56
J. Kuo and C. den Hartog
of root hairs in Heterozostera nigricaulis (as H.
tasmanica) contain cutin or lipid, callose (1–3) βglucans, carbohydrates but no lignin. The major wall
polysaccharide is an apiogalacturonan and that cellulose accounts for most of the glucose.
Cell structure and the arrangement of the cortex
varies with the texture of root types, but roots always have three distinct zones. In soft roots, such
as those of Posidonia (Fig. 2F) and Halophila,
the outer cortex consists of one to several layers of compact cells with thin, non-lignified walls.
The middle cortex has well developed air lacunae separated by a radial partition, which is one
cell thick, containing small intercellular pores,
and the inner cortex consists of compact, small
cells in a regular, concentric arrangement (Roberts
et al., 1985; Kuo and McComb, 1989; Kuo, 1993b).
On the other hand, in hard roots such as those of Thalassodendron (Fig. 2E) and Amphibolis, the outer
cortex consists of several compact cells with slightly
thicker, lignified walls. The middle cortex contains
several layers of thin-walled, irregular cells in two
or more layers that sometimes make up some distinct but irregular air lacunae (Fig. 2D and H). The
inner cortex has two or more layers of small, thin
walled, compact cells (Figs. 2D–H, K and M and
3D–H). These pronounced air lacunae may facilitate gas movement from the root surface to the stele
or vice versa and also from the root base to the root
tip (Tomlinson, 1982; see also Borum et al., Chapter
10). In all seagrass species, the very distinct endodermis consists of a layer of compact cells, which
encloses the stele, as is also the case in many terrestrial plants (Fig. 3D–H). In all species, the walls
of the endodermis are thin to moderately thick, either lignified or not lignified, but they always contain a distinct Casparian strip on the radial walls
(Fig. 3I). An additional suberized layer may occur in
all walls or certain walls of the endodermis of some
species (Fig. 3G). The endodermis of seagrasses just
as in terrestrial flowering plants restricts solute and
water movements between the cortex and the stele
(Kuo and Cambridge, 1978). The stele contains the
vascular tissue with xylem and phloem but no distinct pericycle (Fig. 3H). Xylem elements are either
represented by a few, unthickened walls of poorly
lignified tracheid elements or by a single large lumen (Fig. 3F–H). Several sieve tubes with normal
walls are surrounded by vascular parenchyma cells
(Fig. 3H); these possess wall ingrowths and are rich
in cytoplasm in Zostera (Fig. 3H and J) (Barnabas
and Arnott, 1987).
Tomlinson (1969) studied the development of
roots of Thalassia testudinum and concluded that
there were no water-conducting tissues, except close
to the base of the root, suggesting that roots were of
no special significance in water absorption in this
species. On the other hand, Roberts et al. (1985)
found that in the developing roots of Halophila
ovalis, sieve tubes are differentiated before xylem
elements and the formation of root hairs; Casparian strip formation and xylem differentiation were
approximately synchronous. These findings suggest
that all root structures in H. ovalis are involved in
the uptake and transport of materials.
Roberts (1993) showed that 39% of root epidermal cells in H. ovalis produce root hairs resulting in
an increase in the effective root surface by 215%.
Furthermore, epidermal cells that produced root
hairs contained more cytoplasm and had a greater
number of plasmodesmatal connections with the underlying outer cortical cells than adjacent cells that
did not produce root hairs (Roberts, 1993). These observations suggest that epidermal cells that produce
root hairs play a more active role in nutrient uptake
and exchange than other epidermal cells in H. ovalis.
In addition to their anchoring function, seagrass
roots have other biological functions such as nutrient
uptake from the substratum and providing a suitable environment for microorganisms in the rhizosphere. As in terrestrial monocotyledons, both suberized walls and Casparian strips in exodermis and endodermis can restrict water and solutes transport in
seagrass roots (Kuo and McComb, 1989).
The rhizosphere of many seagrasses has been
found to support a diversity of microorganisms, especially bacteria. They occur mainly on the root surface; tissue penetration rarely occurs, and then only
into the periphery of the host (Kuo and Cambridge,
1978; Kuo et al., 1981; Cambridge and Kuo, 1982,
Kuo, 1993b). Nielsen et al. (2001) reported that bacteria patchily colonize 1–3% of the surface area of
roots and rhizomes of Zostera noltii. Nitrogen fixation associated with roots and rhizomes in this
species accounts for 31% of the nitrogen fixed
in the rhizosphere of Z. noltii. In addition, plantassociated nitrogen fixation could supply 37–1613%
of the nitrogen needed by the sulfate-reducing community (for further discussion see Marb` a et al.,
Chapter 6).
