to biological processes. MATERIALS AND METHODS Plant material (in most instances a green leaf) was air-dried at room temperature, combusted at 800 C in an excess of oxygen, and isotope ratios of the CO2 evolved were measured on a Nier-type mass spectrometer modified according to McKinney et al. (10). Only the organic tissues of calcareous algae were used for the "C/"C measurements. The CaCO3 was removed by reaction with dilute HC1. Results are reported in terms of 8"C relative to a carbonate standard. 613 C R sample 1000 R standard1 where R = mass 45/mass 44 of sample or standard CO,. The standard is carbonate from the fossil skeleton of Belemnitella americana from the Peedee formation of South Carolina (PDB,). Thus a value of --10%o means that "C/Cl ratio of the 1 Research supported by National Science Foundation Grant No. BG-7517. 2Present address: Department of Botany, University of Texas at Austin, Texas 78712. sample is less than that of the standard by 10 per mille or 1%. The precision of measurement is ±0.1%o of the ratio. Sample replication, including all errors of sample preparation, was ± 0.5%0. Details of the procedures are described elsewhere (12).