Correlation between carbon isotope discrimination and transpiration efficiency in lines of the C₄ species Sorghum bicolor in the glasshouse and the field

Transpiration efficiency, W, the ratio of plant carbon produced to water transpired and carbon isotope discrimination of leaf dry matter, Δ(d), were measured together on 30 lines of the C₄ 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 H₂O and the range was 0.8 mmol C mol^-1 H₂O. The mean Δ(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 H₂O and the mean Δ(d) was 4.6‰. Significant positive correlations between W and Δ(d) were observed for plants grown in the glasshouse and in the field. The observed correlations were consistent with theory, opposite to those for C₃ species, and showed that variation in Δ(d) was an integrated measure of long-term variation in the ratio of intercellular to ambient CO₂ partial pressure, p(i)/p(a). Detailed gas exchange measurements of carbon isotope discrimination during CO₂ uptake Δ(A), and p(i)/p(a) were made on leaves of eight S. bicolor lines. The observed relationship between Δ(A) and p(i)/p(a) was linear with a negative slope of 3.7‰ in Δ(A) for a unit change in p(i)/p(a). The slope of this linear relationship between Δ(A) and p(i)/p(a) in C₄ species is dependent on the leakiness of the CO₂ concentrating mechanism of the C₄ pathway. We estimated the leakiness (defined as the fraction of CO₂ released in the bundle sheath by C₄ acid decarboxylations, which is lost by leakage) to be 0.2. We conclude that, although variation in Δ(d) observed in the 30 lines of S. bicolor is smaller than that commonly observed in C₃ 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 environmental interaction.