Fungi were found to have penetrated the epidermal cells of Posidonia australis causing lysis of
the thick polysaccharide material in the walls of
the exodermal cells (Kuo et al., 1981). A suberin
J. Kuo and C. den Hartog
of root hairs in Heterozostera nigricaulis (as H.
tasmanica) contain cutin or lipid, callose (1–3) βglucans, carbohydrates but no lignin. The major wall
polysaccharide is an apiogalacturonan and that cellulose accounts for most of the glucose.
Cell structure and the arrangement of the cortex
varies with the texture of root types, but roots always have three distinct zones. In soft roots, such
as those of Posidonia (Fig. 2F) and Halophila,
the outer cortex consists of one to several layers of compact cells with thin, non-lignified walls.
The middle cortex has well developed air lacunae separated by a radial partition, which is one
cell thick, containing small intercellular pores,
and the inner cortex consists of compact, small
cells in a regular, concentric arrangement (Roberts
et al., 1985; Kuo and McComb, 1989; Kuo, 1993b).
On the other hand, in hard roots such as those of Thalassodendron (Fig. 2E) and Amphibolis, the outer
cortex consists of several compact cells with slightly
thicker, lignified walls. The middle cortex contains
several layers of thin-walled, irregular cells in two
or more layers that sometimes make up some distinct but irregular air lacunae (Fig. 2D and H). The
inner cortex has two or more layers of small, thin
walled, compact cells (Figs. 2D–H, K and M and
3D–H). These pronounced air lacunae may facilitate gas movement from the root surface to the stele
or vice versa and also from the root base to the root
tip (Tomlinson, 1982; see also Borum et al., Chapter
10). In all seagrass species, the very distinct endodermis consists of a layer of compact cells, which
encloses the stele, as is also the case in many terrestrial plants (Fig. 3D–H). In all species, the walls
of the endodermis are thin to moderately thick, either lignified or not lignified, but they always contain a distinct Casparian strip on the radial walls
(Fig. 3I). An additional suberized layer may occur in
all walls or certain walls of the endodermis of some
species (Fig. 3G). The endodermis of seagrasses just
as in terrestrial flowering plants restricts solute and
water movements between the cortex and the stele
(Kuo and Cambridge, 1978). The stele contains the
vascular tissue with xylem and phloem but no distinct pericycle (Fig. 3H). Xylem elements are either
represented by a few, unthickened walls of poorly
lignified tracheid elements or by a single large lumen (Fig. 3F–H). Several sieve tubes with normal
walls are surrounded by vascular parenchyma cells
(Fig. 3H); these possess wall ingrowths and are rich
in cytoplasm in Zostera (Fig. 3H and J) (Barnabas
and Arnott, 1987).
Tomlinson (1969) studied the development of
roots of Thalassia testudinum and concluded that
there were no water-conducting tissues, except close
to the base of the root, suggesting that roots were of
no special significance in water absorption in this
species. On the other hand, Roberts et al. (1985)
found that in the developing roots of Halophila
ovalis, sieve tubes are differentiated before xylem
elements and the formation of root hairs; Casparian strip formation and xylem differentiation were
approximately synchronous. These findings suggest
that all root structures in H. ovalis are involved in
the uptake and transport of materials.
Roberts (1993) showed that 39% of root epidermal cells in H. ovalis produce root hairs resulting in
an increase in the effective root surface by 215%.
Furthermore, epidermal cells that produced root
hairs contained more cytoplasm and had a greater
number of plasmodesmatal connections with the underlying outer cortical cells than adjacent cells that
did not produce root hairs (Roberts, 1993). These observations suggest that epidermal cells that produce
root hairs play a more active role in nutrient uptake
and exchange than other epidermal cells in H. ovalis.
In addition to their anchoring function, seagrass
roots have other biological functions such as nutrient
uptake from the substratum and providing a suitable environment for microorganisms in the rhizosphere. As in terrestrial monocotyledons, both suberized walls and Casparian strips in exodermis and endodermis can restrict water and solutes transport in
seagrass roots (Kuo and McComb, 1989).
The rhizosphere of many seagrasses has been
found to support a diversity of microorganisms, especially bacteria. They occur mainly on the root surface; tissue penetration rarely occurs, and then only
into the periphery of the host (Kuo and Cambridge,
1978; Kuo et al., 1981; Cambridge and Kuo, 1982,
Kuo, 1993b). Nielsen et al. (2001) reported that bacteria patchily colonize 1–3% of the surface area of
roots and rhizomes of Zostera noltii. Nitrogen fixation associated with roots and rhizomes in this
species accounts for 31% of the nitrogen fixed
in the rhizosphere of Z. noltii. In addition, plantassociated nitrogen fixation could supply 37–1613%
of the nitrogen needed by the sulfate-reducing community (for further discussion see Marb` a et al.,
Chapter 6).
Fungi were found to have penetrated the epidermal cells of Posidonia australis causing lysis of
the thick polysaccharide material in the walls of
the exodermal cells (Kuo et al., 1981). A suberin