186 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
1
P680 in PSII, including the primary donor of P680 Z; reaction side of the D1 protein,
which would prevent Q from being reduced Larkum and Wood, 1993. Increases in F ,
o
apparently result, as well, from UV damage to the PSII reaction centers in the seagrasses Cymodocea serrulata, Halodule uninervis, Halophila ovalis, Syringodium isoetifolium
and Zostera capricorni Dawson and Dennison, 1996. F F ratios have been used in
v m
seagrasses to demonstrate photosuppression and PSII responses to UV radiation, light deprivation, and other stressors Table 6.
5. Carbohydrate metabolism
5.1. Major carbohydrate storage compounds The processes by which starch or sucrose is biosynthesized in angiosperms from C3
photosynthesis products triose-P, 3-PGA, and dehydroxyacetone are based on competing reactions that are physically separated within the cell, with starch produced in plastids
and sucrose produced in the cytosol Fig. 3. The relative amount of starch or sucrose produced by plants dependents largely on the available P Goodwin and Mercer, 1983.
i
When high internal P levels are available, more triose-P can be exported into the cytosol
i
to form sucrose; when P is low, triose-P export decreases with concomitant increase in
i
starch production. Beyond this generalization from general plant biochemistry, however, many plant species have demonstrated preference for storage of one compound starch,
sucrose, or other complex carbohydrates such as rafinose or stachyose over another Brocklebank and Hendry, 1989. Furthermore, the primary storage compound can be
influenced by growth status; for example, plants undergoing rapid growth tend to have higher levels of sucrose relative to starch Taiz and Zeiger, 1991.
The dominant storage carbohydrate in most seagrasses is the soluble product, sucrose Drew, 1983; Pirc, 1989; Vermatt and Verhagen, 1996; Touchette, 1999. In the species
that have been examined Enhalus acoroides, Halodule wrightii , Halophila decipiens,
Syringodium filiforme, Thalassia testudinum, Zostera marina, sucrose forms more than 90 of the total soluble carbohydrate pool. Other soluble carbohydrates have included
glucose, fructose, and more complex polysaccharides Drew, 1983. Although lower in abundance, additional soluble carbohydrates in seagrasses include apiose, arabinose,
fucose, galactose, mannose, rhamnose, and xylose Waldron et al., 1989; Webster and Stone, 1994.
On average basis, 24 studies involving 18 seagrass species, total carbohydrates in
21
stem, leaf, root, and rhizome tissues are ca. 95, 100, 135, and 275 mg g dry weight,
respectively Tables 7 and 8. As previously indicated, rhizomatous tissues generally contain most of the stored carbohydrates. Storage of carbohydrate reserves in below-
ground structures may minimize carbon loss from herbivory, and also would ensure that high carbohydrate levels were available to sustain the perennating belowground
structures during dormant periods for example, in warm late summer temperatures, for the north temperate seagrass Zostera marina when aboveground shoot tissues may be
inactive and or senescent Burke et al., 1996. Leaf and rhizome carbohydrate levels are positively correlated for various seagrass species; that is, species with high leaf sugar
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
187
Fig. 3. Sucrose and starch biosynthesis and catabolism in plant cells. Reactions of starch biosynthesis occur in plastids, wherein accumulation of triose-phosphate, especially during photosynthesis, initiates the process.
Sucrose biosynthesis begins with an exchange of P and triose-phosphate at the P-translocator, so that
i
triose-phosphate levels begin to accumulate in the cytosol. Two triose-phosphate dihydroxyacetone 3- phosphate and glyceraldehyde 3-phosphate are then combined to form a single 6-C compound, fructose
1,6-bisphosphate fructose 1,6-bisphosphate, which undergoes dephosphorylation to form fructose 6-phos- phate. This product can then undergo isomerization to form glucose 6-P, which eventually is converted to
UDP-glucose. UDP-glucose combines with available fructose 6-P to form sucrose 6-P, via the enzyme sucrose-P synthase SPS. Sucrose 6-phosphate is dephosphorylated to produce sucrose, which can be used
directly in carbon storage, converted to more complex carbohydrates, or translocated to other tissues to aid in other metabolic processes or storage. To initiate sucrolysis, the enzyme sucrose synthase SS breaks down
sucrose to UDP-glucose and fructose, thus liberating carbon for use in various metabolic pathways developed from Taiz and Zeiger, 1991.
2
content typically accumulate more rhizome carbohydrates r 5 0.54; Fig. 4A. There is
2
a similar though weaker correlation between root and rhizome tissues r 5 0.35; Fig. 4B.
