340
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
sucrose in the same species. Starch may represent up
to two thirds of total NSCC in Z. marina carbohydrates (Burke et al., 1996), and up to 90% in leaf and
root tissue of H. ovalis (Longstaff et al., 1999). In Z.
marina sucrose mobilisation in roots is stimulated
by low-light conditions in winter and translocation
to the shoots is inhibited by anoxic conditions (Zimmerman et al. 1995; Alcoverro et al., 1999).
Marked seasonal changes in NSCC reserves have
been documented in several seagrasses). Alcoverro
et al. (2001) report minimum levels from February
to May in P. oceanica when the plants were growing
rapidly under conditions of high dissolved nutrients
and increasing irradiance and day length, followed
by rapid accumulation in summer when nutrients had
become exhausted but irradiance levels were still
high (Fig. 5D). These reserves were then slowly depleted until the following spring. Vermaat & Verhagen (1996) also report a strong increase in rhizome sugar content in July when growth had largely
ceased but photosynthetic capacity was still maximal. Lazar & Dawes (1991) found the same trend
in Ruppia maritima. Blade growth in that species
occurred in winter, even earlier than in P. oceanica, and high levels of soluble carbohydrates (26.7
to 52% dry weight) did not accumulate until spring
and early summer after blade production was completed but before they died back in late summer and
autumn.
Clearly seagrasses have a wide variety of
metabolic pathways, such as those of cell wall synthesis, and these involve a variety of sugars such
as arabinose, fucose, galactose, mannose, rhamnose
and xylulose (Waldron et al., 1989; Webster and
Stone, 1994).
XIII. Secondary Metabolites
and Chemotaxonomy
In addition to the ubiquitous sucrose, glucose and
fructose, plus two trisaccharides apparently restricted to Phyllospadix spp., several seagrasses accumulate considerable amounts of another type of
soluble carbohydrate, inositols. Of the nine possible
inositols, only 5 are known to occur in plants and
only myo-, l-chiro-, muco- and an 0-methyl-mucoinositol occur in seagrasses. Drew (1984) reviewed
current knowledge of the possible roles of the inositols and their several methyl esters found in plants:
their restricted distribution shows significant taxonomic correlations.
Myo-inositol is found in all living cells in amounts
usually considerably less than 1% dry weight. It
is apparently synthesised by direct cyclization of
photosynthetically-produced glucose (Loewus &
Kelly, 1962) and is then involved mainly in cell
wall synthesis (Loewus, 1965). Leaves and rhizomes
of some seagrasses, particularly the Zosteraceae,
contain relatively large amounts of this compound,
up to a maximum of 2.2% dry weight in Z. noltii
rhizomes.
In most plants the cyclization enzyme is conservative and yields only myo-inositol, which then acts
as the sole precursor for any other inositols they accumulate. Drew (1978, 1983) suggested that, since
the configuration of the glucose molecule would permit the direct formation during cyclization of all the
inositols found in seagrasses, they may be inevitable
by-products of another, less specific, glucose cyclization enzyme. Only members of the predominantly tropical zannichelliacean seagrasses (Cymodoceaceae) accumulate these other inositols, with a
preponderance of l-chiro-inositol. This compound
has been detected in all genera except Halodule,
with a maximum of 6.8% dry weight in C. rotundata leaves. Muco-inositol appears to be slightly less
widely distributed in these seagrasses whilst its Omethyl ester is restricted to the endemic temperate
Australian genus Amphibolis.
Drew (1978) showed that in C. nodosa photosynthetically-fixed
14 C was incorporated only slowly
into these unusual compounds with 1.3% of soluble
14 C in myo-inositol and 1.1% in l-chiro-inositol after
1 hour. However, those other than myo-inositol can
probably accumulate to several percent dry weight
because they are not subsequently utilised even after
all soluble sugars have been respired away during
dark starvation for several days.
The possibility that these compounds might be
involved in an osmoregulatory role was not supported by studies at high and low salinities (Drew,
1978) although respiration, and therefore sucrose
utilisation was increased at both extremes. Tyerman
et al. (1984) also implicated sucrose, and possibly
amino acids, as minor osmoregulants in their study
of the osmotic environment of P. australis leaves.
Proline, an amino acid frequently involved in osmoregulation in halophytes, has been found to increase with increasing salinity in Halodule wrightii,
Ruppia maritima and Thalassia testudinum (Pulich,
1986). Alanine increased similarly in H. engelmanni
(Pulich, 1986). In P. australis and P. oceanica these
amino acids occur only at low levels (Augier, 1982).
