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

Abstract

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. bicolorlines. 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 × environment interaction.