Are C4 Pathway Plants Threatened by Global Climatic Change?
545
ecophysiology over which we have almost no control, but from which we
may learn much. In the preparation of the essay we have come to appreciate the evolutionary significance of the biochemical and physiological
cooperation which allows C4 plants to sequester the photosynthetic carbon
reduction cycle in the high CO2, Cretaceous-like, atmosphere of bundlesheath cells. We have become less concerned about coordination and fine
tuning, and more appreciative of the potential for compensation and compromise that arises as a consequence of metabolic complexity. We conclude
that, so far as making the best use of elevated atmospheric CO2 concentrations is concerned, C4 plants have "been there, done that". Beyond the
biochemical flexibility of CO2 assimilation, we also suspect that the physiological and anatomical requirements for handling high rates of assimilation,
especially at high temperatures, have been adequately selected during
evolution of C4 plants. However, we have to concede that we know almost
nothing about the photosynthetic ecophysiology of C4 plants growing under
x2 current CO2 concentrations. Much experimental research needs to be
done to explore an array of counter intuitive physiological responses, such
as the significance of nitrogen use efficiency (Wong and Osmond 1991).
Most of all, we need to explore the effects of elevated CO2 concentration on
assimilate partitioning and its reproductive outcome through viable seed and
vegetative development (Potvin 1986, 1991).
Although some of the relative advantage that the COz-concentrating
mechanism may confer on the photosynthetic rate of C4 plants in present
atmospheres may diminish in the face of global climatic change, it is by no
means clear that C3 plants will uniformly exploit accelerated CO2 fixation in
the long term. In fact C3 plants face an array of limitations on translocation
and utilisation of assimilates, of respiration at elevated temperatures and of
unpredictable reproductive outcomes (Smith et al. 1989; Potvin 1991) that
may constitute larger threats to fitness than they do to C4 plants. We are
tempted to conclude that the outcomes of the contest between plants with
the C3 and C4 photosynthetic pathway under elevated CO2 will depend more
on the extent of advantages gained by the former than on those surrendered
by the latter.
Indeed, when considering the increase in temperature occurring with
increasing COz, assuming that C4 grasses will not be significantly debilitated
by elevated CO2, and that response to elevated temperature can be projected on the basis of current correlations, we conclude that there may well
be a significant increase in the representation of C4 grasses in the Australian
flora. The southerly and easterly expansion in the range of C 4 grasses
in Australia may match the northerly expansion of the range of North
American grasses such as Echinochloa. It is evident that well-designed
monitoring programs to follow abundance and frequency of C 3 and C 4 plants
in key locations could provide valuable information for photosynthetic
ecophysiology in the decades ahead and serve as useful indicators of the
impact of global climatic change. Our results suggest that it may be more
545
ecophysiology over which we have almost no control, but from which we
may learn much. In the preparation of the essay we have come to appreciate the evolutionary significance of the biochemical and physiological
cooperation which allows C4 plants to sequester the photosynthetic carbon
reduction cycle in the high CO2, Cretaceous-like, atmosphere of bundlesheath cells. We have become less concerned about coordination and fine
tuning, and more appreciative of the potential for compensation and compromise that arises as a consequence of metabolic complexity. We conclude
that, so far as making the best use of elevated atmospheric CO2 concentrations is concerned, C4 plants have "been there, done that". Beyond the
biochemical flexibility of CO2 assimilation, we also suspect that the physiological and anatomical requirements for handling high rates of assimilation,
especially at high temperatures, have been adequately selected during
evolution of C4 plants. However, we have to concede that we know almost
nothing about the photosynthetic ecophysiology of C4 plants growing under
x2 current CO2 concentrations. Much experimental research needs to be
done to explore an array of counter intuitive physiological responses, such
as the significance of nitrogen use efficiency (Wong and Osmond 1991).
Most of all, we need to explore the effects of elevated CO2 concentration on
assimilate partitioning and its reproductive outcome through viable seed and
vegetative development (Potvin 1986, 1991).
Although some of the relative advantage that the COz-concentrating
mechanism may confer on the photosynthetic rate of C4 plants in present
atmospheres may diminish in the face of global climatic change, it is by no
means clear that C3 plants will uniformly exploit accelerated CO2 fixation in
the long term. In fact C3 plants face an array of limitations on translocation
and utilisation of assimilates, of respiration at elevated temperatures and of
unpredictable reproductive outcomes (Smith et al. 1989; Potvin 1991) that
may constitute larger threats to fitness than they do to C4 plants. We are
tempted to conclude that the outcomes of the contest between plants with
the C3 and C4 photosynthetic pathway under elevated CO2 will depend more
on the extent of advantages gained by the former than on those surrendered
by the latter.
Indeed, when considering the increase in temperature occurring with
increasing COz, assuming that C4 grasses will not be significantly debilitated
by elevated CO2, and that response to elevated temperature can be projected on the basis of current correlations, we conclude that there may well
be a significant increase in the representation of C4 grasses in the Australian
flora. The southerly and easterly expansion in the range of C 4 grasses
in Australia may match the northerly expansion of the range of North
American grasses such as Echinochloa. It is evident that well-designed
monitoring programs to follow abundance and frequency of C 3 and C 4 plants
in key locations could provide valuable information for photosynthetic
ecophysiology in the decades ahead and serve as useful indicators of the
impact of global climatic change. Our results suggest that it may be more