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, ∆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 ∆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 ∆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 C3 species, and showed that variation in ∆d was an integrated measure of long-term variation in the ratio of intercellular to ambient CO2 partial pressure, pi/pa. Detailed gas exchange measurements of carbon isotope discrimination during CO2 uptake, ∆A, and pi/pa were made on leaves of eight S. bicolor lines. The observed relationship between ∆A and pi/pa was linear with a negative slope of 3.7‰ in ∆A for a unit change in pi/pa. The slope of this linear relationship between ∆A and pi/pa 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 ∆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 environment interaction. S. Henderson et al. 112 variation in φ, carbon isotope discrimination was, as in C3 species, linearly related to pi/pa. Hence the possibility exists that ∆d can be used as a tool for estimating transpiration efficiency, W, in C4 species. The major disadvantage is the much reduced signal, as the slope of the relationship between ∆d and pi/pa is much less for C4 species than C3 species, which may make it difficult to differentiate between lines. Genetic variation in W and ∆d has been reported in separate studies of C4 species. For instance, Owonubi et al. (1982), Onken et al. (1986) and Hammer et al. (1990) found significant differences in W for S. bicolor lines. A greater number of studies has found genetic variation in pi/pa, CO2 assimilation rate or stomatal conductance which, in turn, could result in genetic variation in W (for S. bicolor, Henzell et al. 1976; Krieg 1983; Blum and Sullivan 1986; Kidambi et al. 1990; Peng et al. 1991; Donatelli et al. 1992; for Zea mays, Duncan and Hesketh 1968; Gaskel and Pearce 1981; for sugarcane, Irvine 1975; and Pennisetum americanum, Blum and Sullivan 1986). Genetic variation in ∆d has also been reported in C4 species; for example, for lines of Zea mays (Samejima 1984), Panicum coloratum (Ohsugi et al. 1988), S. bicolor (Hubick et al. 1990) and Saccharum spp. (Meinzer et al. 1994). O’Leary (1988), however, found no significant variation in ∆d among 120 lines of Z. mays. To test whether variation in ∆d is correlated with variation in W, we grew 30 S. bicolor lines in the glasshouse in large sealed pots and measured W as well as ∆d. The same lines were also grown in the field at Emerald, Central Queensland, to determine variation in ∆d in the field. In the subsequent 2 years, further field measurements were made of ∆d and W on a selected set of eight lines. These eight lines were also grown in the glasshouse in Canberra for detailed measurements of carbon isotope discrimination during CO2 uptake, ∆A, in the laboratory. Materials and methods