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T.G.A. Green and O.L. Lange
remarkably similar and low, maximal A compared with phanerogams on an
area or weight basis. Bryophytes and lichens also have very similar dark
respiration rates which only appear high because A is low. The relatively
high respiration rate does have potentially major effects on net photosynthesis at high temperatures and may even limit distributions in areas of
consistently high temperature, especially where the plants are wet at night.
Respiration-induced depressions in photosynthesis are found in lichens from
Antarctica at temperatures as low as 12 °C (Kappen et al. 1989). It is
suggested that the low A is a consequence of the simple photosynthetic
structure of lichens and bryophytes, in particular the low ratio of the internal
photosynthetic tissues to external surface area. Photosynthetic rates on a
chlorophyll basis are similar to those found in phanerogams. Structurally
imposed, low A was not a disadvantage in early colonization of the land,
which is suggested to have been by cyanobacteria, lichens, and bryophytes,
but had the potential to become a problem when other higher plants evolved.
Phanerogams opened a new series of shade environments that have been
exploited by both lichens and bryophytes.
The two groups are quite distinct in their abilities to deal with liquid
water. Bryophytes have the capacity to produce complex, two-dimensional,
self-supporting photosynthetic tissue, where the CO2 exchange surface is
effectively a "skin" over the surface of stored water. These tissues can be
arranged, not only to generate water storage volumes separated from gas
exchange areas, but also to encourage capillary water movement over, and
between, surfaces. In contrast, lichens have an internal photosynthetic surface which has relatively compact tissues between it and the atmosphere.
Any water storage will tend to block either the compact tissues or the outer
lichen surface. Lichens tend to have lower maximal water content on a dry
weight basis with high risk of A depression at high thallus water content.
This difference is probably the explanation for the dominance of bryophytes
in very wet habitats. Green algal lichens, in contrast, have a major advantage
in dry habitats because of their ability to attain positive net photosynthesis
using only air humidity.
The data base for detailed comparison of photosynthetic processes in
lichens and bryophytes is small. Therefore, the comparisons and conclusions
in this chapter must be regarded as tentative, but they pose areas for future
research.
Acknowledgments. Much of the work reported here was supported by the Deutsche
Forschungsgemeinschaft (Bonn) as a program of the Sonder-forschungsbereich 251 der
Universitat Wiirzburg. The Alexander von Humboldt Stiftung is also thanked for financial
support to TGAG. Julie Cooke is thanked for preparing the text and Frank Bailey for
drawing the figures. The New Zealand Department of Conservation is thanked for their
permission, over many years, to collect and study bryophytes and lichens in reserved and
park areas.
T.G.A. Green and O.L. Lange
remarkably similar and low, maximal A compared with phanerogams on an
area or weight basis. Bryophytes and lichens also have very similar dark
respiration rates which only appear high because A is low. The relatively
high respiration rate does have potentially major effects on net photosynthesis at high temperatures and may even limit distributions in areas of
consistently high temperature, especially where the plants are wet at night.
Respiration-induced depressions in photosynthesis are found in lichens from
Antarctica at temperatures as low as 12 °C (Kappen et al. 1989). It is
suggested that the low A is a consequence of the simple photosynthetic
structure of lichens and bryophytes, in particular the low ratio of the internal
photosynthetic tissues to external surface area. Photosynthetic rates on a
chlorophyll basis are similar to those found in phanerogams. Structurally
imposed, low A was not a disadvantage in early colonization of the land,
which is suggested to have been by cyanobacteria, lichens, and bryophytes,
but had the potential to become a problem when other higher plants evolved.
Phanerogams opened a new series of shade environments that have been
exploited by both lichens and bryophytes.
The two groups are quite distinct in their abilities to deal with liquid
water. Bryophytes have the capacity to produce complex, two-dimensional,
self-supporting photosynthetic tissue, where the CO2 exchange surface is
effectively a "skin" over the surface of stored water. These tissues can be
arranged, not only to generate water storage volumes separated from gas
exchange areas, but also to encourage capillary water movement over, and
between, surfaces. In contrast, lichens have an internal photosynthetic surface which has relatively compact tissues between it and the atmosphere.
Any water storage will tend to block either the compact tissues or the outer
lichen surface. Lichens tend to have lower maximal water content on a dry
weight basis with high risk of A depression at high thallus water content.
This difference is probably the explanation for the dominance of bryophytes
in very wet habitats. Green algal lichens, in contrast, have a major advantage
in dry habitats because of their ability to attain positive net photosynthesis
using only air humidity.
The data base for detailed comparison of photosynthetic processes in
lichens and bryophytes is small. Therefore, the comparisons and conclusions
in this chapter must be regarded as tentative, but they pose areas for future
research.
Acknowledgments. Much of the work reported here was supported by the Deutsche
Forschungsgemeinschaft (Bonn) as a program of the Sonder-forschungsbereich 251 der
Universitat Wiirzburg. The Alexander von Humboldt Stiftung is also thanked for financial
support to TGAG. Julie Cooke is thanked for preparing the text and Frank Bailey for
drawing the figures. The New Zealand Department of Conservation is thanked for their
permission, over many years, to collect and study bryophytes and lichens in reserved and
park areas.