Are C 4 Pathway Plants Threatened by Global Climatic Change?
533
Thus, electron transport in the mesophyll chloroplasts not only powers
regeneration of phosphoenolpyruvate (PEP) in the C4 cycle but also drives
regeneration of PGA in the mesophyll cells so that interdependence between
the C 3 and C 4 cycle exists, irrespective of the type of decarboxylation
pathway (Hatch and Osmond 1976).
Further metabolic communication between the two cycles occurs through
the interconversion of PGA and PEP catalyzed by phosphoglycerate mutase
and enolase (Furbank and Leegood 1984). The relationship between the
PGA and PEP pools is shown in Fig. 25.2. This figure demonstrates the
strong positive correlation between the two metabolite pools in leaves of
Zea mays L. and Amaranthus edulis L. under different flux conditions
generated by short-term variations in irradiance, CO2 partial pressures and
leaf temperatures (Leegood and von Caemmerer 1988, 1989; Labate et al.
1990). In both, the size of the PGA pool was found to be closely related to
the rate of PGA production. Since PEP carboxylase is a highly regulated
enzyme, with a high affinity for bicarbonate and a relatively low affinity for
PEP (O'Leary 1982), the amount of PEP is likely to be of crucial importance
120
100 cD
o 0
0
•
,I •
•
- 80 c• .. ~ 0
C\J
E
0
0
• •
o. 0
0
E 60
~
0
:::i.
• ·a 0
0
'-'"
0
• cz. 00
a..
••• ° °
w
•
a.. 40
•
LI. •
~ oLl.
20 -
eLI.
Ll. 0
•
0
I
0
100
200
300
400
500
PGA (Ilmol m- 2 )
Fig. 25.2. The relationship between the steady-state contents of PGA anp PEP in leaves
of Zea mays and Amaranthus edulis with varying intercellular partial pressure of CO2 (Pi),
irradiimce and leaf temperature. The data are from Leegood and von Caemmerer (1988,
1989) for Z. mays L. [Pi varied (0), irradiance varied (0)] and A. edulis [Pi varied (e),
irradiance varied (.)], and from Labate et al. (1990) for Z. mays [leaf temperature varied
(.6.)]. Data from Labate et al. (1990) are converted from nmol mg Chl- 1 to !lmol m- 2
assuming 363 mg Chi m- 2 for Zea mays. (Lee good and von Caemmerer 1988)
533
Thus, electron transport in the mesophyll chloroplasts not only powers
regeneration of phosphoenolpyruvate (PEP) in the C4 cycle but also drives
regeneration of PGA in the mesophyll cells so that interdependence between
the C 3 and C 4 cycle exists, irrespective of the type of decarboxylation
pathway (Hatch and Osmond 1976).
Further metabolic communication between the two cycles occurs through
the interconversion of PGA and PEP catalyzed by phosphoglycerate mutase
and enolase (Furbank and Leegood 1984). The relationship between the
PGA and PEP pools is shown in Fig. 25.2. This figure demonstrates the
strong positive correlation between the two metabolite pools in leaves of
Zea mays L. and Amaranthus edulis L. under different flux conditions
generated by short-term variations in irradiance, CO2 partial pressures and
leaf temperatures (Leegood and von Caemmerer 1988, 1989; Labate et al.
1990). In both, the size of the PGA pool was found to be closely related to
the rate of PGA production. Since PEP carboxylase is a highly regulated
enzyme, with a high affinity for bicarbonate and a relatively low affinity for
PEP (O'Leary 1982), the amount of PEP is likely to be of crucial importance
120
100 cD
o 0
0
•
,I •
•
- 80 c• .. ~ 0
C\J
E
0
0
• •
o. 0
0
E 60
~
0
:::i.
• ·a 0
0
'-'"
0
• cz. 00
a..
••• ° °
w
•
a.. 40
•
LI. •
~ oLl.
20 -
eLI.
Ll. 0
•
0
I
0
100
200
300
400
500
PGA (Ilmol m- 2 )
Fig. 25.2. The relationship between the steady-state contents of PGA anp PEP in leaves
of Zea mays and Amaranthus edulis with varying intercellular partial pressure of CO2 (Pi),
irradiimce and leaf temperature. The data are from Leegood and von Caemmerer (1988,
1989) for Z. mays L. [Pi varied (0), irradiance varied (0)] and A. edulis [Pi varied (e),
irradiance varied (.)], and from Labate et al. (1990) for Z. mays [leaf temperature varied
(.6.)]. Data from Labate et al. (1990) are converted from nmol mg Chl- 1 to !lmol m- 2
assuming 363 mg Chi m- 2 for Zea mays. (Lee good and von Caemmerer 1988)