Transpiration efficiency, W, the ratio of plant carbon
produced to water transpired and carbon isotope discrimination of leaf dry
matter, Δd, 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
Δd 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
Δd was 4.6‰. Significant positive
correlations between W and
Δd 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 Δd 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, ΔA, and
pi/pa
were made on leaves of eight S. bicolorlines. The
observed relationship between ΔA and
pi/pa
was linear with a negative slope of 3.7‰ in
ΔA for a unit change in
pi/pa.
The slope of this linear relationship between ΔA
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 Δd
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.
Genetic and Environmental Variation in Transpiration Efficiency and Its Correlation with Carbon Isotope Discrimination and Specific Leaf Area in Peanut