A. W. D. Larkum, E. A. Drew, and P. J. Ralph
sucrose in the same species. Starch may represent up
to two thirds of total NSCC in Z. marina carbohydrates (Burke et al., 1996), and up to 90% in leaf and
root tissue of H. ovalis (Longstaff et al., 1999). In Z.
marina sucrose mobilisation in roots is stimulated
by low-light conditions in winter and translocation
to the shoots is inhibited by anoxic conditions (Zimmerman et al. 1995; Alcoverro et al., 1999).
Marked seasonal changes in NSCC reserves have
been documented in several seagrasses). Alcoverro
et al. (2001) report minimum levels from February
to May in P. oceanica when the plants were growing
rapidly under conditions of high dissolved nutrients
and increasing irradiance and day length, followed
by rapid accumulation in summer when nutrients had
become exhausted but irradiance levels were still
high (Fig. 5D). These reserves were then slowly depleted until the following spring. Vermaat & Verhagen (1996) also report a strong increase in rhizome sugar content in July when growth had largely
ceased but photosynthetic capacity was still maximal. Lazar & Dawes (1991) found the same trend
in Ruppia maritima. Blade growth in that species
occurred in winter, even earlier than in P. oceanica, and high levels of soluble carbohydrates (26.7
to 52% dry weight) did not accumulate until spring
and early summer after blade production was completed but before they died back in late summer and
autumn.
Clearly seagrasses have a wide variety of
metabolic pathways, such as those of cell wall synthesis, and these involve a variety of sugars such
as arabinose, fucose, galactose, mannose, rhamnose
and xylulose (Waldron et al., 1989; Webster and
Stone, 1994).
XIII. Secondary Metabolites
and Chemotaxonomy
In addition to the ubiquitous sucrose, glucose and
fructose, plus two trisaccharides apparently restricted to Phyllospadix spp., several seagrasses accumulate considerable amounts of another type of
soluble carbohydrate, inositols. Of the nine possible
inositols, only 5 are known to occur in plants and
only myo-, l-chiro-, muco- and an 0-methyl-mucoinositol occur in seagrasses. Drew (1984) reviewed
current knowledge of the possible roles of the inositols and their several methyl esters found in plants:
their restricted distribution shows significant taxonomic correlations.
Myo-inositol is found in all living cells in amounts
usually considerably less than 1% dry weight. It
is apparently synthesised by direct cyclization of
photosynthetically-produced glucose (Loewus &
Kelly, 1962) and is then involved mainly in cell
wall synthesis (Loewus, 1965). Leaves and rhizomes
of some seagrasses, particularly the Zosteraceae,
contain relatively large amounts of this compound,
up to a maximum of 2.2% dry weight in Z. noltii
rhizomes.
In most plants the cyclization enzyme is conservative and yields only myo-inositol, which then acts
as the sole precursor for any other inositols they accumulate. Drew (1978, 1983) suggested that, since
the configuration of the glucose molecule would permit the direct formation during cyclization of all the
inositols found in seagrasses, they may be inevitable
by-products of another, less specific, glucose cyclization enzyme. Only members of the predominantly tropical zannichelliacean seagrasses (Cymodoceaceae) accumulate these other inositols, with a
preponderance of l-chiro-inositol. This compound
has been detected in all genera except Halodule,
with a maximum of 6.8% dry weight in C. rotundata leaves. Muco-inositol appears to be slightly less
widely distributed in these seagrasses whilst its Omethyl ester is restricted to the endemic temperate
Australian genus Amphibolis.
Drew (1978) showed that in C. nodosa photosynthetically-fixed
14 C was incorporated only slowly
into these unusual compounds with 1.3% of soluble
14 C in myo-inositol and 1.1% in l-chiro-inositol after
1 hour. However, those other than myo-inositol can
probably accumulate to several percent dry weight
because they are not subsequently utilised even after
all soluble sugars have been respired away during
dark starvation for several days.
The possibility that these compounds might be
involved in an osmoregulatory role was not supported by studies at high and low salinities (Drew,
1978) although respiration, and therefore sucrose
utilisation was increased at both extremes. Tyerman
et al. (1984) also implicated sucrose, and possibly
amino acids, as minor osmoregulants in their study
of the osmotic environment of P. australis leaves.
Proline, an amino acid frequently involved in osmoregulation in halophytes, has been found to increase with increasing salinity in Halodule wrightii,
Ruppia maritima and Thalassia testudinum (Pulich,
1986). Alanine increased similarly in H. engelmanni
(Pulich, 1986). In P. australis and P. oceanica these
amino acids occur only at low levels (Augier, 1982).