530
25.2 Low Atmospheric CO2 Concentrations and Evolution
of C4 Pathway Photosynthesis
S. Henderson et al.
Low atmospheric CO2 concentrations in the geologically recent past are
commonly regarded as a major selective pressure leading to the evolution
of the C4 pathway of photosynthesis (Ehleringer et al. 1991). Values of
1500 to 30001111-1 CO2 are thought to have prevailed throughout much of
angiosperm evolution during the Cretaceous (Budyko et al. 1987). Minima
approaching present day concentrations are thought to have occurred in the
Paleocene (7 x 10 7 years ago) and the Miocene (2 x 10 7 years ago), and
these are thought to be the periods in which C4 plants evolved. However, we
need to bear in mind that stomatal closure in plants of xeric habitats may
have exaggerated the selective pressure of low internal CO2 concentrations
over a longer period of time.
These speculations are confirmed by a few palaeontogical data. Fossil
grass leaves from the Miocene in western North America are anatomically
similar to extant C4-pathway grasses (Thomasson 1987; Tidwell and
Nambudiri 1989). Isotopic evidence from paleosols (carbonate deposits in
soils formed about plant roots) suggest a dramatic change from predominantly C 3 to predominantly C4 vegetation in the Padawar Plateau of Pakistan,
again in the Miocene, some 10 7 years ago (Quade et al. 1989). This change
is believed to mark a change in seasonality of rainfall, and the development
of the Asiatic monsoon as a major feature of global climate, adding credence
to the notion that seasonal water stress was an important factor associated
with evolution of C4-pathway photosynthesis.
Reliable data for atmospheric CO2 concentration extends back only
160000 years. Figure 25.1 shows the concentration of CO 2 in ice of the
Vostock and Siple cores, and that measured recently in the atmosphere itself
(at Manna Loa, Hawaii, Boden et al. 1990). Although atmospheric CO 2
concentration has increased more or less logarithmically for about 10 5 years,
the present concern arises because the rate of change in atmospheric CO 2
concentration is now possibly the most rapid in the history of angiosperm
evolution. The Vostock core shows that CO2 concentrations as low as
2001111-1 prevailed at several times in the last 160000 years and that an
increase from 200 to 300Il11-1 has occurred twice, with a half-time of about
10 000 years. At the present time, with an atmospheric CO2 concentration
midpoint between 300 to 400 1l11- \ the rate of change is 100 to 1000 times
more rapid. The increase in global atmospheric CO2 concentration is possibly
greater and more rapid than anything experienced by land plants since the
end of the Cretaceous.
The notion that low atmospheric CO2 concentrations should play such
a dominant role in evolution of C4 plants is based on the extensive biochemical, physiological, and anatomical evidence for the central importance
of a CO2 concentrating mechanism in C4-pathway photosynthesis. First
25.2 Low Atmospheric CO2 Concentrations and Evolution
of C4 Pathway Photosynthesis
S. Henderson et al.
Low atmospheric CO2 concentrations in the geologically recent past are
commonly regarded as a major selective pressure leading to the evolution
of the C4 pathway of photosynthesis (Ehleringer et al. 1991). Values of
1500 to 30001111-1 CO2 are thought to have prevailed throughout much of
angiosperm evolution during the Cretaceous (Budyko et al. 1987). Minima
approaching present day concentrations are thought to have occurred in the
Paleocene (7 x 10 7 years ago) and the Miocene (2 x 10 7 years ago), and
these are thought to be the periods in which C4 plants evolved. However, we
need to bear in mind that stomatal closure in plants of xeric habitats may
have exaggerated the selective pressure of low internal CO2 concentrations
over a longer period of time.
These speculations are confirmed by a few palaeontogical data. Fossil
grass leaves from the Miocene in western North America are anatomically
similar to extant C4-pathway grasses (Thomasson 1987; Tidwell and
Nambudiri 1989). Isotopic evidence from paleosols (carbonate deposits in
soils formed about plant roots) suggest a dramatic change from predominantly C 3 to predominantly C4 vegetation in the Padawar Plateau of Pakistan,
again in the Miocene, some 10 7 years ago (Quade et al. 1989). This change
is believed to mark a change in seasonality of rainfall, and the development
of the Asiatic monsoon as a major feature of global climate, adding credence
to the notion that seasonal water stress was an important factor associated
with evolution of C4-pathway photosynthesis.
Reliable data for atmospheric CO2 concentration extends back only
160000 years. Figure 25.1 shows the concentration of CO 2 in ice of the
Vostock and Siple cores, and that measured recently in the atmosphere itself
(at Manna Loa, Hawaii, Boden et al. 1990). Although atmospheric CO 2
concentration has increased more or less logarithmically for about 10 5 years,
the present concern arises because the rate of change in atmospheric CO 2
concentration is now possibly the most rapid in the history of angiosperm
evolution. The Vostock core shows that CO2 concentrations as low as
2001111-1 prevailed at several times in the last 160000 years and that an
increase from 200 to 300Il11-1 has occurred twice, with a half-time of about
10 000 years. At the present time, with an atmospheric CO2 concentration
midpoint between 300 to 400 1l11- \ the rate of change is 100 to 1000 times
more rapid. The increase in global atmospheric CO2 concentration is possibly
greater and more rapid than anything experienced by land plants since the
end of the Cretaceous.
The notion that low atmospheric CO2 concentrations should play such
a dominant role in evolution of C4 plants is based on the extensive biochemical, physiological, and anatomical evidence for the central importance
of a CO2 concentrating mechanism in C4-pathway photosynthesis. First