We note, as well, an interesting trend between light compensation point I and leaf
c
soluble carbohydrate levels in seagrasses Fig. 5. If I values represent the light level at
c
which photosynthesis and respiration rates are equivalent, then plants with high I values
c
would have either relatively high respiration rates or low photosynthesis rates — both would indicate a lower carbon efficiency. Accordingly, seagrasses with high I levels
c
188 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205 Table 7
Nonstructural carbohydrate levels reported in photosynthetic tissues of seagrass species; data include species,
a
tissue aboveground, leaf, stem, and soluble carbohydrates
Species Tissue
Soluble carbohydrates Source
Temperate
21
Amphibolis antarctica Leaf
11 mg g dw
Drew 1982
21
Amphibolis antarctica Stem
7.3 mg g dw
Drew 1982
21
Phyllospadix scouleri Leaf
134 mg g dw
Neighbors and Horn 1991
21
Phyllospadix torreyi Aboveground
228 mg g dw
Drew 1982
21
Ruppia maritima Leaf
119–334 mg g dw
Lazer and Dawes 1991
21
Zostera marina Leaf
42 mg g fw
Zimmerman et al. 1995
21
Zostera marina Leaf
34 mg g fw
Zimmerman et al. 1997
21
Zostera marina Leaf
144 mg g dw
Drew 1982
b 21
Zostera noltii Aboveground
1–70 mg g dw
Vermatt and Verhagen 1996
21
Grand mean61 S.E. Leaf n54 species
128.7644.3 mg g dw
21
Stem n51 species 7.3 mg g
dw
21
Aboveground n52 species 29.266.3 mg g
dw Tropical subtropical
21
Cymodocea nodosa Leaf
55 mg g dw
Drew 1982
21
Cymodocea serrulata Leaf
92 mg g dw
Tomasko 1993
21
Enhalus acoroides Leaf
25 mg g dw
Drew 1982
21
Halodule pinifolia Leaf
163 mg g dw
Tomasko 1993
21
Halodule uninervis Leaf
290 mg g dw
Tomasko 1993
21
Halodule wrightii Leaf
30 mg g dw
Drew 1982
21
Halophila decipiens Leaf
180 mg g dw
Drew 1982
21
Halophila engelmanii Leaf
52–124 mg g dw
Dawes et al. 1987
21
Halophila engelmanii Stem
72–151 mg g dw
Dawes et al. 1987
b 21
Halophila ovalis Leaf
15– 42 mg g dw
Longstaff et al. 1999
21
Syringodium filiforme Leaf
185 mg g dw
Ray and Stevens 1996
21
Syringodium filiforme Leaf
22 mg g dw
Drew 1982
21
Syringodium isoetifolium Leaf
169 mg g dw
Tomasko 1993
21
Thalassia hemprichii Leaf
92 mg g dw
Tomasko 1993
21
Thalassia testudinum Leaf
99–161 mg g dw
Durako and Moffler 1985
21
Thalassia testudinum Leaf
50–70 mg C g dw
Lee and Dunton 1996
21
Thalassia testudinum Leaf
50 mg C g dw
Lee and Dunton 1997
21
Thalassia testudinum Leaf
60 mg g dw
Drew 1982
21
Thalassodendron ciliatum Leaf
24 mg g dw
Drew 1982
21
Thalassodendron ciliatum Stem
52 mg g dw
Drew 1982
21
Grand mean61 S.E. Leaf n514 species
102.5620.3 mg g dw
21
Stem n52 species 81.7629.2 mg g
dw
a 21
Note that, where possible, values were converted from percent dry weight to mg g dry weight dw.
Other values are reported as fresh weight fw. The data are given as means 6 1 S.E. for temperate and tropical subtropical seagrasses, or as ranges if means were not available. Grand means confined to
21
consideration of data reported with the most common unit representation, mg g dw, were calculated based
on mid-range estimates for those cases.
b 21
21
Starch was also observed in these two species 0–20 mg g dw in Zostera noltii; 73–91 mg g
dw for Halophila ovalis.
