CORRELATION BETWEEN CARBON-ISOTOPE DISCRIMINATION AND TRANSPIRATION EFFICIENCY IN LINES OF THE C-4 SPECIES SORGHUM-BICOLOR IN THE GLASSHOUSE AND THE FIELD

Transpiration efficiency, W, the ratio of plant carbon produced to wat er transpired and carbon isotope discrimination of leaf dry matter, De lta(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 t he 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 Delta(d) was 3.0 parts per t housand and the observed range was 0.4 parts per thousand. In the fiel d, the mean W was lower at 2.8 mmol C mol H2O and the mean Delta(d) wa s 4.6 parts per thousand. Significant positive correlations between W and Delta(d) were observed for plants grown in the glasshouse and in t he field. The observed correlations were consistent with theory, oppos ite to those for C-4 species, and showed that variation in Delta(d) wa s an integrated measure of long-term variation in the ratio of interce llular to ambient CO2 partial pressure, p(i)/p(a). Detailed gas exchan ge measurements of carbon isotope discrimination during CO2 uptake, De lta(A) and p(i)/p(a) were made on leaves of eight S. bicolor lines. Th e observed relationship between Delta(A) and p(i)/p(a) was linear with a negative slope of 3.7 parts per thousand in Delta(A) for a unit cha nge in p(i)/p(a). The slope of this linear relationship between Delta( A) and p(i)/p(a) in C-4 species is dependent on the leakiness of the C O2 concentrating mechanism of the C pathway, We estimated the leakines s (defined as the fraction of CO2 released in the bundle sheath by C-4 acid decarboxylations, which is lost by leakage) to be 0.2. We conclu de that, although variation in Delta(d) observed in the 30 lines of S. bicolor is smaller than that commonly observed in C-4 species, it als o reflects variation in transpiration efficiency, W. Among the eight l ines examined in detail and in the environments used, there was consid erable genotype x environment interaction.