16 Photosynthesis in Poikilohydric Plants:
A Comparison of Lichens and Bryophytes
T.G.A. Green and O.L. Lange
16.1 Introduction
Poikilohydric plants are those in which water status is completely dependent
on their environment (Walter 1931) so that, in terrestrial habitats, the water
vapor partial pressure of the plant body comes into equilibrium with the
humidity of the atmosphere. These plants must be able to tolerate some
desiccation although the minimum water potentials reached will depend on
the environment and some poikilohydric plants may not be desiccationtolerant (Green et al. 1991). In contrast, homoiohydric, terrestrial, vascular
plants maintain their water status within fairly tight limits. Poikilohydry is
exhibited by some ferns, club mosses, and even several phanerogams, but
the major groups are algae, cyanobacteria (e.g., cyanobacterial mats), fungi
and, of course, lichens, and bryophytes. Taxonomically, lichens and
bryophytes are widely separated since, under presently accepted systems of
classification (Raven et al. 1986), they fall into two separate Kingdoms;
lichens in the Mycota and bryophytes in the Plantae. The situation is further
complicated by the phycobionts of lichens which may be in the Monera
(cyanobacteria) or Protista (chlorophyta) or in both, when multiple
photobionts are present.
Despite the taxonomic divergence, lichens and bryophytes are, to a
greater or lesser extent, present in almost every terrestrial ecosystem and
may dominate some plant communities. The single factor that both lichens
and bryophytes are poikilohydric may be sufficient to explain their cooccurrence in so many habitats. Although the two groups often coexist,
there is a general tendency for bryophytes to dominate in wetter
environments, such as in rainforests, and lichens to predominate in dry
areas, such as deserts.
Poikilohydry also affects the ecophysiology of photosynthesis for both
lichens and bryophytes. There is a general perception that both groups have
low photosynthetic rates compared to phanerogams. In this chapter an
attempt will be made to analyze bryophytes and lichens in terms of their
photosynthetic features: the mechanisms and how morphology interacts with
CO2 exchange. The analysis is not a complete review but builds on past
reviews including those of Proctor (1979, 1980, 1981, 1982, 1984, 1990),
A Comparison of Lichens and Bryophytes
T.G.A. Green and O.L. Lange
16.1 Introduction
Poikilohydric plants are those in which water status is completely dependent
on their environment (Walter 1931) so that, in terrestrial habitats, the water
vapor partial pressure of the plant body comes into equilibrium with the
humidity of the atmosphere. These plants must be able to tolerate some
desiccation although the minimum water potentials reached will depend on
the environment and some poikilohydric plants may not be desiccationtolerant (Green et al. 1991). In contrast, homoiohydric, terrestrial, vascular
plants maintain their water status within fairly tight limits. Poikilohydry is
exhibited by some ferns, club mosses, and even several phanerogams, but
the major groups are algae, cyanobacteria (e.g., cyanobacterial mats), fungi
and, of course, lichens, and bryophytes. Taxonomically, lichens and
bryophytes are widely separated since, under presently accepted systems of
classification (Raven et al. 1986), they fall into two separate Kingdoms;
lichens in the Mycota and bryophytes in the Plantae. The situation is further
complicated by the phycobionts of lichens which may be in the Monera
(cyanobacteria) or Protista (chlorophyta) or in both, when multiple
photobionts are present.
Despite the taxonomic divergence, lichens and bryophytes are, to a
greater or lesser extent, present in almost every terrestrial ecosystem and
may dominate some plant communities. The single factor that both lichens
and bryophytes are poikilohydric may be sufficient to explain their cooccurrence in so many habitats. Although the two groups often coexist,
there is a general tendency for bryophytes to dominate in wetter
environments, such as in rainforests, and lichens to predominate in dry
areas, such as deserts.
Poikilohydry also affects the ecophysiology of photosynthesis for both
lichens and bryophytes. There is a general perception that both groups have
low photosynthetic rates compared to phanerogams. In this chapter an
attempt will be made to analyze bryophytes and lichens in terms of their
photosynthetic features: the mechanisms and how morphology interacts with
CO2 exchange. The analysis is not a complete review but builds on past
reviews including those of Proctor (1979, 1980, 1981, 1982, 1984, 1990),