Chapter 3 Seagrass Morphology, Anatomy, and Ultrastructure
81
(Birch, 1981); H. engelmanni (McMillan, 1987;
Jewett-Smith and McMillan, 1990); H. decipiens
(McMillan, 1988; McMillan and Soong, 1989; Kuo
and Kirkman, 1995); H. tricostata (Kuo et al., 1993)
and H. beccarii (Muta Harah et al., 1999, 2000). The
seedlings of all Halophila species display unusual
early growth of ‘hypocotylar hairs’ or ‘anchoring
hairs’ from the surface of the hypocotylar collar,
which appear to anchor the seed before the emergence of the radicle (Fig. 16A and B). A similar
phenomenon also occurs in fresh water monocotyledonous seedlings (Kaul, 1978), in some aquatic dicotyledonous seedlings and in a few terrestrial plants
(Arber, 1925).
2. Zosteraceae
Anatomical studies on seed and seedling development in Zostera marina and Z. japonica have
been published by Taylor (1957a, b) and Yamashita (1973). The rhomboid fruit of Phyllospadix
(Fig. 14K) has a dormancy period of about 6
weeks. During germination, the shoots emerge first
(Fig. 16D and E) and roots usually emerge more
than six months later (Kuo et al., 1990b). Nutrient reserves in the Phyllospadix hypocotyl disappear about
4 months after germination.
3. Posidoniaceae
Fruits of Australian Posidonia mature in the early
summer, 3 months after anthesis (Fig. 13H and I)
and germinate as soon as being released without a
distinct dormancy period (Fig. 14G). Roots are produced shortly after. Starch and nutrient reserves in
the hypocotyl exhausted by eight months after germination, but the hypocotyl (seed) may still be attached to young plants (Fig. 16C) for more than 2
years (Kuo and Kirkman, 1996). On the other hand,
seeds of P. oceanica germinated in culture media
within 15–18 days with up to 96.2% germination
rate, and no less than 75% of the seedlings survived
and grew continuously for 10 months (Balestri et al.,
1998).
4. Cymodoceaceae
In the tropical species Cymodocea rotundata, germination can occur throughout the year. Seeds of
Halodule may be dormant for long periods, possibly extending over many years, while those of Syringodium show only short-term dormancy. However, seeds of Halodule and Syringodium are able
to germinate for more than 3 years after they have
been released from the parent plant. The longevity of
seeds of these species may well exceed that of other
seagrasses (McMillan, 1983a).
Finally, it should mentioned that Orth et al. (2000)
reviewed seed dormancy, seed bank and germination
of seagrasses and the possible applications of this
knowledge for the conservation and restoration of
seagrass beds (see also Orth et al., Chapter 5).
V. Water Movement in Seagrass Leaves
and Roots
By using apoplastic tracers, Barnabas (1988, 1989,
1991, 1994a) showed that sea water freely enters
from the medium and moves amongst the leaf blade
and root tissues of Thalassodendron ciliatum and
Halodule uninervis. However, water movement is
restricted by the suberin of the vascular bundles in
the blades and by the hypodermis and endodermis
in the roots. Furthermore, in contrast to leaf blades,
leaf sheaths have a distinct ‘suberin-like’ cuticle that
prevents seawater entering the sheath tissue and acts
as a protection of the meristem tissues and developing leaves. As mentioned in Section III.C.1, Tyerman (1989) showed that the leaf sheath in Posidonia australis provides for the developing leaf an
(lower) osmotic environment different from the seawater environment, which the leaf experiences when
it emerges from the leaf sheath. Indeed, Arai et al.
(1991) showed that spherical protoplasts isolated
from young leaf blades of several seagrass species,
i.e. Zostera marina, Z. japonica and Phyllospadix
iwatensis, which were protected from seawater by
the leaf sheath, had little resistance to seawater. Nonspherical protoplasts from mature leaf blades of the
same species were resistant to wide ranges of osmotic potential and salinity.
