Overview: Perspectives in Ecophysiological Research of Photosynthesis
559
lichens (Green and Lange, Chap. 16). Thus, it is with aquatic plants where
basically new observations on carbon fixation mechanisms were made in
recent years (Raven, Chap. 15).
Just as exciting is the world of poikilohydric plants, which received
general scientific attention following the work of Otto Lange (1953, 1965).
Presently, molecular biologists are searching for protein features in poikilohydric plants in order to achieve improved desiccation tolerance in crop
plants. However, even in comparing two groups of plants which are ecologically closely related, such as mosses and lichens, their whole-plant gas
exchange processes turn out to be quite complicated. Both plant types
inhabit similar niches; the lichens probably extending more to the dry side
and the mosses to the wet side of this spectrum. Mosses exhibit a low gasphase resistance and a high cellular resistance for carbon dioxide movement,
while lichens exhibit a high gas phase and a low cellular resistance for the
carbon-concentrating mechanism.
The distribution and competition of C3 and C4 plants is a problem similar
to that of the moss/lichen comparison. It has fascinated ecologists and
physiologists since the recognition of the C4 pathway. Study of competition
and distribution of C3 and C4 plants was greatly facilitated by use of carbon
stable isotope natural abundance levels in plant tissues. These allow not only
the identification of photosynthetic pathway types, but also other aspects of
photosynthesis, stomatal function, and water use. For example, it has been
possible to estimate season-long comparative water-use efficiencies of
different plants occurring in the same environments, or water-use behavior
of stems and leaves of the same plants. Assessment of deuterium natural
abundance levels in plant tissues makes it possible to make inferences about
the source water for plants in different habitat positions. Some surprises
have emerged; for example, streamside trees do not always obtain their
water from the stream (Ehleringer, Chap. 18). The combined information
on carbon, water, and nitrogen relations makes the use of stable isotopes an
attractive approach. However, many problems remain unsolved, such as the
heavy water abundance in parasitic plants relative to their hosts (Ziegler,
Chap. 19).
26.6 Photosynthesis and Global Climate Change:
Making Global Predictions
The question being put to ecophysiologists most commonly at present relates
to the expected changes of photosynthesis and plant growth with global
climate change, which includes elevated CO2 and temperatures. One of the
central issues is whether long-term exposure of plants to elevated CO 2
results in a down-regulation of photosynthesis, i.e., that photosynthetic rates
559
lichens (Green and Lange, Chap. 16). Thus, it is with aquatic plants where
basically new observations on carbon fixation mechanisms were made in
recent years (Raven, Chap. 15).
Just as exciting is the world of poikilohydric plants, which received
general scientific attention following the work of Otto Lange (1953, 1965).
Presently, molecular biologists are searching for protein features in poikilohydric plants in order to achieve improved desiccation tolerance in crop
plants. However, even in comparing two groups of plants which are ecologically closely related, such as mosses and lichens, their whole-plant gas
exchange processes turn out to be quite complicated. Both plant types
inhabit similar niches; the lichens probably extending more to the dry side
and the mosses to the wet side of this spectrum. Mosses exhibit a low gasphase resistance and a high cellular resistance for carbon dioxide movement,
while lichens exhibit a high gas phase and a low cellular resistance for the
carbon-concentrating mechanism.
The distribution and competition of C3 and C4 plants is a problem similar
to that of the moss/lichen comparison. It has fascinated ecologists and
physiologists since the recognition of the C4 pathway. Study of competition
and distribution of C3 and C4 plants was greatly facilitated by use of carbon
stable isotope natural abundance levels in plant tissues. These allow not only
the identification of photosynthetic pathway types, but also other aspects of
photosynthesis, stomatal function, and water use. For example, it has been
possible to estimate season-long comparative water-use efficiencies of
different plants occurring in the same environments, or water-use behavior
of stems and leaves of the same plants. Assessment of deuterium natural
abundance levels in plant tissues makes it possible to make inferences about
the source water for plants in different habitat positions. Some surprises
have emerged; for example, streamside trees do not always obtain their
water from the stream (Ehleringer, Chap. 18). The combined information
on carbon, water, and nitrogen relations makes the use of stable isotopes an
attractive approach. However, many problems remain unsolved, such as the
heavy water abundance in parasitic plants relative to their hosts (Ziegler,
Chap. 19).
26.6 Photosynthesis and Global Climate Change:
Making Global Predictions
The question being put to ecophysiologists most commonly at present relates
to the expected changes of photosynthesis and plant growth with global
climate change, which includes elevated CO2 and temperatures. One of the
central issues is whether long-term exposure of plants to elevated CO 2
results in a down-regulation of photosynthesis, i.e., that photosynthetic rates