Chapter 14 Photosynthesis in Seagrasses
341
Tyerman (1989) presented evidence that sucrose
serves an osmoregulatory role as a compatible solute in the cytoplasm of Zostera capricorni and the
same may be true for P. australis (Tyerman et al.,
1984).
These are the only reports to date on the possible involvement of organic compounds in seagrass
osmoregulation. However, the presence of considerable quantities of various inositol derivatives in mangroves (Popp, 1984) suggests that this diverse group
of plants growing exclusively in wet, saline environments, similar to those of the seagrasses, may use
these unusual compounds to alleviate their considerable osmoregulatory problems.
Several types of secondary metabolites have been
studied in seagrasses, usually from a chemotaxonomic viewpoint. Attaway et al. (1970) found that
the normal alkanes of several genera represented less
than 0.01% dry weight but their distribution parallelled current taxonomic schemes of the seagrasses
with Halodule (Diplanthera) and Syringodium distinguished from each other and even more clearly
from Thalassia and Halophila. Both groups were
very distinct from Ruppia. Cluster analysis of highresolution GCMS analyses of the sterols and fatty
acids of a number of species from tropical Australia
(Gillan et al., 1984) also confirmed significant segregation of the zannichelliacean genera Cymodocea
and Halodule from the hydrocharitacean genera
Thalassia and Enhalus. However, Halophila, a genus
from the latter family but with very different morphology, was separated at a much higher level from
all the other seagrasses analysed. That study was
aimed at tracing seagrass-derived material throughout the coastal ecosystem. A similar approach in
temperate Australia by Nicholls et al. (1982) showed
that the lipids, comprising up to 7.6% dry weight, of
P. australis and Heterozostera tasmanica, had patterns of monocarboxylic, dicarboxylic and hydroxy
acids sufficiently different to allow even detritus of
these two species to be distinguished.
Taxonomic questions at the species level in seagrasses have also been approached chemically by
McMillan et al. (1981) and McMillan (1983) using
secondary products such as the flavonones and their
sulphonated derivatives from Amphibolis, Halodule,
Halophila, Posidonia and Zostera.
XIV. Summary
Photosynthesis in seagrasses is constrained by their
aquatic existence. Apart from living in seawater,
with its high ion concentrations (the effect of
which on photosynthesis is little known) seagrasses
share many of the same features as freshwater hydrophytes. The most influential effect, on photosynthesis, is the diffusive boundary layer and the
limitations it imposes on the uptake of C i . As suggested by Maberly and Madsen (2002), many of
the unique features of aquatic plants originate from
adaptations to overcome these limitations. In hydrophytes the adaptations which occur are (i) obtaining CO 2 directly from the sediment via roots
(well documented in some freshwater hydrophytes,
but not in seagrasses), (ii) possessing the ability to
undertake CAM or C 4 -like metabolism and, most
common of all, (iii), being able to exploit reserves of
HCO
−
3 , as well as accessing atmospheric CO 2 (a device large denied to seagrasses). Aerenchyma is also
a shared feature with freshwater aquatics as an adaptation to reduce respiratory load (Williams and Barber, 1961) and to carry oxygen to underground parts
and CO 2 to the leaves (see Borum et al., Chapter 10).
Although photosynthesis can contribute only small
amounts of gas exchange to the aerenchyma system
(the major pathways of exchange of C i and O 2 being
across the epidermis), it nevertheless has a powerful effect on pressurising the aerenchymal system.
The effects of this pressurisation on photosynthesis
and other physiological processes have been little
studied.
Fluorescence techniques are currently providing
a powerful tool for studying photosynthesis in situ,
and this will be a much-needed impetus to study,
since seagrasses in contrast to freshwater aquatics
cannot easily be studied in tank cultures. First of all
there is a need to carry out detailed comparisons
on the relationship between rates measured by other
techniques (e.g. O 2 evolution) and fluorescence techniques. Then there are a host of individual questions to be asked and answered, such as the effect of
flow, the position along the leaf, the operation of the
xanthophyll cycle in photoprotection, and the influence of a changing spectral light climate through the
canopy on photosynthesis (Zimmerman, Chapter 13)
and other influences on light harvesting processes. In
this regard the recent addition of the Imaging PAM
should have a marked effect in speeding up this area
of research.
