Photosynthesis in Poikilohydric Plants: A Comparison of Lichens and Bryophytes
331
Ventilated bryophytes have the additional diffusion resistance of the
pore structure between the photosynthetic cell surface and the atmosphere.
Values are available only for the thalloid liverworts, being 1.6scm- 1 (Marchantia foliacea, Green and Snelgar 1982a) and between 0.6 and 5.0scm- 1
(various thalloid liverworts, Proctor 1981). The only estimate reported for
cellular CO 2 diffusion resistance in these species is 68 s cm -1 (Green and
Snelgar 1982a), again much larger than that of the gas pathway resistance.
Ventilated tissues are susceptible to blockage by water (Table 16.4) and
it has been calculated that solid thalli are advantageous in habitats with
abundant liquid water (Green and Snelgar 1982a).
16.3.2 Lichens
The photosynthetic gas exchange surface of lichens is the surface of
the photobiont cells. Lichens, unlike bryophytes, cannot construct twodimensional surfaces entirely of photosynthetic tissue. This is a consequence
of the nature of the photobiont, often unicellular or, at best, filamentous or
in small groups. Any photosynthetic cells will always be surrounded by
fungal tissue to maintain structural integrity. In the majority of lichens
(heteromerous) the cells form a layer beneath the upper, fungal cortex (Fig.
16.2). The cortex is a compact structure in comparison to the lichen medulla,
which may have up to 18% air space (Collins and Farrar 1978). Although
water can be lost from the entire cortex surface, in many cases CO2 exchange
has been found to be confined to surfaces with special structures such as the
cyphellae or pseudo-cyphellae of the genera Sticta and Pseudocyphellaria
(Green and Snelgar 1982b; Green et al. 1982), or small pores (Peveling
1970; Hale 1981) estimated to be 0.1-0.2% of the cortex surface for R.
maciformis (Cowan et al. 1992), or to air spaces between fungal tissue. The
values of the diffusion resistances at the various points along the CO 2
diffusion pathway have been calculated for Pseudocyphellaria and Sticta
species (Snelgar et al. 1981) and measured for Ramalina maciformis (Cowan
et al. 1992). In both cases, at optimal water content, the resistance of the
gas diffusion pathways was found to be similar to non-gas-phase diffusion
resistances in the photobiont. In a comparison with a herbaceous higher
plant (Phaseolus vulgaris), the photobiont resistance to CO 2 uptake was
lower than the internal resistance of a mesophyll cell (Cowan et al. 1992).
Both Green and Snelgar (1981b) and Cowan et al. (1992) found that the gas
diffusion resistances had little effect on A (except at high thallus water
content). This is a consequence of the A response to external CO 2 concentration, which was nonlinear at 350j..lIC02 1- 1 air, with A often at about
90% of maximum rate. Thus, the majority of optimally wetted lichens
contrast with the situation for bryophytes where the CO 2 response is still
almost linear at normal ambient CO2 levels (Silvola 1985). The hypothesis
still seems to hold that the lichen thallus structure creates a moist, low VPD
331
Ventilated bryophytes have the additional diffusion resistance of the
pore structure between the photosynthetic cell surface and the atmosphere.
Values are available only for the thalloid liverworts, being 1.6scm- 1 (Marchantia foliacea, Green and Snelgar 1982a) and between 0.6 and 5.0scm- 1
(various thalloid liverworts, Proctor 1981). The only estimate reported for
cellular CO 2 diffusion resistance in these species is 68 s cm -1 (Green and
Snelgar 1982a), again much larger than that of the gas pathway resistance.
Ventilated tissues are susceptible to blockage by water (Table 16.4) and
it has been calculated that solid thalli are advantageous in habitats with
abundant liquid water (Green and Snelgar 1982a).
16.3.2 Lichens
The photosynthetic gas exchange surface of lichens is the surface of
the photobiont cells. Lichens, unlike bryophytes, cannot construct twodimensional surfaces entirely of photosynthetic tissue. This is a consequence
of the nature of the photobiont, often unicellular or, at best, filamentous or
in small groups. Any photosynthetic cells will always be surrounded by
fungal tissue to maintain structural integrity. In the majority of lichens
(heteromerous) the cells form a layer beneath the upper, fungal cortex (Fig.
16.2). The cortex is a compact structure in comparison to the lichen medulla,
which may have up to 18% air space (Collins and Farrar 1978). Although
water can be lost from the entire cortex surface, in many cases CO2 exchange
has been found to be confined to surfaces with special structures such as the
cyphellae or pseudo-cyphellae of the genera Sticta and Pseudocyphellaria
(Green and Snelgar 1982b; Green et al. 1982), or small pores (Peveling
1970; Hale 1981) estimated to be 0.1-0.2% of the cortex surface for R.
maciformis (Cowan et al. 1992), or to air spaces between fungal tissue. The
values of the diffusion resistances at the various points along the CO 2
diffusion pathway have been calculated for Pseudocyphellaria and Sticta
species (Snelgar et al. 1981) and measured for Ramalina maciformis (Cowan
et al. 1992). In both cases, at optimal water content, the resistance of the
gas diffusion pathways was found to be similar to non-gas-phase diffusion
resistances in the photobiont. In a comparison with a herbaceous higher
plant (Phaseolus vulgaris), the photobiont resistance to CO 2 uptake was
lower than the internal resistance of a mesophyll cell (Cowan et al. 1992).
Both Green and Snelgar (1981b) and Cowan et al. (1992) found that the gas
diffusion resistances had little effect on A (except at high thallus water
content). This is a consequence of the A response to external CO 2 concentration, which was nonlinear at 350j..lIC02 1- 1 air, with A often at about
90% of maximum rate. Thus, the majority of optimally wetted lichens
contrast with the situation for bryophytes where the CO 2 response is still
almost linear at normal ambient CO2 levels (Silvola 1985). The hypothesis
still seems to hold that the lichen thallus structure creates a moist, low VPD