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A. W. D. Larkum, E. A. Drew, and P. J. Ralph
high respiratory load imposed by a leaf anatomy with
the major photosynthetic layer limited to the epidermis and a large underground (non-photosynthetic)
component. In terms of maximum photosynthetic
rate, seagrass leaves are not the highest of photosynthetic tissues/communities (Krause-Jensen and
Sand-Jensen, 1998) but they are in the mid to upper
range. It should also be remembered that such rates
often focus on rates under optimum conditions.
A number of other studies have focussed on such
important topics as photosynthetic performance with
depth (e.g. Olesen et al., 2002) and variations along
a single leaf (e.g. Enriquez et al., 2002), as well as
many studies on such environmental factors as temperature and salinity.
VI. Photosynthetic Efficiency,
Light-Harvesting and the Package Effect
Seagrasses like all aquatic plants are shade-adapted
(Reiskind et al., 1989; Bowes et al., 2002), i.e. they
show light saturation at fairly low irradiances and
have a high α (initial slope of the P vs E curve).
Like all angiosperms they have the ability to vary
their photosynthetic apparatus to optimise use of the
available light but have only a small ability to do
this in the short-term (that is in minutes to hours) by
state transitions (whereby light capture by the two
photosystems is manipulated—see section VIII.I).
As shade plants they optimise to rather high levels
of light harvesting proteins per photosystem (high
absorption cross-section) and can also vary the number of photosystems per unit of thylakoid membrane and the number of chloroplasts per cell (Major
and Dunton, 2000, 2002: Cummings and Zimmerman, 2003). However, unlike many land plants with
complex photosynthetic anatomies (eg palisade and
spongy mesophyll) which allows them to harvest
light more efficiently (Lee et al. 1990), seagrasses
rely almost entirely on a photosynthetic epidermis,
limiting their ability to efficiently harvest available
light (Cummings and Zimmerman, 2003).
Light harvesting in seagrasses has been studied
by Major and Dunton (2000, 2002), in Thalassia testudinum and by Cummings and Zimmerman (2003)
in T. testudinum and Z. marina. All these studies
show that photoacclimation is largely brought about
by changes in the chlorophyll content per unit surface
area (or unit weight). Chlorophyll content was shown
to vary up to five fold (Cummings and Zimmerman,
2003). Major and Dunton (2002) showed that the
unit size (absorption cross-section) of photosystem
I increased under low light but found that neither
photosystem density (per unit chlorophyll) or the
unit size (absorption cross-section) of photosystem
II changed, again consistent with a shade strategy.
One way for increased photosynthesis is to increase
the absorptance of a leaf by increasing the number
of chloroplasts or by rearranging chloroplasts in or
out of the light path (Schwarz et al., 2002). As the
number of chloroplasts in the light path increases
the absorption of light approaches a black body absorber (Larkum and Barrett, 1991). The result is that
chloroplasts deep in the tissue (or on the under side of
leaves which undergo little displacement) receive a
very-much modified spectral radiation, rich in green
light. This is a consequence of the package effect,
the tendency for densely packed chlorophyll to absorb greater amounts of violet and red light, compared to green light. A photosynthetic system without suitable pigments such as phycobiliproteins to
harvest green light can nevertheless harvest most
of the available light, but only at the expense of an
inefficient use of the available photosynthetic apparatus, in this case deeper chloroplasts which work at
low efficiency. The package effect has been directly
demonstrated in two seagrasses, T. testudinum and
Z. marina (Cummings and Zimmerman (2003; see
also Zimmerman, Chapter 13).
Parts of the leaves, usually the younger parts,
deeper down in the canopy are also subject to this effect since the package effect means that red and blue
light are differentially absorbed, dependent on shoot
density and current velocity, in the upper canopy region (Zimmerman, 2003; Zimmerman, Chapter 13).
Since seagrasses show reasonable photosynthetic efficiencies, on the basis of incident photons, compared with other plants (Major and Dunton, 2002,
Cummings and Zimmerman, 2003) they clearly provide the necessary photosynthetic machinery for optimum light absorption despite the metabolic costs
of providing that machinery.
The total primary productivity of seagrasses
varies from quite high to moderate, in comparison
with the most highly productive land plants and algae (Larkum, 1981; Duarte and Chiscano, 1999).
This has generally been seen as caused by deployment of a large underground rhizome and root system, rather than leaves inefficient in photosynthesis.
