Photosynthesis in Poikilohydric Plants: A Comparison of Lichens and Bryophytes
323
bonate seems never to be used as an inorganic carbon source (Bain and
Proctor 1980). CO2 compensation points range from 45-1601lIC021-1 air
(Dilks 1976; Proctor 1981) and, for Marchantia berteroana, are much lower
in 2% oxygen and show the expected increase with rise in temperature (Fig.
16.1, top). Photosynthesis is also depressed at 21% oxygen compared to 2%
oxygen and the depression increases at higher temperatures (Fig. 16.1,
bottom).
Lichens, on the other hand, appear atypical when compared to C 3 phanerogams. Carbon isotope ratios show a wide range from -14%0, for homoiomerous, cyanobacterial species to below -30%0 for some green species
(Lange and Ziegler 1986; Lange et a!. 1988). This is a very broad range
compared to values typical for C3 or C4 phanerogams and similar to that
found for CAM plants. The values have been explained in terms of different
thallus diffusion resistances (Lange et a!. 1988) but this may need reevaluation in the light of the discovery of carbon-concentrating mechanisms
(CCM) in lichens (Badger et a!. 1993, and later this section). CO 2 compensation points range from typical C3 values to very low values «10 IlIC021-1
air), more in the range of C4 or Cr C4 intermediate higher plants (Snelgar
and Green 1980; Green and Snelgar 1981a; Bauer 1984). The sensitivity of
A to oxygen level also varies and is low, as expected, when CO 2 compensation is low, but the situation is not simple (Green et a!. 1985). Green and
Snelgar (1981a) attempted to explain low CO2 compensation values as a
result of CO2 refixation within the lichen thallus. CO 2 refixation may well
occur but cannot produce a lower compensation value than that of the
photobiont, the CO2 compensation concentration at the photobiont must
always be the same, or lower than the entire lichen value (Cowan et a!.
1992). Although it is not yet totally clear how it is achieved, it is obvious
that lichens may be able to photosynthesize at much lower CO 2 levels than
bryophytes. The existence of COrconcentrating mechanisms is well known
for free-living green algae and cyanobacteria (Badger and Price 1992) and
has also been suggested for the photobionts of lichens (Raven et a!. 1990;
Maguas and Griffiths 1992). The existence of CCM in lichens has recently
been demonstrated by Badger et a1. (1993), and seems to be particularly
active in cyanobacterial lichens and, to a smaller extent (perhaps only 10%
of the cyanobacterial rate), in green lichens. The occurrence of CCM would
certainly explain the low CO2 compensation points and low CO 2 saturation
in lichens.
16.2.3 Dark Respiration Rates
Dark respiration rates, on a dry weight basis (R), of lichens and bryophytes
show similar response patterns to thallus water content and temperature.
Typically, R increases with the degree of hydration until a maximum value is
reached at a thallus water content that varies between species but is often in
323
bonate seems never to be used as an inorganic carbon source (Bain and
Proctor 1980). CO2 compensation points range from 45-1601lIC021-1 air
(Dilks 1976; Proctor 1981) and, for Marchantia berteroana, are much lower
in 2% oxygen and show the expected increase with rise in temperature (Fig.
16.1, top). Photosynthesis is also depressed at 21% oxygen compared to 2%
oxygen and the depression increases at higher temperatures (Fig. 16.1,
bottom).
Lichens, on the other hand, appear atypical when compared to C 3 phanerogams. Carbon isotope ratios show a wide range from -14%0, for homoiomerous, cyanobacterial species to below -30%0 for some green species
(Lange and Ziegler 1986; Lange et a!. 1988). This is a very broad range
compared to values typical for C3 or C4 phanerogams and similar to that
found for CAM plants. The values have been explained in terms of different
thallus diffusion resistances (Lange et a!. 1988) but this may need reevaluation in the light of the discovery of carbon-concentrating mechanisms
(CCM) in lichens (Badger et a!. 1993, and later this section). CO 2 compensation points range from typical C3 values to very low values «10 IlIC021-1
air), more in the range of C4 or Cr C4 intermediate higher plants (Snelgar
and Green 1980; Green and Snelgar 1981a; Bauer 1984). The sensitivity of
A to oxygen level also varies and is low, as expected, when CO 2 compensation is low, but the situation is not simple (Green et a!. 1985). Green and
Snelgar (1981a) attempted to explain low CO2 compensation values as a
result of CO2 refixation within the lichen thallus. CO 2 refixation may well
occur but cannot produce a lower compensation value than that of the
photobiont, the CO2 compensation concentration at the photobiont must
always be the same, or lower than the entire lichen value (Cowan et a!.
1992). Although it is not yet totally clear how it is achieved, it is obvious
that lichens may be able to photosynthesize at much lower CO 2 levels than
bryophytes. The existence of COrconcentrating mechanisms is well known
for free-living green algae and cyanobacteria (Badger and Price 1992) and
has also been suggested for the photobionts of lichens (Raven et a!. 1990;
Maguas and Griffiths 1992). The existence of CCM in lichens has recently
been demonstrated by Badger et a1. (1993), and seems to be particularly
active in cyanobacterial lichens and, to a smaller extent (perhaps only 10%
of the cyanobacterial rate), in green lichens. The occurrence of CCM would
certainly explain the low CO2 compensation points and low CO 2 saturation
in lichens.
16.2.3 Dark Respiration Rates
Dark respiration rates, on a dry weight basis (R), of lichens and bryophytes
show similar response patterns to thallus water content and temperature.
Typically, R increases with the degree of hydration until a maximum value is
reached at a thallus water content that varies between species but is often in