560
E.-D. Schulze and M.M. Caldwell
at elevated CO2 are lower than would be expected based on short-term
assessments of photosynthetic rates as a function of CO2. If down-regulation
occurs, plants might constitute much less of a sink for CO 2 than previously
thought. These and other questions are the subject of much controversy and
certainly the case is not decided. Other changes are also expected. For
example, there are indications that, depending on the mineral nutrition of
plants, the photosynthetic protein content will decrease (Stitt 1991, 1993b).
This would be consistent with a down-regulation. In this book, the biochemical issues are of less concern than the whole-plant behavior of photosynthesis using mangroves and C4 plants are case examples.
The mangroves are taken as representing plants in a stressful environment. It appears (Ball and Passioura, Chap. 12) that mangroves probably
will save water rather than increase their photosynthetic rates. However,
this will not change the distribution of mangroves in the saline portions of
their distribution but would likely alter species composition in the less saline
habitats. This is an example of how difficult it is to make predictions
on the effects of global change. One must begin with species-specific predictions and pursue broader generalizations with considerable caution and
circumspection.
An equally difficult question relates to the future of C4 plants (Henderson
et aI., Chap. 25). One might expect that C3 plants will gain dominance
because they profit in their photosynthetic rates with increasing CO 2 while
C4 plants do not. However, the picture is again more complicated because in
addition to CO2, temperatures are also expected to rise. This may lessen the
increase of photosynthetic rates of C3 plants while having a negligible effect
on C4 plants. At present, there are many examples of C4 species spreading
at the expense of C3 species.
The chapters by Ball and Passioura (Chap. 12) and by Henderson et al.
(Chap. 22) venture into the future with predictions of whole- plant performance. However, in order to do this, we need not only to extrapolate
processes but also to scale up from leaves to canopies (Beyschlag et aI.,
Chap. 20) and landscapes (Tenhunen, Chap. 21). The problem of scaling
has led to a whole new area of global modeling in the ecophysiology of
photosynthesis. It is quite clear that the integration of our knowledge and
contribution to global climate predictions will only be achieved through
modeling. However, there is the risk of transmitting, and sometimes amplifying, errors when scaling up. Thus, scaling problems remain a major
challenge at present. In the future, however, it may in fact be possible
to directly measure global processes. Conway et al. (1988) determined
atmospheric CO2 concentrations in remote areas, and thereby made the
seasonal pulses of photosynthesis and respiration of the globe visible (Fig.
26.1). Wintertime increases in CO2 concentration alternate with CO2 depletion during summers of the northern hemisphere. These fluctuations in
the southern hemisphere are smaller and opposite in phase in the northern
hemisphere. The differences between maximum and minimum CO2 concen-
E.-D. Schulze and M.M. Caldwell
at elevated CO2 are lower than would be expected based on short-term
assessments of photosynthetic rates as a function of CO2. If down-regulation
occurs, plants might constitute much less of a sink for CO 2 than previously
thought. These and other questions are the subject of much controversy and
certainly the case is not decided. Other changes are also expected. For
example, there are indications that, depending on the mineral nutrition of
plants, the photosynthetic protein content will decrease (Stitt 1991, 1993b).
This would be consistent with a down-regulation. In this book, the biochemical issues are of less concern than the whole-plant behavior of photosynthesis using mangroves and C4 plants are case examples.
The mangroves are taken as representing plants in a stressful environment. It appears (Ball and Passioura, Chap. 12) that mangroves probably
will save water rather than increase their photosynthetic rates. However,
this will not change the distribution of mangroves in the saline portions of
their distribution but would likely alter species composition in the less saline
habitats. This is an example of how difficult it is to make predictions
on the effects of global change. One must begin with species-specific predictions and pursue broader generalizations with considerable caution and
circumspection.
An equally difficult question relates to the future of C4 plants (Henderson
et aI., Chap. 25). One might expect that C3 plants will gain dominance
because they profit in their photosynthetic rates with increasing CO 2 while
C4 plants do not. However, the picture is again more complicated because in
addition to CO2, temperatures are also expected to rise. This may lessen the
increase of photosynthetic rates of C3 plants while having a negligible effect
on C4 plants. At present, there are many examples of C4 species spreading
at the expense of C3 species.
The chapters by Ball and Passioura (Chap. 12) and by Henderson et al.
(Chap. 22) venture into the future with predictions of whole- plant performance. However, in order to do this, we need not only to extrapolate
processes but also to scale up from leaves to canopies (Beyschlag et aI.,
Chap. 20) and landscapes (Tenhunen, Chap. 21). The problem of scaling
has led to a whole new area of global modeling in the ecophysiology of
photosynthesis. It is quite clear that the integration of our knowledge and
contribution to global climate predictions will only be achieved through
modeling. However, there is the risk of transmitting, and sometimes amplifying, errors when scaling up. Thus, scaling problems remain a major
challenge at present. In the future, however, it may in fact be possible
to directly measure global processes. Conway et al. (1988) determined
atmospheric CO2 concentrations in remote areas, and thereby made the
seasonal pulses of photosynthesis and respiration of the globe visible (Fig.
26.1). Wintertime increases in CO2 concentration alternate with CO2 depletion during summers of the northern hemisphere. These fluctuations in
the southern hemisphere are smaller and opposite in phase in the northern
hemisphere. The differences between maximum and minimum CO2 concen-