(Raven, 1984). However, as shown by Duarte and
Chicano (1999) the biomass ratios and production
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
high respiratory load imposed by a leaf anatomy with
the major photosynthetic layer limited to the epidermis and a large underground (non-photosynthetic)
component. In terms of maximum photosynthetic
rate, seagrass leaves are not the highest of photosynthetic tissues/communities (Krause-Jensen and
Sand-Jensen, 1998) but they are in the mid to upper
range. It should also be remembered that such rates
often focus on rates under optimum conditions.
A number of other studies have focussed on such
important topics as photosynthetic performance with
depth (e.g. Olesen et al., 2002) and variations along
a single leaf (e.g. Enriquez et al., 2002), as well as
many studies on such environmental factors as temperature and salinity.
VI. Photosynthetic Efficiency,
Light-Harvesting and the Package Effect
Seagrasses like all aquatic plants are shade-adapted
(Reiskind et al., 1989; Bowes et al., 2002), i.e. they
show light saturation at fairly low irradiances and
have a high α (initial slope of the P vs E curve).
Like all angiosperms they have the ability to vary
their photosynthetic apparatus to optimise use of the
available light but have only a small ability to do
this in the short-term (that is in minutes to hours) by
state transitions (whereby light capture by the two
photosystems is manipulated—see section VIII.I).
As shade plants they optimise to rather high levels
of light harvesting proteins per photosystem (high
absorption cross-section) and can also vary the number of photosystems per unit of thylakoid membrane and the number of chloroplasts per cell (Major
and Dunton, 2000, 2002: Cummings and Zimmerman, 2003). However, unlike many land plants with
complex photosynthetic anatomies (eg palisade and
spongy mesophyll) which allows them to harvest
light more efficiently (Lee et al. 1990), seagrasses
rely almost entirely on a photosynthetic epidermis,
limiting their ability to efficiently harvest available
light (Cummings and Zimmerman, 2003).
Light harvesting in seagrasses has been studied
by Major and Dunton (2000, 2002), in Thalassia testudinum and by Cummings and Zimmerman (2003)
in T. testudinum and Z. marina. All these studies
show that photoacclimation is largely brought about
by changes in the chlorophyll content per unit surface
area (or unit weight). Chlorophyll content was shown
to vary up to five fold (Cummings and Zimmerman,
2003). Major and Dunton (2002) showed that the
unit size (absorption cross-section) of photosystem
I increased under low light but found that neither
photosystem density (per unit chlorophyll) or the
unit size (absorption cross-section) of photosystem
II changed, again consistent with a shade strategy.
One way for increased photosynthesis is to increase
the absorptance of a leaf by increasing the number
of chloroplasts or by rearranging chloroplasts in or
out of the light path (Schwarz et al., 2002). As the
number of chloroplasts in the light path increases
the absorption of light approaches a black body absorber (Larkum and Barrett, 1991). The result is that
chloroplasts deep in the tissue (or on the under side of
leaves which undergo little displacement) receive a
very-much modified spectral radiation, rich in green
light. This is a consequence of the package effect,
the tendency for densely packed chlorophyll to absorb greater amounts of violet and red light, compared to green light. A photosynthetic system without suitable pigments such as phycobiliproteins to
harvest green light can nevertheless harvest most
of the available light, but only at the expense of an
inefficient use of the available photosynthetic apparatus, in this case deeper chloroplasts which work at
low efficiency. The package effect has been directly
demonstrated in two seagrasses, T. testudinum and
Z. marina (Cummings and Zimmerman (2003; see
also Zimmerman, Chapter 13).
Parts of the leaves, usually the younger parts,
deeper down in the canopy are also subject to this effect since the package effect means that red and blue
light are differentially absorbed, dependent on shoot
density and current velocity, in the upper canopy region (Zimmerman, 2003; Zimmerman, Chapter 13).
Since seagrasses show reasonable photosynthetic efficiencies, on the basis of incident photons, compared with other plants (Major and Dunton, 2002,
Cummings and Zimmerman, 2003) they clearly provide the necessary photosynthetic machinery for optimum light absorption despite the metabolic costs
of providing that machinery.
The total primary productivity of seagrasses
varies from quite high to moderate, in comparison
with the most highly productive land plants and algae (Larkum, 1981; Duarte and Chiscano, 1999).
This has generally been seen as caused by deployment of a large underground rhizome and root system, rather than leaves inefficient in photosynthesis.
(Raven, 1984). However, as shown by Duarte and
Chicano (1999) the biomass ratios and production