380
J.R. Ehleringer
18.5.3.2 C4 Plants
Farquhar (1983) developed an expression for carbon isotope discrimination
in C4 plants,
where b3 and b4 are the fractionation of gaseous CO2 by Rubisco and the
fixation of bicarbonate by PEP carboxylase, respectively, and is the
leakage of CO2 from the bundle sheath cells. Depending on the value of,
the relationship with cJca could be positive, negative, or zero. The value of
cannot be measured directly, but only indirectly estimated by biochemical
or gas-exchange techniques. Early gas-exchange studies suggested that the
value of was close to 0.34, resulting in a zero slope between Ll and cJca for
C4 plants (Evans et al. 1986). Alternative biochemical calculations suggested
that should be in the range of 0.1-0.13 (Hatch and Osmond 1976; Jenkins
et al. 1989). The most recent estimates of for a broad range of C4 species
are 0.21 (Henderson et al. 1992), resulting in a negative relation between Ll
and cJca .
There is limited information on variation in Ll of C4 species. O'Leary
(1988) and Hubick et al. (1989) report variation on the order of 1-1.5%0 for
two crop species. While this may appear to be a small amount of variation,
at = 0.21, it does represent significant variation in Ci/Ca (Henderson et al.
1992). Bowman et al. (1989) reported that salt stress would induce an
equivalent change in online Ll values and attributed increased Ll values to an
increased bundle sheath leakage during stress. More recently, Walker and
Sinclair (1992) have reported changes in Ll of Atriplex along salinity gradients
in Australian deserts. The extent to which the changes in Ll values in
the field represent genetic differences and/or environmental acclimation is
unclear at this time. Should the differences in Ll of native C4 species represent genetic differences, then it is possible that Ci values and life-history
patterns may vary in a manner analogous to that of C3 plants, but opposite
in sign since Ll and Ci are negatively related at = 0.21.
18.5.4 Intercellular CO2 and Life History in C3 Plants
The Ll values vary among C3 species in the Sonoran Desert (Fig. 18.11).
Carbon isotope discrimination was positively correlated with expected soil
moisture availability, suggesting that species in wetter habitats had higher Ci
values. Of ecological interest, Ll values of species within a microbabitat were
inversely related to the life expectancy of the shrub. Longer-lived species
has lower Ci values than shorter-lived species, indicating a more conservative
efficiency of water use than in shorter-lived species. Similar negative correlations between life expectancy and Ll have also been observed among
J.R. Ehleringer
18.5.3.2 C4 Plants
Farquhar (1983) developed an expression for carbon isotope discrimination
in C4 plants,
where b3 and b4 are the fractionation of gaseous CO2 by Rubisco and the
fixation of bicarbonate by PEP carboxylase, respectively, and
leakage of CO2 from the bundle sheath cells. Depending on the value of
the relationship with cJca could be positive, negative, or zero. The value of
or gas-exchange techniques. Early gas-exchange studies suggested that the
value of
C4 plants (Evans et al. 1986). Alternative biochemical calculations suggested
that
et al. 1989). The most recent estimates of
are 0.21 (Henderson et al. 1992), resulting in a negative relation between Ll
and cJca .
There is limited information on variation in Ll of C4 species. O'Leary
(1988) and Hubick et al. (1989) report variation on the order of 1-1.5%0 for
two crop species. While this may appear to be a small amount of variation,
at
1992). Bowman et al. (1989) reported that salt stress would induce an
equivalent change in online Ll values and attributed increased Ll values to an
increased bundle sheath leakage during stress. More recently, Walker and
Sinclair (1992) have reported changes in Ll of Atriplex along salinity gradients
in Australian deserts. The extent to which the changes in Ll values in
the field represent genetic differences and/or environmental acclimation is
unclear at this time. Should the differences in Ll of native C4 species represent genetic differences, then it is possible that Ci values and life-history
patterns may vary in a manner analogous to that of C3 plants, but opposite
in sign since Ll and Ci are negatively related at
18.5.4 Intercellular CO2 and Life History in C3 Plants
The Ll values vary among C3 species in the Sonoran Desert (Fig. 18.11).
Carbon isotope discrimination was positively correlated with expected soil
moisture availability, suggesting that species in wetter habitats had higher Ci
values. Of ecological interest, Ll values of species within a microbabitat were
inversely related to the life expectancy of the shrub. Longer-lived species
has lower Ci values than shorter-lived species, indicating a more conservative
efficiency of water use than in shorter-lived species. Similar negative correlations between life expectancy and Ll have also been observed among