It has been demonstrated biochemically and physiologically that the plasmalemma (PM) of leaf cells
in seagrasses plays an important role in osmoregulation. The salt-tolerant H
+ -ATPase generates an
electrochemical membrane potential in the seagrass
plasmalemma preventing Na
+ from entering into the
cytoplasm (Pak et al., 1995). Several intracellular
ATPase enzymes including PEPC for CO 2 fixation,
ATPase in mitochondria and ATPase in the tonoplast,
81
(Birch, 1981); H. engelmanni (McMillan, 1987;
Jewett-Smith and McMillan, 1990); H. decipiens
(McMillan, 1988; McMillan and Soong, 1989; Kuo
and Kirkman, 1995); H. tricostata (Kuo et al., 1993)
and H. beccarii (Muta Harah et al., 1999, 2000). The
seedlings of all Halophila species display unusual
early growth of ‘hypocotylar hairs’ or ‘anchoring
hairs’ from the surface of the hypocotylar collar,
which appear to anchor the seed before the emergence of the radicle (Fig. 16A and B). A similar
phenomenon also occurs in fresh water monocotyledonous seedlings (Kaul, 1978), in some aquatic dicotyledonous seedlings and in a few terrestrial plants
(Arber, 1925).
2. Zosteraceae
Anatomical studies on seed and seedling development in Zostera marina and Z. japonica have
been published by Taylor (1957a, b) and Yamashita (1973). The rhomboid fruit of Phyllospadix
(Fig. 14K) has a dormancy period of about 6
weeks. During germination, the shoots emerge first
(Fig. 16D and E) and roots usually emerge more
than six months later (Kuo et al., 1990b). Nutrient reserves in the Phyllospadix hypocotyl disappear about
4 months after germination.
3. Posidoniaceae
Fruits of Australian Posidonia mature in the early
summer, 3 months after anthesis (Fig. 13H and I)
and germinate as soon as being released without a
distinct dormancy period (Fig. 14G). Roots are produced shortly after. Starch and nutrient reserves in
the hypocotyl exhausted by eight months after germination, but the hypocotyl (seed) may still be attached to young plants (Fig. 16C) for more than 2
years (Kuo and Kirkman, 1996). On the other hand,
seeds of P. oceanica germinated in culture media
within 15–18 days with up to 96.2% germination
rate, and no less than 75% of the seedlings survived
and grew continuously for 10 months (Balestri et al.,
1998).
4. Cymodoceaceae
In the tropical species Cymodocea rotundata, germination can occur throughout the year. Seeds of
Halodule may be dormant for long periods, possibly extending over many years, while those of Syringodium show only short-term dormancy. However, seeds of Halodule and Syringodium are able
to germinate for more than 3 years after they have
been released from the parent plant. The longevity of
seeds of these species may well exceed that of other
seagrasses (McMillan, 1983a).
Finally, it should mentioned that Orth et al. (2000)
reviewed seed dormancy, seed bank and germination
of seagrasses and the possible applications of this
knowledge for the conservation and restoration of
seagrass beds (see also Orth et al., Chapter 5).
V. Water Movement in Seagrass Leaves
and Roots
By using apoplastic tracers, Barnabas (1988, 1989,
1991, 1994a) showed that sea water freely enters
from the medium and moves amongst the leaf blade
and root tissues of Thalassodendron ciliatum and
Halodule uninervis. However, water movement is
restricted by the suberin of the vascular bundles in
the blades and by the hypodermis and endodermis
in the roots. Furthermore, in contrast to leaf blades,
leaf sheaths have a distinct ‘suberin-like’ cuticle that
prevents seawater entering the sheath tissue and acts
as a protection of the meristem tissues and developing leaves. As mentioned in Section III.C.1, Tyerman (1989) showed that the leaf sheath in Posidonia australis provides for the developing leaf an
(lower) osmotic environment different from the seawater environment, which the leaf experiences when
it emerges from the leaf sheath. Indeed, Arai et al.
(1991) showed that spherical protoplasts isolated
from young leaf blades of several seagrass species,
i.e. Zostera marina, Z. japonica and Phyllospadix
iwatensis, which were protected from seawater by
the leaf sheath, had little resistance to seawater. Nonspherical protoplasts from mature leaf blades of the
same species were resistant to wide ranges of osmotic potential and salinity.
It has been demonstrated biochemically and physiologically that the plasmalemma (PM) of leaf cells
in seagrasses plays an important role in osmoregulation. The salt-tolerant H
+ -ATPase generates an
electrochemical membrane potential in the seagrass
plasmalemma preventing Na
+ from entering into the
cytoplasm (Pak et al., 1995). Several intracellular
ATPase enzymes including PEPC for CO 2 fixation,
ATPase in mitochondria and ATPase in the tonoplast,