540
S. Henderson et a1.
more than 15 dicotyledonous families). C4 species comprise about half of
the approximately 10 000 grass species known worldwide (Hattersley 1987;
Hattersley and Watson 1992). The latter authors concluded that the diversification of grasses at subfamily level may have been mainly associated with
divergence in photosynthesis in response to declining CO2 concentrations
and associated changes in temperature and rainfall. It is striking that the
climatic variables that seem likely to have been important in the evolutionary
diversification of grasses, are the same as those which are now being altered
anthropogenically; CO2, temperature and precipitation. Temperature and to
a lesser extent the seasonality of rainfall seem to have been most important
for the divergence of C3 and C4 plants in response to lowering of atmospheric
cb2 partial pressure, while amount of rainfall (in tropical and subtropical
climates) may have been most significant for diversification of the C4 pathway itself (Hattersley and Watson 1992). By no means all tropical plants are
C4, but where C4 photosynthesis evolved in a group (such as the grasses),
the outcome of intense competition between species, otherwise similarly
adapted, seems to have favored the C4 pathway.
Comparative present-day distributions of C3 and C4 grasses, for example
for North America (Teeri and Stowe 1976), Australia (Hattersley 1983),
Japan (Takeda et al. 1985) and Argentina (Cavagnaro 1988), suggest that
temperature is the key climatic variable which determines the presence or
absence of species with the two photosynthetic pathways. The correlation is
often best with minimum summer temperature as shown for Australian
grasses (Fig. 25.5a). Biochemical diversity within C4 grasses, on the other
hand, is best correlated with rainfall parameters. Thus percent C4 acid
decarboxylation type in Australian C4 grass floras correlates most closely
with median annual rainfall or median summer (December + January +
February) rainfall (Fig. 25.5b). It is fascinating to reflect upon the water
relations parameters which might underscore Fig. 25.5b.
There may well be no simple explanation for Fig. 25.5a. Although it
is commonly accepted that C4 plants may outcompete C3 plants at high
temperatures because the CO2 concentrating mechanism renders them
insensitive to increased photorespiration, other factors could be just as,
or even more important. Thus, minimum temperature during the growth
season might limit migration of C4 species of tropical origin because minimum
(night) temperatures limit development of the photosynthetic apparatus
(Slack et al. 1974) or translocation and/or utilization of assimilates (Potvin
et al. 1985). This would provide an alternative to the explanation that
limitations on the spread of C4 species is due to the increasing competitiveness of C 3 plant growth rates at lower temperatures which prevail at higher
latitudes and altitudes.
In either case, reproductive success presumably underlies the presence or
absence of species in a particular habitat, and almost nothing is known
of, for example, relationships between Summer minimum temperature and
reproductive success in the following Autumn. Although floral emergence in
S. Henderson et a1.
more than 15 dicotyledonous families). C4 species comprise about half of
the approximately 10 000 grass species known worldwide (Hattersley 1987;
Hattersley and Watson 1992). The latter authors concluded that the diversification of grasses at subfamily level may have been mainly associated with
divergence in photosynthesis in response to declining CO2 concentrations
and associated changes in temperature and rainfall. It is striking that the
climatic variables that seem likely to have been important in the evolutionary
diversification of grasses, are the same as those which are now being altered
anthropogenically; CO2, temperature and precipitation. Temperature and to
a lesser extent the seasonality of rainfall seem to have been most important
for the divergence of C3 and C4 plants in response to lowering of atmospheric
cb2 partial pressure, while amount of rainfall (in tropical and subtropical
climates) may have been most significant for diversification of the C4 pathway itself (Hattersley and Watson 1992). By no means all tropical plants are
C4, but where C4 photosynthesis evolved in a group (such as the grasses),
the outcome of intense competition between species, otherwise similarly
adapted, seems to have favored the C4 pathway.
Comparative present-day distributions of C3 and C4 grasses, for example
for North America (Teeri and Stowe 1976), Australia (Hattersley 1983),
Japan (Takeda et al. 1985) and Argentina (Cavagnaro 1988), suggest that
temperature is the key climatic variable which determines the presence or
absence of species with the two photosynthetic pathways. The correlation is
often best with minimum summer temperature as shown for Australian
grasses (Fig. 25.5a). Biochemical diversity within C4 grasses, on the other
hand, is best correlated with rainfall parameters. Thus percent C4 acid
decarboxylation type in Australian C4 grass floras correlates most closely
with median annual rainfall or median summer (December + January +
February) rainfall (Fig. 25.5b). It is fascinating to reflect upon the water
relations parameters which might underscore Fig. 25.5b.
There may well be no simple explanation for Fig. 25.5a. Although it
is commonly accepted that C4 plants may outcompete C3 plants at high
temperatures because the CO2 concentrating mechanism renders them
insensitive to increased photorespiration, other factors could be just as,
or even more important. Thus, minimum temperature during the growth
season might limit migration of C4 species of tropical origin because minimum
(night) temperatures limit development of the photosynthetic apparatus
(Slack et al. 1974) or translocation and/or utilization of assimilates (Potvin
et al. 1985). This would provide an alternative to the explanation that
limitations on the spread of C4 species is due to the increasing competitiveness of C 3 plant growth rates at lower temperatures which prevail at higher
latitudes and altitudes.
In either case, reproductive success presumably underlies the presence or
absence of species in a particular habitat, and almost nothing is known
of, for example, relationships between Summer minimum temperature and
reproductive success in the following Autumn. Although floral emergence in