82
J. Kuo and C. den Hartog
are not salt-tolerant thus osmoregulation may not
take place within the cytoplasm in Z. marina cells. It
should be stressed that (PM) H
+ -ATPase and (PM)
Na
+ /H
+ are found neither in freshwater Vallisneria
nor the terrestrial monocotyledon rice Oryza, despite
both species sharing similar anatomical features as
seagrasses, indicating that possession of this important enzyme permits seagrasses to grow in high Na
+
concentrations (Muramatsu et al., 2002). The role of
ion transport is also discussed by Larkum et al. in
Chapter 14 in relation to photosynthetic inorganic
carbon uptake by leaves.
VI. Morphological, Anatomical and
Ultrastructural Modifications in Relation
to Environmental Conditions
Several seagrasses show considerable morphological variations from a wide range of geographic
distribution. Some species have different reproductive strategies under different environmental
circumstances. For example, Zostera marina is
largely perennial in the sublittoral, but in the intertidal belt many populations appear to be annual.
The Z. marina population in the Sea of Cortez is
annual; its development is totally restricted to the
cooler season, while along the Pacific coast of the
same latitude the species is fully perennial. Other
morphological variations concern the broad-leaved
P. oceanica from Algiers (Semroud et al., 1992),
Halodule wrightii from southern Brazil (Creed,
1999), Halodule spp. from Malaysia (Japar Sidik
et al., 1999) and Halophila spinulosa with three
leaflets instead of two at the nodes of the erect stem
in Malaysia (Japar Sidik et al., 2000). Further it has
become clear from transplant experiments that morphologically similar plants of the same taxon may be
physiologically different, with relation to tolerating
low or high temperatures, salinity fluctuations, day
length, etc.
Iyer and Barnabas (1993) demonstrated that leaf
epidermal cells of low-salinity Zostera capensis,
compared to seawater plants, had larger central vacuoles; fewer chloroplasts; reduced plasmalemma
area; fewer mitochondria; thinner walls; reduced
extra cytoplasmic volume; and symplastic connections with mesophyll cells. Cell walls of high-salinity
plants histochemically show a stronger acidity gradient indicating a pH driven anion exchange mechanism in the wall. Similar results were found for
Ruppia maritima (Jagels and Barnabas, 1989).
Light intensity also has effects on seagrass morphology and anatomy. Abal et al. (1994) showed that
plants of Zostera capricorni Aschers. grown under
high light conditions (50–100% light) had smaller
shoots, higher biomass and productivity, larger air lacunae, but less negative δ
13 C values, lower nitrogen
content, less chlorophyll and more ultraviolet light
absorbing pigment than plants grown under low light
conditions (<20% light). Olesen et al. (2002) also
discussed light capture efficiency with water depth
for Cymodocea nodosa.
Cooper and McRoy (1988) found that numbers
of fibers per bundle and bundle frequency are relatively constant in the three co-existing Phyllospadix
species: P. torreyi, P. serrulatus and P. scouleri at different tidal heights at Cape Arago, Oregon, U.S.A.,
but leaf and epidermal cell thickness of P. torreyi and
P. serrulatus significantly decrease as tidal height increases.
Seagrasses cannot grow to such great depths as
their algal counterparts and there has been much discussion of this difference (Duarte, 1991), an observation made by Hutchinson (1975) for hydrophytes
in general. It has been related to the extra physiological/biochemical load of having roots and rhizomes (Hemminga, 1998; Olesen et al., 2002) but
it has also been ascribed to poor ability to change
anatomical adaptations such as air lacunae (Beer
and Waisel, 1982). In this respect, it is noteworthy
that aquatic freshwater angiosperms have rarely been
found at depths greater than 10 m; compared with
them many seagrasses show a better performance.
Schwarz and Hellblom (2002) showed that chloroplasts of Halophila stipulacea become clumped in
shallow water. West (1990) showed that leaf width
and leaf length of Posidonia australis had low variability between shallow (0.5 m) and deep (8.5 m)
sites; however, unit frond weight was significantly
higher in shallow sites and leaf area per shoot was
significantly lower in deep sites. Some seagrass
species can grow in clear deep water by reducing the leaf thickness, hypodermal fiber frequency
and grana density in chloroplasts, and by increasing the phenolic contents in mesophyll cells in P.
oceanica (Colombo et al., 1983). On the other hand,
Halophila decipiens and Thalassodendron ciliatum
can grow from the intertidal zone to a depth of
more than 60 m, apparently without any morphological or anatomical modification (Kuo and
J. Kuo and C. den Hartog
are not salt-tolerant thus osmoregulation may not
take place within the cytoplasm in Z. marina cells. It
should be stressed that (PM) H
+ -ATPase and (PM)
Na
+ /H
+ are found neither in freshwater Vallisneria
nor the terrestrial monocotyledon rice Oryza, despite
both species sharing similar anatomical features as
seagrasses, indicating that possession of this important enzyme permits seagrasses to grow in high Na
+
concentrations (Muramatsu et al., 2002). The role of
ion transport is also discussed by Larkum et al. in
Chapter 14 in relation to photosynthetic inorganic
carbon uptake by leaves.
