P
Rp
R
sRs
R
Rp
s 1 (3)a
Ra
R
sRs
R
Ra
s 1 (4)wherePis the carbon isotope composition of the plant sample, athe carbon isotope composition of air,
Rsthe molar abundance ratio of^13 C/^12 C of the standard, and RpandRathe molar abundance ratios of
(^13) C/ (^12) C of the plant sample and air, respectively.
The reference material in determinations of carbon isotope ratios has traditionally been in CO 2 gen-
erated from a fossil PDB. The carbon isotope composition () is standardized against PDB; atmospheric
CO 2 has a value of 8‰ relative to PDB [92].
The carbon isotopic technique can also be used to quantify internal CO 2 levels of leaves on a long-
term basis. Internal CO 2 levels (Ci) represent a balance between AandT. The existence of variation in Ci
confirms the existence of genotypic differences in TE. Carbon isotope discrimination and TE are related
through independent relationships with Pi/Pa[10]. This depends to different extents on the way in which
plants coordinate leaf conductance to water vapor with the capacity for photosynthetic CO 2 uptake. Vari-
ation in coordination of leaf gsandAcan give rise to variation in Pi/Pa[10]. This, in turn, results in vari-
ation in TE and carbon isotope discrimination. It has been stated that if plant breeding is to affect de-
tectable changes in TE of dry matter production, (1 Pi/Pa) needs to be modified substantially [93]. In
theory, greater TE will be associated with low if the leaf-to-air vpd remains constant [10].
Farquhar et al. [90] have suggested that can be expressed based on gas exchange as follows:
a
Pa
P
a
Pi
bP
P
a
ia(ba)
P
P
a
i (5)
whereais the fractionation due to diffusion in air, which is about 4.4‰ [94]; bthe net fractionation
caused by carboxylation, which is about 27‰ [29]; and PaandPiare the ambient and intercellular par-
tial pressures of CO 2 , respectively.
840 SUBBARAO AND JOHANSEN
Figure 1 Carbon isotope discrimination, , versus the ratio of intercellular and ambient partial pressures of
CO 2 (Pi/Pa) when both are measured simultaneously in a gas exchange system. Peanut and barley are C 3 species
andAmaranthus edulisis a C 4 species (From Ref. 38.)