Short-term carbon-isotope discrimination in C₃-C₄ intermediate species

Short-term discrimination in assimilation of stable isotopes of carbon was measured for leaves of the C₃ species Phaseolus vulgaris L. cv. Hawkesbury Wonder and Flaveria pringlei Gandoger, the C₄ species Amaranthus edulis Speg., and the C₃-C₄ intermediate species Panicum milioides Nees ex. Trin, Flaveria floridana Johnson, and Flaveria anomala B.L. Robinson. Discriminations in the C₃ and C₄ species were similar to those expected from theoretical considerations. When ambient CO₂ pressure was 330 μbar the mean discriminations in the C₃ species and Panicum milioides were similar, whereas the mean discriminations in F. floridana and F. anomala were less than discrimination in C₃ species and Panicum milioides. When ambient CO₂ pressure was 100 μbar the mean discriminations in Panicum milioides and F. anomala were greater, and that in F. floridana was less, than that in Phaseolus vulgaris. We conclude that the pattern of discrimination in Panicum milioides is consistent with the presence of a glycine shuttle; in F. floridana and F. anomala, discrimination is consistent with the presence of a C₄ pathway coupled with the operation of a glycine shuttle.