324
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
II. Photosynthetic Adaptations
to Marine Submergence
The clear photosynthetic adaptation to marine submergence, also seen in hydrophytes (Sculthorpe,
1967), is the conversion of the leaf epidermis to the
primary site of photosynthesis, accompanied by the
loss of stomata and extreme reduction of the cuticle
(Larkum et al., 1989). This is presumably an adaptation to gaseous movement in a liquid medium rather
than a gaseous medium, where the rates of diffusion in the latter are reduced by a factor of ca 10,000
(Larkum et al., 1989; Borum et al., Chapter 10). This
Abbreviations
α – initial rate of photosynthesis for a P vs E curve
AF – Absorptance (absorption factor).
AZ – acetozolamide
C 4 – primary photosynthetic CO 2 fixation involving production
of C 4 acids
CA – carbonic anhydrase
CAM – crassulacean acid metabolism;
CCM – carbon concentrating mechanism;
C i – inorganic carbon sources for photosynthesis;
δ
13 C – fractionation of
13 C against
12 C compared to belemnite
standard
D1 – major polypeptide of PSII which binds Q A and partly P 680
E c – irradiance at the compensation point (older symbol, I c )
F – fractional yield of maximum fluorescence in actinic light
DBL – diffusive boundary layer
F m – maximum fluorescence yield
F m
– maximum fluorescence yield in actinic light
F o – minimum (dark) fluorescence yield
F t – fluorescence yield at time T in actinic light
F v – variable fluorescence yield
E – photon flux density
E K – E at which slope of light limiting region of the light curve
(α) intercepts Pmax (older term, I k )
ETR – electron transport rate
GC-MS – gas chromatography-mass spectrometry
MAP – Mehler ascorbate peroxidase (pathway)
NADP – nicotinamide dinucleotide phosphate
NPQ – non-photochemical quenching
NSCC – non-structural carbohydrate carbon
P max – maximum rate of photosynthesis
PAM – pulse amplitude modulated (fluorometry)
PAR – photosynthetically active radiation
PQ – photochemical quenching
PSI – photosystem I
PSII – photosystem II
qE – energy dependent quenching of fluorescence yield
qN – quotient of non-photochemical quenching
qP – quotient of photochemical quenching
Q A – plastoquinone acceptor of PSII
Q B – secondary plastoquinone acceptor of PSII
RLC – rapid light curve
Rubisco – ribulose-1,5-bisphosphate carboxylase/oxygenase
UV-B – ultraviolet B radiation
primary adaptation has been accompanied by three
very important adaptations also found in a number of
other submerged plants and some, but not all, algae:
(i) development of biochemical mechanisms for
enhanced inorganic carbon uptake, similar to
the carbon concentrating mechanisms (CCMs) of
some algae,
(ii) limitation of photosynthesis to the outer cell
layers of the leaf or lamina and reduction in the
non-photosynthetic tissues of the leaf or lamina
(iii) development of aerenchyma.
There may also be other adaptations which are
poorly documented at present such as reduction in
photorespiration and the Mehler ascorbate peroxidase (MAP) pathway (see Section VII) in order to
counteract the high levels of oxygen generated internally in the leaf as a result of the photosynthetic epidermis and high diffusive boundary layers at the leaf
surface (see e.g. Maberly and Madsen, 2002). However, both mechanisms (photorespiration and MAP
pathway) play an important role in protecting seagrass leaves from damage from excess light, under
exactly these conditions, so a balance has to occur.
However, in freshwater hydrophytes the tendency is
to thinner leaves with often just two cell layers in
leaves, both photosynthetic. This is not an option in
most seagrasses (Halophila spp. are an exception)
because of wave action, which necessitates a strong,
tough photosynthetic lamina.
These aspects are discussed in greater detail in the
following sections
III. The Diffusive Boundary Layer (DBL)
Substantial diffusive boundary layers are found
at most respiratory or photosynthetic surfaces of
submerged plants, animals or bacteria (Jørgensen,
2001). The size and stability of these layers have not
been explored fully in seagrasses (but see Larkum
et al., 1989); however, their existence has been shown
in the freshwater aquatic, Potomageton spp (SandJensen and Revsbech, 1987) and it is certain that
they exist in seagrasses. As a consequence of the
DBL and the anatomy of seagrass leaves, water motion will be a critical factor in determining the rate
of photosynthesis in seagrasses (Larkum et al., 1989
and this fact has been established experimentally in
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
II. Photosynthetic Adaptations
to Marine Submergence
The clear photosynthetic adaptation to marine submergence, also seen in hydrophytes (Sculthorpe,
1967), is the conversion of the leaf epidermis to the
primary site of photosynthesis, accompanied by the
loss of stomata and extreme reduction of the cuticle
(Larkum et al., 1989). This is presumably an adaptation to gaseous movement in a liquid medium rather
than a gaseous medium, where the rates of diffusion in the latter are reduced by a factor of ca 10,000
(Larkum et al., 1989; Borum et al., Chapter 10). This
Abbreviations
α – initial rate of photosynthesis for a P vs E curve
AF – Absorptance (absorption factor).