In terms of the biochemistry of photosynthesis,
it is clear that there is much still to be done and
that seagrass research in this area has lagged behind that in freshwater aquatics. The roles of βcarboxylation, MAP and photorespiration deserve
341
Tyerman (1989) presented evidence that sucrose
serves an osmoregulatory role as a compatible solute in the cytoplasm of Zostera capricorni and the
same may be true for P. australis (Tyerman et al.,
1984).
These are the only reports to date on the possible involvement of organic compounds in seagrass
osmoregulation. However, the presence of considerable quantities of various inositol derivatives in mangroves (Popp, 1984) suggests that this diverse group
of plants growing exclusively in wet, saline environments, similar to those of the seagrasses, may use
these unusual compounds to alleviate their considerable osmoregulatory problems.
Several types of secondary metabolites have been
studied in seagrasses, usually from a chemotaxonomic viewpoint. Attaway et al. (1970) found that
the normal alkanes of several genera represented less
than 0.01% dry weight but their distribution parallelled current taxonomic schemes of the seagrasses
with Halodule (Diplanthera) and Syringodium distinguished from each other and even more clearly
from Thalassia and Halophila. Both groups were
very distinct from Ruppia. Cluster analysis of highresolution GCMS analyses of the sterols and fatty
acids of a number of species from tropical Australia
(Gillan et al., 1984) also confirmed significant segregation of the zannichelliacean genera Cymodocea
and Halodule from the hydrocharitacean genera
Thalassia and Enhalus. However, Halophila, a genus
from the latter family but with very different morphology, was separated at a much higher level from
all the other seagrasses analysed. That study was
aimed at tracing seagrass-derived material throughout the coastal ecosystem. A similar approach in
temperate Australia by Nicholls et al. (1982) showed
that the lipids, comprising up to 7.6% dry weight, of
P. australis and Heterozostera tasmanica, had patterns of monocarboxylic, dicarboxylic and hydroxy
acids sufficiently different to allow even detritus of
these two species to be distinguished.
Taxonomic questions at the species level in seagrasses have also been approached chemically by
McMillan et al. (1981) and McMillan (1983) using
secondary products such as the flavonones and their
sulphonated derivatives from Amphibolis, Halodule,
Halophila, Posidonia and Zostera.
XIV. Summary
Photosynthesis in seagrasses is constrained by their
aquatic existence. Apart from living in seawater,
with its high ion concentrations (the effect of
which on photosynthesis is little known) seagrasses
share many of the same features as freshwater hydrophytes. The most influential effect, on photosynthesis, is the diffusive boundary layer and the
limitations it imposes on the uptake of C i . As suggested by Maberly and Madsen (2002), many of
the unique features of aquatic plants originate from
adaptations to overcome these limitations. In hydrophytes the adaptations which occur are (i) obtaining CO 2 directly from the sediment via roots
(well documented in some freshwater hydrophytes,
but not in seagrasses), (ii) possessing the ability to
undertake CAM or C 4 -like metabolism and, most
common of all, (iii), being able to exploit reserves of
HCO
−
3 , as well as accessing atmospheric CO 2 (a device large denied to seagrasses). Aerenchyma is also
a shared feature with freshwater aquatics as an adaptation to reduce respiratory load (Williams and Barber, 1961) and to carry oxygen to underground parts
and CO 2 to the leaves (see Borum et al., Chapter 10).
Although photosynthesis can contribute only small
amounts of gas exchange to the aerenchyma system
(the major pathways of exchange of C i and O 2 being
across the epidermis), it nevertheless has a powerful effect on pressurising the aerenchymal system.
The effects of this pressurisation on photosynthesis
and other physiological processes have been little
studied.
Fluorescence techniques are currently providing
a powerful tool for studying photosynthesis in situ,
and this will be a much-needed impetus to study,
since seagrasses in contrast to freshwater aquatics
cannot easily be studied in tank cultures. First of all
there is a need to carry out detailed comparisons
on the relationship between rates measured by other
techniques (e.g. O 2 evolution) and fluorescence techniques. Then there are a host of individual questions to be asked and answered, such as the effect of
flow, the position along the leaf, the operation of the
xanthophyll cycle in photoprotection, and the influence of a changing spectral light climate through the
canopy on photosynthesis (Zimmerman, Chapter 13)
and other influences on light harvesting processes. In
this regard the recent addition of the Imaging PAM
should have a marked effect in speeding up this area
of research.
In terms of the biochemistry of photosynthesis,
it is clear that there is much still to be done and
that seagrass research in this area has lagged behind that in freshwater aquatics. The roles of βcarboxylation, MAP and photorespiration deserve