334
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
necessitating the replacement of the damaged D1
protein. An essential aspect of understanding the dynamic nature of diurnal rhythms is the ability of PSII
to recover during the afternoon or overnight, i.e. a
recovery from photoinhibition over a period of hours
in the shade or overnight.
Seagrasses growing near the intertidal region will
be exposed to high light stress, which can result in
either down-regulation or, if irradiance is sufficiently
high, can result in photoinhibition. Photoinhibition
can be defined as damage to the PSII reaction centres,
mainly to D1 protein, requiring several hours of low
light (or overnight) for repair. Down-regulation is
associated with increased NPQ. Photoinhibition is
generally associated with reduced F v /F m and with
increase in F o .
E. Xanthophyll Cycle
The xanthophyll cycle is well understood in terrestrial plants (see eg. Demmig-Adams and Adams
(1993). However, recent evidence indicates that the
xanthophyll cycle is an integral component of the
seagrasses as well (Ralph et al., 2002). In the xanthophyll cycle, increased energy dissipation (NPQ)
is linked to an increase of the pH gradient across
the thylakoid membrane, which in turn triggers the
xanthophyll cycle (Fig. 2). The xanthophyll cycle
is dependent on three xanthophylls: light triggers
the conversion of the low-light form of violaxanthin to the epoxidised (high-light) form, zeaxanthin,
and the intermediate, antheraxanthin. Zostera marina shows distinct changes in xanthophyll pigments
composition that is linked to diurnal light fluctuations and these changes are also correlated to a
down-regulation of photochemical efficiency, a decline in F m
and an increase in NPQ (Ralph et al.
2002). Once the minimum saturating irradiance was
exceeded, all violaxanthin was converted to antheraxanthin and zeaxanthin. Unusually high levels of
antheraxanthin were found in Z. marina suggesting
incomplete conversion to zeaxanthin.
F. Photoinhibition and UVB Inhibition
As described above photoinhibition, is defined as
damage to PSII, principally due to damage to D1
protein (section VIII.D). This is in contrast to downregulation, which is reversible in a matter of hours
and is sometimes referred to as dynamic photoinhibition. If the rate of D1 repair is less than the rate
of damage, then photoinhibition is said to have occurred (Flanigan and Critchley 1996). Photoinhibition occurs most severely with shade-adapted plants,
such as seagrasses, and recovery requires a relatively
long time (6 h to several days, depending strongly
on ambient temperature). Photoinhibition is characterized by a sustained decline in effective quantum
yield. High light can cause substantial damage to
PSII of seagrasses (Ralph and Burchett 1995) and
a large component of this is due to damage to D1
protein (Flanigan and Critchley 1996). Flanigan and
Critchley (1996) found the maximum rate of D1
synthesis occurred at 350 µmol photons m
−2 s
−1
in Z. capricorni, whilst photoinhibition occurred at
about 1100 µmol photons m
−2 s
−1 . This suggested
that D1 synthesis in Z. capricorni is not used in photoprotection and is not proportional to irradiance.
Ultra-violet radiation (280–400 nm) has been
found to have substantial impacts on seagrasses
(Larkum and Wood 1993; Dawson and Dennison
1996; Figueroa et al. 2002). Deep-water seagrasses
are not acclimated to UV radiation and therefore are
most sensitive. Shallow water seagrasses tolerate UV
radiation. Larkum and Wood (1993) suggested the
primary impact of UV-B (280–320) on P. australis
was the oxidizing side of PSII, as it is in cyanobacteria and terrestrial plants (Vass et al., 2000; Larkum
et al., 2001). Dawson and Dennison (1996) found H.
ovalis and Halodule uninervis to be the most sensitive species to UV radiation. Thick-tissued species
(Z. capricorni, C. serrulate and Syringodium isoetifolium) were only marginally impacted by elevated
UV irradiance. UV has been shown to effect the
F/F m
, ETR max and light-limited slope of the RLC
(α). It has also been suggested that UV can affect
down-regulation, as well as photoinhibitory sites
(Figueroa et al., 2002).
G. Low Light Stress
Several investigations have examined the effect of
light starvation, or more specifically, how shading
limits seagrass distribution (Longstaff et al., 1999).
Once a seagrass receives, at its midday maximum,
less than 100–500 µmol photons m
−2 s
−1 (5–25% of
surface irradiance), its respiration demands exceeds
the rate of carbon fixation (Abal et al., 1994). When
most of the downwelling irradiance is absorbed by
the overlying water column, as well as increased
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
necessitating the replacement of the damaged D1
protein. An essential aspect of understanding the dynamic nature of diurnal rhythms is the ability of PSII
to recover during the afternoon or overnight, i.e. a
recovery from photoinhibition over a period of hours
in the shade or overnight.