AZ – acetozolamide
C 4 – primary photosynthetic CO 2 fixation involving production
of C 4 acids
CA – carbonic anhydrase
CAM – crassulacean acid metabolism;
CCM – carbon concentrating mechanism;
C i – inorganic carbon sources for photosynthesis;
δ
13 C – fractionation of
13 C against
12 C compared to belemnite
standard
D1 – major polypeptide of PSII which binds Q A and partly P 680
E c – irradiance at the compensation point (older symbol, I c )
F – fractional yield of maximum fluorescence in actinic light
DBL – diffusive boundary layer
F m – maximum fluorescence yield
F m
– maximum fluorescence yield in actinic light
F o – minimum (dark) fluorescence yield
F t – fluorescence yield at time T in actinic light
F v – variable fluorescence yield
E – photon flux density
E K – E at which slope of light limiting region of the light curve
(α) intercepts Pmax (older term, I k )
ETR – electron transport rate
GC-MS – gas chromatography-mass spectrometry
MAP – Mehler ascorbate peroxidase (pathway)
NADP – nicotinamide dinucleotide phosphate
NPQ – non-photochemical quenching
NSCC – non-structural carbohydrate carbon
P max – maximum rate of photosynthesis
PAM – pulse amplitude modulated (fluorometry)
PAR – photosynthetically active radiation
PQ – photochemical quenching
PSI – photosystem I
PSII – photosystem II
qE – energy dependent quenching of fluorescence yield
qN – quotient of non-photochemical quenching
qP – quotient of photochemical quenching
Q A – plastoquinone acceptor of PSII
Q B – secondary plastoquinone acceptor of PSII
RLC – rapid light curve
Rubisco – ribulose-1,5-bisphosphate carboxylase/oxygenase
UV-B – ultraviolet B radiation
primary adaptation has been accompanied by three
very important adaptations also found in a number of
other submerged plants and some, but not all, algae:
(i) development of biochemical mechanisms for
enhanced inorganic carbon uptake, similar to
the carbon concentrating mechanisms (CCMs) of
some algae,
(ii) limitation of photosynthesis to the outer cell
layers of the leaf or lamina and reduction in the
non-photosynthetic tissues of the leaf or lamina
(iii) development of aerenchyma.
There may also be other adaptations which are
poorly documented at present such as reduction in
photorespiration and the Mehler ascorbate peroxidase (MAP) pathway (see Section VII) in order to
counteract the high levels of oxygen generated internally in the leaf as a result of the photosynthetic epidermis and high diffusive boundary layers at the leaf
surface (see e.g. Maberly and Madsen, 2002). However, both mechanisms (photorespiration and MAP
pathway) play an important role in protecting seagrass leaves from damage from excess light, under
exactly these conditions, so a balance has to occur.
However, in freshwater hydrophytes the tendency is
to thinner leaves with often just two cell layers in
leaves, both photosynthetic. This is not an option in
most seagrasses (Halophila spp. are an exception)
because of wave action, which necessitates a strong,
tough photosynthetic lamina.
These aspects are discussed in greater detail in the
following sections
III. The Diffusive Boundary Layer (DBL)
Substantial diffusive boundary layers are found
at most respiratory or photosynthetic surfaces of
submerged plants, animals or bacteria (Jørgensen,
2001). The size and stability of these layers have not
been explored fully in seagrasses (but see Larkum
et al., 1989); however, their existence has been shown
in the freshwater aquatic, Potomageton spp (SandJensen and Revsbech, 1987) and it is certain that
they exist in seagrasses. As a consequence of the
DBL and the anatomy of seagrass leaves, water motion will be a critical factor in determining the rate
of photosynthesis in seagrasses (Larkum et al., 1989
and this fact has been established experimentally in