Are C4 Pathway Plants Threatened by Global Climatic Change?
537
25.3.3 Translocation of Carbohydrate
In C4 plants the C3 cycle in bundle-sheath cells is exposed to high CO 2
concentrations which support greater rates of sucrose synthesis than in C 3
species (Kalt-Torres et al. 1987; Furbank and Foyer 1988), but the extra
carbohydrate does not accumulate because the rate of assimilate export is
greater than in C3 leaves (Stephenson et al. 1976; Wardlaw 1982; KaltTorres et al. 1987). The higher capacity for sucrose synthesis in maize may
be supported by the tenfold lower sensitivity of enzymes of sucrose synthesis
to their regulators (Stitt and Heldt 1985). The higher capacity for sucrose
translocation in C4 plants may be supported by the closer proximity of
mesophyll cells to the vascular system. Indeed, several authors have used
this or related anatomical properties for taxonomic and predictive purposes
(Crookston and Moss 1974; Hattersley and Watson 1975).
Thus we also believe that C4 plants will cope with elevated CO2 because
they have mechanisms to deal with high rates of carbohydrate synthesis from
a C3 cycle exposed to high concentrations of CO2. Indeed, short-term
exposures (h) of C4 leaves to x2 current CO2 concentrations had little effect
on the amount of carbohydrate accumulated (Wardlaw 1982; BlechschmidtSchneider et al. 1989). On the other hand, the ability of C 3 plants to adapt
and utilize high CO2 concentrations is often limited by their capability to
store, translocate, or use the extra carbohydrate produced under high CO 2
concentrations (Stitt 1992).
25.3.4 Water Use Efficiency
Another advantage which the C4 photosynthetic pathway confers under
present CO2 concentrations is a higher water use efficiency compared to C 3
species, and it is often proposed that this advantage may diminish under
elevated CO2. However, available data suggest that C 4 plants may retain
most of this advantage (Table 25.2). The instantaneous water use efficiency
(ratio of CO2 assimilation rate to transpiration rate for single leaves) or
whole plant transpiration efficiency (unit dry matter produced per unit water
used) increases when both C3 plants and C4 plants are grown at x2 current
CO2 concentration. In C3 plants this is generally due to increases in the CO 2
assimilation rate, whereas in C4 plants it is due to a decrease in stomatal
conductance at elevated CO 2 ,
25.3.5 Nitrogen Use Efficiency
The nitrogen use efficiency of C4 plants is generally held to be superior to C 3
plants because Rubisco functions under CO2 saturation in the bundle sheath
(Brown 1978) and less Rubisco protein is required to sustain high rates of
537
25.3.3 Translocation of Carbohydrate
In C4 plants the C3 cycle in bundle-sheath cells is exposed to high CO 2
concentrations which support greater rates of sucrose synthesis than in C 3
species (Kalt-Torres et al. 1987; Furbank and Foyer 1988), but the extra
carbohydrate does not accumulate because the rate of assimilate export is
greater than in C3 leaves (Stephenson et al. 1976; Wardlaw 1982; KaltTorres et al. 1987). The higher capacity for sucrose synthesis in maize may
be supported by the tenfold lower sensitivity of enzymes of sucrose synthesis
to their regulators (Stitt and Heldt 1985). The higher capacity for sucrose
translocation in C4 plants may be supported by the closer proximity of
mesophyll cells to the vascular system. Indeed, several authors have used
this or related anatomical properties for taxonomic and predictive purposes
(Crookston and Moss 1974; Hattersley and Watson 1975).
Thus we also believe that C4 plants will cope with elevated CO2 because
they have mechanisms to deal with high rates of carbohydrate synthesis from
a C3 cycle exposed to high concentrations of CO2. Indeed, short-term
exposures (h) of C4 leaves to x2 current CO2 concentrations had little effect
on the amount of carbohydrate accumulated (Wardlaw 1982; BlechschmidtSchneider et al. 1989). On the other hand, the ability of C 3 plants to adapt
and utilize high CO2 concentrations is often limited by their capability to
store, translocate, or use the extra carbohydrate produced under high CO 2
concentrations (Stitt 1992).
25.3.4 Water Use Efficiency
Another advantage which the C4 photosynthetic pathway confers under
present CO2 concentrations is a higher water use efficiency compared to C 3
species, and it is often proposed that this advantage may diminish under
elevated CO2. However, available data suggest that C 4 plants may retain
most of this advantage (Table 25.2). The instantaneous water use efficiency
(ratio of CO2 assimilation rate to transpiration rate for single leaves) or
whole plant transpiration efficiency (unit dry matter produced per unit water
used) increases when both C3 plants and C4 plants are grown at x2 current
CO2 concentration. In C3 plants this is generally due to increases in the CO 2
assimilation rate, whereas in C4 plants it is due to a decrease in stomatal
conductance at elevated CO 2 ,
25.3.5 Nitrogen Use Efficiency
The nitrogen use efficiency of C4 plants is generally held to be superior to C 3
plants because Rubisco functions under CO2 saturation in the bundle sheath
(Brown 1978) and less Rubisco protein is required to sustain high rates of