532
S. Henderson et al.
1989). Although we know something of the population size and area for
many C4 species, little is known of contemporary evolution of C 4 plants.
However, in the grass Alloteropsis C. Presl., there is evidence from recent
collections that evolution of C4 photosynthesis is in process (P.W. Hattersley
and C. Long 1992, unpubl.).
25.3 Physiological Flexibility in C4 Plants Under High CO2 Concentrations
It is often assumed that the advantages which the C4 photosynthetic pathway
confers on plants over the C3 photosynthetic pathway will be somewhat
diminished under elevated global concentrations of CO2, This conclusion is
based on observations that short-term CO2 assimilation by leaves of C4
species is almost completely saturated at current CO2 concentrations. In
leaves of C3 species it continues to increase with increasing external CO2
concentration as Rubisco becomes increasingly saturated by CO2, Increased
rates of carboxylation and lower rates of photo respiration result in larger
net CO 2 assimilation rates and improved quantum efficiencies at high temperatures for C3 plants. However, growth and ecological advantage are not
simply related to photosynthetic assimilation. For example, the advantages
which elevated CO2 concentrations may confer on rate of C 3 photosynthesis
can be negated by their limited ability to use or store the extra carbohydrate
produced. In this section we shall argue that although there are no obvious
benefits of elevated CO2 concentrations to plants that possess the C4 photosynthetic pathway, the physiology of these plants may be well suited to these
environments.
25.3.1 Coordination of Metabolism
The dramatic specialization of photosynthetic functions between mesophyll
and bundle-sheath cells in C4 plants means that CO2 fixation depends upon
regulation of metabolite transport both within and between cells. The C4
cycle can be considered simply as a multifaceted, ATP-driven CO2 pump
which concentrates CO2 in the bundle sheath. Yet its very complexity seems
to confer a large degree of flexibility. For this system to operate efficiently,
CO 2 supply must match consumption by the C3 cycle with the avoidance of
excessive leakage of CO 2 from the bundle sheath. In NADP malic enzyme
species, such as maize and sorghum, the C4 cycle is obligatorily coupled to
the C 3 cycle because NADPH, generated in the reactions catalyzed by
NADP malic enzyme, is re-oxidized in the bundle-sheath chloroplast by the
reduction of glycerate-3-phosphate (PGA). However, in all the C4 plants, a
PGA/triose-P shuttle operates between the mesophyll and bundle sheath.
S. Henderson et al.
1989). Although we know something of the population size and area for
many C4 species, little is known of contemporary evolution of C 4 plants.
However, in the grass Alloteropsis C. Presl., there is evidence from recent
collections that evolution of C4 photosynthesis is in process (P.W. Hattersley
and C. Long 1992, unpubl.).
25.3 Physiological Flexibility in C4 Plants Under High CO2 Concentrations
It is often assumed that the advantages which the C4 photosynthetic pathway
confers on plants over the C3 photosynthetic pathway will be somewhat
diminished under elevated global concentrations of CO2, This conclusion is
based on observations that short-term CO2 assimilation by leaves of C4
species is almost completely saturated at current CO2 concentrations. In
leaves of C3 species it continues to increase with increasing external CO2
concentration as Rubisco becomes increasingly saturated by CO2, Increased
rates of carboxylation and lower rates of photo respiration result in larger
net CO 2 assimilation rates and improved quantum efficiencies at high temperatures for C3 plants. However, growth and ecological advantage are not
simply related to photosynthetic assimilation. For example, the advantages
which elevated CO2 concentrations may confer on rate of C 3 photosynthesis
can be negated by their limited ability to use or store the extra carbohydrate
produced. In this section we shall argue that although there are no obvious
benefits of elevated CO2 concentrations to plants that possess the C4 photosynthetic pathway, the physiology of these plants may be well suited to these
environments.
25.3.1 Coordination of Metabolism
The dramatic specialization of photosynthetic functions between mesophyll
and bundle-sheath cells in C4 plants means that CO2 fixation depends upon
regulation of metabolite transport both within and between cells. The C4
cycle can be considered simply as a multifaceted, ATP-driven CO2 pump
which concentrates CO2 in the bundle sheath. Yet its very complexity seems
to confer a large degree of flexibility. For this system to operate efficiently,
CO 2 supply must match consumption by the C3 cycle with the avoidance of
excessive leakage of CO 2 from the bundle sheath. In NADP malic enzyme
species, such as maize and sorghum, the C4 cycle is obligatorily coupled to
the C 3 cycle because NADPH, generated in the reactions catalyzed by
NADP malic enzyme, is re-oxidized in the bundle-sheath chloroplast by the
reduction of glycerate-3-phosphate (PGA). However, in all the C4 plants, a
PGA/triose-P shuttle operates between the mesophyll and bundle sheath.