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, pI/pa. 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 21�C to 34�C). 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.

Transpiration efficiency, W, the ratio of plant carbon produced to water transpired and carbon isotope discrimination of leaf dry matter, ∆ d , were measured together on 30 lines of the C 4 species, Sorghum bicolor, in the glasshouse and on eight lines grown in the field. In the glasshouse, the mean W observed was 4.9 mmol C mol -1 H 2 O and the range was 0.8 mmol C mol -1 H 2 O. The mean ∆ d was 3.0‰ and the observed range was 0.4‰. In the field, the mean W was lower at 2.8 mmol C mol -1 H 2 O and the mean ∆ d was 4.6‰. Significant positive correlations between W and ∆ d were observed for plants grown in the glasshouse and in the field. The observed correlations were consistent with theory, opposite to those for C 3 species, and showed that variation in ∆ d was an integrated measure of long-term variation in the ratio of intercellular to ambient CO 2 partial pressure, p i /p a . Detailed gas exchange measurements of carbon isotope discrimination during CO 2 uptake, ∆ A , and p i /p a were made on leaves of eight S. bicolor lines. The observed relationship between ∆ A and p i /p a was linear with a negative slope of 3.7‰ in ∆ A for a unit change in p i /p a . The slope of this linear relationship between ∆ A and p i /p a in C 4 species is dependent on the leakiness of the CO 2 concentrating mechanism of the C 4 pathway. We estimated the leakiness (defined as the fraction of CO 2 released in the bundle sheath by C 4 acid decarboxylations, which is lost by leakage) to be 0.2. We conclude that, although variation in ∆ d observed in the 30 lines of S. bicolor is smaller than that commonly observed in C 3 species, it also reflects variation in transpiration efficiency, W. Among the eight lines examined in detail and in the environments used, there was considerable genotype x environment interaction.

The carbon isotope composition of terrestrial C4 plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO2 to the carboxylation sites, but is also influenced by the final carboxylation of ribulose‐1,5‐bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C4 photosynthesis. As a new feature, the model considers four types of limitation of the overall CO2 assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO2, irradiance and water stress on short‐term discrimination by leaves of several C4 species measured simultaneously with CO2 gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C4 plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C4 plants, evolutionary and genetical aspects, and some ecological implications are reviewed.

Carbon-isotope discrimination (A) is used to distinguish between different photosynthetic pathways. It has also been shown that variation in A occurs among varieties of C3 species, but not as yet, in C4 species. We now report that A also varies among genotypes of sorghum (Sorghum bicolor Moench), a C4 species.The discrimination in leaves of field-grown plants of 12 diverse genotypes of sorghum was measured and compared with their grain yields. Discrimination varied significantly among genotypes, and there was a significant negative correlation between grain yield and A. The variation in A may be caused by genetic differences in either leakiness of the bundle-sheath cells or by differences in the ratio of assimilation rate to stomatal conductance. At the leaf level, the former should be related to light-use efficiency of carbon fixation and the latter should be related to transpiration efficiency. Both could relate to the yield of the crop.

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