VI. Morphological, Anatomical and
Ultrastructural Modifications in Relation
to Environmental Conditions
Several seagrasses show considerable morphological variations from a wide range of geographic
distribution. Some species have different reproductive strategies under different environmental
circumstances. For example, Zostera marina is
largely perennial in the sublittoral, but in the intertidal belt many populations appear to be annual.
The Z. marina population in the Sea of Cortez is
annual; its development is totally restricted to the
cooler season, while along the Pacific coast of the
same latitude the species is fully perennial. Other
morphological variations concern the broad-leaved
P. oceanica from Algiers (Semroud et al., 1992),
Halodule wrightii from southern Brazil (Creed,
1999), Halodule spp. from Malaysia (Japar Sidik
et al., 1999) and Halophila spinulosa with three
leaflets instead of two at the nodes of the erect stem
in Malaysia (Japar Sidik et al., 2000). Further it has
become clear from transplant experiments that morphologically similar plants of the same taxon may be
physiologically different, with relation to tolerating
low or high temperatures, salinity fluctuations, day
length, etc.
Iyer and Barnabas (1993) demonstrated that leaf
epidermal cells of low-salinity Zostera capensis,
compared to seawater plants, had larger central vacuoles; fewer chloroplasts; reduced plasmalemma
area; fewer mitochondria; thinner walls; reduced
extra cytoplasmic volume; and symplastic connections with mesophyll cells. Cell walls of high-salinity
plants histochemically show a stronger acidity gradient indicating a pH driven anion exchange mechanism in the wall. Similar results were found for
Ruppia maritima (Jagels and Barnabas, 1989).
Light intensity also has effects on seagrass morphology and anatomy. Abal et al. (1994) showed that
plants of Zostera capricorni Aschers. grown under
high light conditions (50–100% light) had smaller
shoots, higher biomass and productivity, larger air lacunae, but less negative δ
13 C values, lower nitrogen
content, less chlorophyll and more ultraviolet light
absorbing pigment than plants grown under low light
conditions (<20% light). Olesen et al. (2002) also
discussed light capture efficiency with water depth
for Cymodocea nodosa.
Cooper and McRoy (1988) found that numbers
of fibers per bundle and bundle frequency are relatively constant in the three co-existing Phyllospadix
species: P. torreyi, P. serrulatus and P. scouleri at different tidal heights at Cape Arago, Oregon, U.S.A.,
but leaf and epidermal cell thickness of P. torreyi and
P. serrulatus significantly decrease as tidal height increases.
Seagrasses cannot grow to such great depths as
their algal counterparts and there has been much discussion of this difference (Duarte, 1991), an observation made by Hutchinson (1975) for hydrophytes
in general. It has been related to the extra physiological/biochemical load of having roots and rhizomes (Hemminga, 1998; Olesen et al., 2002) but
it has also been ascribed to poor ability to change
anatomical adaptations such as air lacunae (Beer
and Waisel, 1982). In this respect, it is noteworthy
that aquatic freshwater angiosperms have rarely been
found at depths greater than 10 m; compared with
them many seagrasses show a better performance.
Schwarz and Hellblom (2002) showed that chloroplasts of Halophila stipulacea become clumped in
shallow water. West (1990) showed that leaf width
and leaf length of Posidonia australis had low variability between shallow (0.5 m) and deep (8.5 m)
sites; however, unit frond weight was significantly
higher in shallow sites and leaf area per shoot was
significantly lower in deep sites. Some seagrass
species can grow in clear deep water by reducing the leaf thickness, hypodermal fiber frequency
and grana density in chloroplasts, and by increasing the phenolic contents in mesophyll cells in P.
oceanica (Colombo et al., 1983). On the other hand,
Halophila decipiens and Thalassodendron ciliatum
can grow from the intertidal zone to a depth of
more than 60 m, apparently without any morphological or anatomical modification (Kuo and