tend to have low leaf carbohydrates. Although this approach is simplistic considering the number of factors that can influence carbohydrate levels, nonetheless, seagrasses with I
c 22
21
values less than 50 mE m s
have leaf soluble carbohydrates typically in excess of 50
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
189 Table 8
Soluble carbohydrate levels reported in non-photosynthetic tissues of seagrass species. Data include species,
a
tissues belowground, rhizome, root, and soluble carbohydrates
Species Tissue
Soluble carbohydrates Source
Temperate
21
Amphibolis antarctica Belowground
33 mg g dw
Tomasko 1993
21
Amphibolis antarctica Rhizome
11.4 mg g dw
Drew 1982
21
Amphibolis antarctica Root
19 mg g dw
Drew 1982
21
Phyllospadix torreyi Belowground
234 mg g dw
Drew 1982
21
Posidonia australis Rhizome
390 mg g dw
Ralph et al. 1992
21
Ruppia maritima Rhizome
118–520 mg g dw
Lazar and Dawes 1991
21
Ruppia maritima Root
95–236 mg g dw
Lazar and Dawes 1991
21
Zostera marina Root
5 mg g fw
Zimmerman et al. 1997
21
Zostera marina Root
3.5 mg g fw
Zimmerman et al. 1995
21
Zostera marina Rhizome
282 mg g dw
Drew 1982
21
Zostera marina Root
43 mg g dw
Drew 1982
b 21
Zostera noltii Belowground
20–150 mg g dw
Vermatt and Verhagen 1996
21
Grand mean 6 S.E. Root n53 species
75.6645.3 mg g dw
21
Rhizome n54 species 250.6682.8 mg g
dw
21
Belowground n53 species 120.3662.8 mg g
dw Tropical subtropical
21
Cymodocea nodosa Rhizome
213 mg g dw
Drew 1982
21
Cymodocea nodosa Root
151 mg g dw
Drew 1982
21
Cymodocea serrulata Belowground
367 mg g dw
Tomasko 1993
21
Enhalus acoroides Rhizome
84 mg g dw
Drew 1982
21
Enhalus acoroides Root
227 mg g dw
Drew 1982
21
Halodule pinifolia Belowground
182 mg g dw
Tomasko 1993
21
Halodule uninervis Belowground
45 mg g dw
Tomasko 1993
21
Halodule wrightii Rhizome
210 mg g dw
Drew 1982
21
Halodule wrightii Root
168 mg g dw
Drew 1982
21
Halophila decipiens Rhizome
454 mg g dw
Drew 1982
21
Halophila decipiens Root
96 mg g dw
Drew 1982
21
Halophila engelmanii Rhizome
202–347 mg g dw
Dawes et al. 1987
b 21
Halophila ovalis Rhizome
40–126 mg g dw
Longstaff et al. 1999
b 21
Halophila ovalis Root
8–30 mg g dw
Longstaff et al. 1999
21
Syringodium filiforme Rhizome
280 mg g dw
Rey and Stephens 1996
21
Syringodium filiforme Rhizome
804 mg g dw
Drew 1982
21
Syringodium filiforme Root
473 mg g dw
Drew 1982
21
Thalassia hemprichii Belowground
45 mg g dw
Tomasko 1993
21
Thalassia testudinum Rhizome
194–318 mg g dw
Durako and Moffler 1985
21
Thalassia testudinum Rhizome
110–200 mg C g dw
Lee and Dunton 1996
21
Thalassia testudinum Rhizome
130 mg C g dw
Lee and Dunton 1997
21
Thalassia testudinum Root
94–151 mg g dw
Durako and Moffler 1985
21
Thalassia testudinum Root
65–100 mg C g dw
Lee and Dunton 1996
21
Thalassia testudinum Root
71 mg C g dw
Lee and Dunton 1997
21
Thalassia testudinum Rhizome
263 mg g dw
Drew 1982
21
Thalassia testudinum Root
200 mg g dw
Drew 1982
21
Thalassodendron ciliatum Rhizome
139 mg g dw
Drew 1982
21
Thalassodendron ciliatum Root
160 mg g dw
Drew 1982 Grand mean 6 S.E.
21
Root n58 species 181.9646.8 mg g
dw
21
Rhizome n59 species 250.8652.5 mg g
dw
21
Belowground n54 species 159.7676.2 mg g
dw
a 21
Some values were converted from percent dry weight to mg g dry weight [dw]. Data are given as means
6 1 S.E. for temperate and tropical subtropical seagrasses, or as ranges if means were not available. Grand
21
means confined to consideration of data reported with the most common unit representation, mg g dw were
calculated using mid-range values for those cases.
b 21
Starch was also observed in these species 10–40 mg g dw for Zostera noltii belowground tissues,
21 21
57–85 mg g dw for Halophila ovalis rhizome, and 92–118 mg g
dw for Halophila ovalis roots.
190 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
Fig. 4. Relationship between rhizome soluble carbohydrate levels and A leaf soluble carbohydrates; or B root soluble carbohydrates. Correlations were derived from published literature values, and letters represent
different species see Table 8. In the leaf carbohydrate regression, the far right S .f. value was omitted from the
analysis because of its extremely high rhizome carbon content more than two-fold higher than all other values.
21
mg g dry weight, whereas plants with higher I values tend to have much lower leaf
c
carbohydrate content. 5.2. Sucrose metabolism
The key enzyme involved in sucrose biosynthesis is sucrose-P synthase SPS; Fig. 3.
