Photosynthesis in Poikilohydric Plants: A Comparison of Lichens and Bryophytes
325
determine R. In higher plants, maintenance respiration is often modeled
entirely from crude protein content or tissue nitrogen content (see Merino
1987; Buwalda 1991). Using these relationships, rates of 70-100mg CO2
g-lNh- 1 would give a value of around 0.7-1.0mg CO 2 g-ldwh- 1 (calculated at 1 % tissue nitrogen and 15°C). Crude protein contents of bryophytes are only slightly below those of other plant types in a tundra biome
although lichens may be lower (Table 7.9 in Longton 1988). From this
viewpoint, a similarity in R between lichens and bryophytes does not seem
so surprising since it would simply indicate similar, underlying metabolic
processes. There is a need for more respiration data for lichens and bryophytes that have been calculated on a nitrogen or protein basis.
In conclusion, the respiration rates of both lichens and bryophytes are
very high relative to A, but this is because of the low A maxima, not
because of unusually high respiration. Because respiration increases rapidly
with temperature, it becomes a major load at moderate to high temperatures; it has been suggested that this load is severe enough to exclude
bryophytes from lowland tropical areas (Frahm 1990a,b). From the previous
discussion, such an exclusion would also be expected to apply to lichens.
16.2.4 Lichens and Bryophytes as Shade Plants
Valanne (1984) has suggested that the characteristics of shade plants seem
to be obligate within the photosynthetic apparatus of mosses. All mosses
investigated had large amounts of the light-harvesting chlorophyll alb protein
complex, a low chlorophyll alb ratio and rapid in vivo fluorescence induction
kinetics. Thylakoid structure resembled that of higher, shade plants. The
cyanobacterial lichen, Pseudocyphellaria dissimilis, also shows a suite of
characters typical of shade plants (Green et al. 1991). Certainly the light
compensation and light saturation values for bryophytes and lichens are low
when compared to phanerogam sun plants (on an area and dw basis).
Extreme low values can be found in some cases such as saturation of A at
20flmolm-2s-1 PPD for P. dissimilis (Green et al. 1991) and very low
values for compensation and saturation in submerged mosses (Priddle 1980).
It follows that, if shade adaptation is entrenched, species growing in
higher light must utilize mechanisms to protect against damage. The most
obvious mechanism is the poikilohydric nature of lichens and mosses which
allows them to dry out and thus be metabolically inert. Dry lichens are
protected against photoinhibition (Demmig-Adams et al. 1990) and Ramatina maciformis, a lichen which occurs in open, desert habitats, seems to be
adapted to low light and temperatures, these being the conditions to which it
is exposed in the mornings when it is active and before it dries out (Lange
1969a). Antarctic lichens have also been shown to have peak photosynthesis
in dim light and to show photoinhibition if exposed to high light (Kappen et
al. 1991). Green lichens can apparently withstand excessive light by the use
325
determine R. In higher plants, maintenance respiration is often modeled
entirely from crude protein content or tissue nitrogen content (see Merino
1987; Buwalda 1991). Using these relationships, rates of 70-100mg CO2
g-lNh- 1 would give a value of around 0.7-1.0mg CO 2 g-ldwh- 1 (calculated at 1 % tissue nitrogen and 15°C). Crude protein contents of bryophytes are only slightly below those of other plant types in a tundra biome
although lichens may be lower (Table 7.9 in Longton 1988). From this
viewpoint, a similarity in R between lichens and bryophytes does not seem
so surprising since it would simply indicate similar, underlying metabolic
processes. There is a need for more respiration data for lichens and bryophytes that have been calculated on a nitrogen or protein basis.
In conclusion, the respiration rates of both lichens and bryophytes are
very high relative to A, but this is because of the low A maxima, not
because of unusually high respiration. Because respiration increases rapidly
with temperature, it becomes a major load at moderate to high temperatures; it has been suggested that this load is severe enough to exclude
bryophytes from lowland tropical areas (Frahm 1990a,b). From the previous
discussion, such an exclusion would also be expected to apply to lichens.
16.2.4 Lichens and Bryophytes as Shade Plants
Valanne (1984) has suggested that the characteristics of shade plants seem
to be obligate within the photosynthetic apparatus of mosses. All mosses
investigated had large amounts of the light-harvesting chlorophyll alb protein
complex, a low chlorophyll alb ratio and rapid in vivo fluorescence induction
kinetics. Thylakoid structure resembled that of higher, shade plants. The
cyanobacterial lichen, Pseudocyphellaria dissimilis, also shows a suite of
characters typical of shade plants (Green et al. 1991). Certainly the light
compensation and light saturation values for bryophytes and lichens are low
when compared to phanerogam sun plants (on an area and dw basis).
Extreme low values can be found in some cases such as saturation of A at
20flmolm-2s-1 PPD for P. dissimilis (Green et al. 1991) and very low
values for compensation and saturation in submerged mosses (Priddle 1980).
It follows that, if shade adaptation is entrenched, species growing in
higher light must utilize mechanisms to protect against damage. The most
obvious mechanism is the poikilohydric nature of lichens and mosses which
allows them to dry out and thus be metabolically inert. Dry lichens are
protected against photoinhibition (Demmig-Adams et al. 1990) and Ramatina maciformis, a lichen which occurs in open, desert habitats, seems to be
adapted to low light and temperatures, these being the conditions to which it
is exposed in the mornings when it is active and before it dries out (Lange
1969a). Antarctic lichens have also been shown to have peak photosynthesis
in dim light and to show photoinhibition if exposed to high light (Kappen et
al. 1991). Green lichens can apparently withstand excessive light by the use