Chapter 14 Photosynthesis in Seagrasses
335
scattering of light by suspended particles, this results in seagrasses being exposed to extreme shaded
conditions (see Zimmerman, Chapter 13). In theory, low-light acclimation could be effected in seagrasses, by maintenance of a large population of inactivated PSII reaction centres. The advantage would
be that these inactivated centres could function as effective energy quenchers of trapped light energy in
fluctuating light climates. This would lower effective quantum yield, yet prevent photoinhibitory damage (Ralph, 1999). In a green alga, Longstaff et al.
(1999) showed that chlorophyll pigment content responded rapidly to changes in the available light,
which provided appropriate configuration of the light
harvesting complex (LHC) to maintain photon capture at near optimal levels. Photochemical efficiency
(F v /F m ) was not effected during light starvation, as
pigment adjusted to ensure maximum efficiency.
Major and Dunton (2002) investigated photoacclimation in Thalassia testudinum using PAM fluorescence and other techniques. Maximum quantum
yield (F m ) was found to be elevated in low-light
plants relative to high light plants, which were found
to have increased F o . They developed a model of
seagrass photo-acclimation, based on light harvesting capacity optimised to lower irradiance, resulting
in a reduction of P max (light-saturated photosynthesis). Light harvesting was thought to increase by increasing chlorophyll content, thereby increasing the
absorption cross-section and the efficiency of PSII
(F v /F m ) (Major and Dunton 2002). However, when
leaf absorptance is maximized, increasing chlorophyll content has no effect on light harvesting. Thus
leaves with low chlorophyll levels may modulate
light harvesting by changing chlorophyll content,
highly pigmented leaves cannot.
Several important questions still require further
research; these include understanding the process
of low light stress linked to increased turbidity, and
shading by surface structures such as docks and excess shading by epiphytes as a result of eutrophication (Borowitzka et al., Chapter 19; Ralph et al.,
Chapter 24).
H. Photosynthetic Light Climate along the Leaf
Scale is an important consideration in all assessments of primary productivity. Photosynthesis is often used as a surrogate for assessing primary productivity, and photosynthesis varies across scales
from the upper edge of an intertidal meadow to the
deepest growing edge (Major and Dunton 2002).
There is also variation in photosynthesis among individual plants, and we are now recognizing that
there is variation in photosynthesis along a single
leaf blade. The density of a seagrass meadow can
cause self-shading, which will also influence the
photosynthesis of a leaf (Enr´ ıquez et al., 2002).
Durako and Kunzelman (2002) examined shootto-landscape scaling. Shoot-scale variation was assessed which included both within shoot, and
between shoot variation. The youngest leaf of a
T. testudinum plant was found to have the lowest
F v /F m , while leaves ranked 2 (second oldest leaf)
and 3, had higher F v /F m . Light-adapted maximum
fluorescence (F
m ) decreased towards the tip of the
leaf, indicating an increase in NPQ with higher light
exposure.
Seagrass leaf blades can remain attached for over
6 months, causing gradual changes in light climate
as the leaf moves through different vertical positions
and acquires different degrees of epiphytic cover.
The age of a leaf blade strongly influences its photokinetic responses (Enriquez et al., 2002). Several
investigations have recognised that photosynthesis
is not constant along a leaf (Ralph and Gademann,
1999; Durako and Kunzelman, 2002; Enr´ ıquez et al.,
2002). Since seagrasses grow from the basal meristem, this region has the least chlorophyll and also
receives the least incident irradiance (Enriquez et al.,
2002). As the leaf ages, each section moves progressively to higher light climates, so cells within
a leaf must be shade acclimated initially, then progressively photo-acclimated to higher light (Zimmerman, Chapter 13). Enr´ ıquez et al. (2002) found
that sections of a leaf can experience a wide range
of irradiance, which can change by three orders of
magnitude over the leaf ’s lifetime. Enr´ ıquez et al.
(2002) examined age-dependent loss of F v /F m in T.
testudinum. From the base to about 4 cm, maximum
quantum yield increased, and then gradually declined possibly due to exposure to irradiances about
saturation. RLCs of P. australis showed three regions
of distinct photochemical patterns along a leaf: the
basal region (2–4 cm) with low ETR, E k and high
F v /F m ; middle region (8–22 cm) with higher ETR,
E k and F v /F m ; and finally the apical region with the
highest ETR, E k and F v /F m . This shows a gradation
in photokinetics along a leaf blade (Ralph and Gademann 1999). In contrast, Ralph et al. (2002) found
that F v /F m , qP and NPQ did not vary along Z. marina
leaf blades, which was confirmed by constant levels
335
scattering of light by suspended particles, this results in seagrasses being exposed to extreme shaded
conditions (see Zimmerman, Chapter 13). In theory, low-light acclimation could be effected in seagrasses, by maintenance of a large population of inactivated PSII reaction centres. The advantage would
be that these inactivated centres could function as effective energy quenchers of trapped light energy in
fluctuating light climates. This would lower effective quantum yield, yet prevent photoinhibitory damage (Ralph, 1999). In a green alga, Longstaff et al.