Seagrasses growing near the intertidal region will
be exposed to high light stress, which can result in
either down-regulation or, if irradiance is sufficiently
high, can result in photoinhibition. Photoinhibition
can be defined as damage to the PSII reaction centres,
mainly to D1 protein, requiring several hours of low
light (or overnight) for repair. Down-regulation is
associated with increased NPQ. Photoinhibition is
generally associated with reduced F v /F m and with
increase in F o .
E. Xanthophyll Cycle
The xanthophyll cycle is well understood in terrestrial plants (see eg. Demmig-Adams and Adams
(1993). However, recent evidence indicates that the
xanthophyll cycle is an integral component of the
seagrasses as well (Ralph et al., 2002). In the xanthophyll cycle, increased energy dissipation (NPQ)
is linked to an increase of the pH gradient across
the thylakoid membrane, which in turn triggers the
xanthophyll cycle (Fig. 2). The xanthophyll cycle
is dependent on three xanthophylls: light triggers
the conversion of the low-light form of violaxanthin to the epoxidised (high-light) form, zeaxanthin,
and the intermediate, antheraxanthin. Zostera marina shows distinct changes in xanthophyll pigments
composition that is linked to diurnal light fluctuations and these changes are also correlated to a
down-regulation of photochemical efficiency, a decline in F m
and an increase in NPQ (Ralph et al.
2002). Once the minimum saturating irradiance was
exceeded, all violaxanthin was converted to antheraxanthin and zeaxanthin. Unusually high levels of
antheraxanthin were found in Z. marina suggesting
incomplete conversion to zeaxanthin.
F. Photoinhibition and UVB Inhibition
As described above photoinhibition, is defined as
damage to PSII, principally due to damage to D1
protein (section VIII.D). This is in contrast to downregulation, which is reversible in a matter of hours
and is sometimes referred to as dynamic photoinhibition. If the rate of D1 repair is less than the rate
of damage, then photoinhibition is said to have occurred (Flanigan and Critchley 1996). Photoinhibition occurs most severely with shade-adapted plants,
such as seagrasses, and recovery requires a relatively
long time (6 h to several days, depending strongly
on ambient temperature). Photoinhibition is characterized by a sustained decline in effective quantum
yield. High light can cause substantial damage to
PSII of seagrasses (Ralph and Burchett 1995) and
a large component of this is due to damage to D1
protein (Flanigan and Critchley 1996). Flanigan and
Critchley (1996) found the maximum rate of D1
synthesis occurred at 350 µmol photons m
−2 s
−1
in Z. capricorni, whilst photoinhibition occurred at
about 1100 µmol photons m
−2 s
−1 . This suggested
that D1 synthesis in Z. capricorni is not used in photoprotection and is not proportional to irradiance.
Ultra-violet radiation (280–400 nm) has been
found to have substantial impacts on seagrasses
(Larkum and Wood 1993; Dawson and Dennison
1996; Figueroa et al. 2002). Deep-water seagrasses
are not acclimated to UV radiation and therefore are
most sensitive. Shallow water seagrasses tolerate UV
radiation. Larkum and Wood (1993) suggested the
primary impact of UV-B (280–320) on P. australis
was the oxidizing side of PSII, as it is in cyanobacteria and terrestrial plants (Vass et al., 2000; Larkum
et al., 2001). Dawson and Dennison (1996) found H.
ovalis and Halodule uninervis to be the most sensitive species to UV radiation. Thick-tissued species
(Z. capricorni, C. serrulate and Syringodium isoetifolium) were only marginally impacted by elevated
UV irradiance. UV has been shown to effect the
F/F m
, ETR max and light-limited slope of the RLC
(α). It has also been suggested that UV can affect
down-regulation, as well as photoinhibitory sites
(Figueroa et al., 2002).
G. Low Light Stress
Several investigations have examined the effect of
light starvation, or more specifically, how shading
limits seagrass distribution (Longstaff et al., 1999).
Once a seagrass receives, at its midday maximum,
less than 100–500 µmol photons m
−2 s
−1 (5–25% of
surface irradiance), its respiration demands exceeds
the rate of carbon fixation (Abal et al., 1994). When
most of the downwelling irradiance is absorbed by
the overlying water column, as well as increased