Short-Term Measurements of Carbon Isotope Discrimination in Several C₄ Species

Abstract

Carbon isotope discrimination (Δ) and leaf gas-exchange were measured simultaneously for a number of C₄ species. Linear relationships were found between A and the ratio of intercellular to ambient partial pressures of CO₂, p_I/p_a. These data were used to estimate the fraction of CO₂ released by C₄-acid decarboxylation in the bundle sheath, which subsequently leaks out to the mesophyll. We define this fraction as the leakiness of the system and it is also a measure of the extent to which phosphoenolpyruvate (PEP) carboxylations exceed ribulose 1,5-bisphosphate (RuBP) carboxylations.

For Sorghum bicolor and Amaranthus edulis, leakiness was estimated at 0.2 and was constant over a wide range of irradiances (between 480 and 1600 μmol quanta m^-2 s^-1), intercellular CO₂ pressures (between 30 and 350 μbar) and leaf temperatures (from 21ºC to 34ºC). At irradiances less than 240 μmol quanta m^-2 s^-1, leakiness appeared to increase. For a number of dicotyledonous and monocotyledonous species, of the various C₄-decarboxylation types, leakiness was also estimated at 0.2. Contrary to expectation, amongst the 11 species examined, those with suberised lamellae did not show lower values of leakiness than those without suberised lamellae. For one NAD-ME and one PCK monocot, the estimates of leakiness were significantly higher at 0.30 and 0.25, respectively. Long-term discrimination (assessed from carbon isotope composition of leaf dry matter) did not correlate well with these short- term measures of discrimination. We suggest that this may be due to differences between species in fractionations occurring after photosynthesis.