(1999) showed that chlorophyll pigment content responded rapidly to changes in the available light,
which provided appropriate configuration of the light
harvesting complex (LHC) to maintain photon capture at near optimal levels. Photochemical efficiency
(F v /F m ) was not effected during light starvation, as
pigment adjusted to ensure maximum efficiency.
Major and Dunton (2002) investigated photoacclimation in Thalassia testudinum using PAM fluorescence and other techniques. Maximum quantum
yield (F m ) was found to be elevated in low-light
plants relative to high light plants, which were found
to have increased F o . They developed a model of
seagrass photo-acclimation, based on light harvesting capacity optimised to lower irradiance, resulting
in a reduction of P max (light-saturated photosynthesis). Light harvesting was thought to increase by increasing chlorophyll content, thereby increasing the
absorption cross-section and the efficiency of PSII
(F v /F m ) (Major and Dunton 2002). However, when
leaf absorptance is maximized, increasing chlorophyll content has no effect on light harvesting. Thus
leaves with low chlorophyll levels may modulate
light harvesting by changing chlorophyll content,
highly pigmented leaves cannot.
Several important questions still require further
research; these include understanding the process
of low light stress linked to increased turbidity, and
shading by surface structures such as docks and excess shading by epiphytes as a result of eutrophication (Borowitzka et al., Chapter 19; Ralph et al.,
Chapter 24).
H. Photosynthetic Light Climate along the Leaf
Scale is an important consideration in all assessments of primary productivity. Photosynthesis is often used as a surrogate for assessing primary productivity, and photosynthesis varies across scales
from the upper edge of an intertidal meadow to the
deepest growing edge (Major and Dunton 2002).
There is also variation in photosynthesis among individual plants, and we are now recognizing that
there is variation in photosynthesis along a single
leaf blade. The density of a seagrass meadow can
cause self-shading, which will also influence the
photosynthesis of a leaf (Enr´ ıquez et al., 2002).
Durako and Kunzelman (2002) examined shootto-landscape scaling. Shoot-scale variation was assessed which included both within shoot, and
between shoot variation. The youngest leaf of a
T. testudinum plant was found to have the lowest
F v /F m , while leaves ranked 2 (second oldest leaf)
and 3, had higher F v /F m . Light-adapted maximum
fluorescence (F
m ) decreased towards the tip of the
leaf, indicating an increase in NPQ with higher light
exposure.
Seagrass leaf blades can remain attached for over
6 months, causing gradual changes in light climate
as the leaf moves through different vertical positions
and acquires different degrees of epiphytic cover.
The age of a leaf blade strongly influences its photokinetic responses (Enriquez et al., 2002). Several
investigations have recognised that photosynthesis
is not constant along a leaf (Ralph and Gademann,
1999; Durako and Kunzelman, 2002; Enr´ ıquez et al.,
2002). Since seagrasses grow from the basal meristem, this region has the least chlorophyll and also
receives the least incident irradiance (Enriquez et al.,
2002). As the leaf ages, each section moves progressively to higher light climates, so cells within
a leaf must be shade acclimated initially, then progressively photo-acclimated to higher light (Zimmerman, Chapter 13). Enr´ ıquez et al. (2002) found
that sections of a leaf can experience a wide range
of irradiance, which can change by three orders of
magnitude over the leaf ’s lifetime. Enr´ ıquez et al.
(2002) examined age-dependent loss of F v /F m in T.
testudinum. From the base to about 4 cm, maximum
quantum yield increased, and then gradually declined possibly due to exposure to irradiances about
saturation. RLCs of P. australis showed three regions
of distinct photochemical patterns along a leaf: the
basal region (2–4 cm) with low ETR, E k and high
F v /F m ; middle region (8–22 cm) with higher ETR,
E k and F v /F m ; and finally the apical region with the
highest ETR, E k and F v /F m . This shows a gradation
in photokinetics along a leaf blade (Ralph and Gademann 1999). In contrast, Ralph et al. (2002) found
that F v /F m , qP and NPQ did not vary along Z. marina
leaf blades, which was confirmed by constant levels