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
191
Fig. 5. Relationship between compensation-level irradiance I and leaf soluble carbohydrates, including 15
c
observations on nine seagrass species from the published literature. Letters represent different species see
22 21
Tables 3 and 8. Note that plants with relatively high I values .50 mE m s
tend to have low soluble
c 21
carbohydrate content ,50 mg g dry weight, whereas species with lower I values have the potential to
c
accumulate more soluble carbohydrates. Plants with high I values likely have higher respiration-to-photo-
c
synthesis ratios and, thus, may have a higher carbon demand.
SPS activity has been used as an indicator for sucrose export from source tissues, exemplified by an increase in SPS activity in developing leaf tissue during the transition
in function from a carbon sink to a carbon source Giaquinta, 1979; Stitt, 1994; Zimmerman et al., 1995b. In Zostera marina, for example, SPS activity has been shown
to increase with leaf age, which is consistent with leaf maturation from carbon sink to source Zimmerman et al., 1995b.
In many terrestrial plants with sucrose as the major storage carbohydrate, SPS activity is strongly influenced by light activation and P inhibition; and soluble carbohydrate
i
levels appear to fluctuate between light and dark conditions Huber et al., 1989a. However, the level of SPS activation by light may depend strongly on substrate
availability group I versus group II species; see Huber et al., 1989b. In contrast, SPS activity in starch-accumulating terrestrial plants appears to be unaffected by light or P
i
content, and soluble carbohydrate levels remain relatively stable between light and dark transitions group III species; Huber et al., 1989b. By contrast, Zimmerman et al.
1995b demonstrated that regulation of SPS activity in the seagrass Zostera marina failed to fall into the typical group classifications designated for terrestrial plants. SPS
activity in Z
. marina was similar to that reported for starch accumulating plants group III, in demonstrating limited response to light availability. But Z
. marina generally does not accumulate high starch content Smith et al., 1988; Zimmerman et al., 1996;
192 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
Touchette, 1999. Moreover, SPS activity in Z . marina leaf tissues appears to be
22 21
unaffected by extended periods of light deprivation up to 2 weeks at ,10 mE m s
Touchette et al., 1999, indicating lack of a SPS-light response in this seagrass. Although SPS is highly regulated by physiological processes, certain environmental
factors also can influence its activity. In Zostera marina, leaf SPS activities have been
1
correlated with changes in water-column NH , CO availability, photosynthesis,
4 2
salinity, temperature, and grazing Zimmerman et al., 1996, 1997; Touchette, 1999. In terrestrial plants, SPS activity and or sucrose levels tend to increase with decreasing
temperature Kaurin et al., 1981; Guy et al., 1992; Hurry et al., 1994. By contrast, in Z .
marina a positive relationship has been observed between SPS activity and temperature, and between SPS activity and salinity Fig. 6.
Terrestrial plants that have shown an inverse relationship between SPS activity and temperature appear to increase sucrose accumulation as a cryoprotectant during cooler
Fig. 6. Relationship between A temperature and B salinity and sucrose-P synthase SPS activity in leaf tissue of the seagrass Zostera marina
. Data are given as means61 S.E. from Touchette, 1999.
B .W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
193
periods Santarius, 1973; Carpenter et al., 1986, with sucrose functioning as an osmolyte in reducing water stress Guy et al., 1992. However, halophytes in shallow
marine systems sometimes encounter dynamic changes in environmental salinity for example, during warm periods with increased evaporation, which can significantly alter
the osmotic potential of their tissues Reed and Stewart, 1985; Weimberg, 1987. Increased SPS activity which has been observed, for example, in Z
. marina with increased temperature or salinity thus may indicate an osmotic adjustment response for
marine angiosperms, analogous to increased SPS activity as a cryoprotectant response in terrestrial non-halophytic plants Touchette, 1999.
Sucrose metabolism and degradation in sink tissues involve two key enzymes as invertase which hydrolyzes sucrose into glucose and sucrose synthase SS, which can
also carry out the initial step in sucrolysis but forms UDP-glucose and fructose as products; Fig. 3. Activities of these enzymes have been related to sucrose import into
sink tissues and sucrose entry into metabolism Claussen, 1983; Sung et al., 1988; Stitt, 1994; Koch and Nolte, 1995. Based on data from general plant physiology, invertase
activity tends to be highest during active growth and declines following maturation Sung et al., 1994; Koch and Nolte, 1995. In contrast, high SS activity can occur in
sink storage tissues of all ages, and SS appears to be the primary sucrose-metabolizing enzyme in mature storage tissues. SS activity and, thus, sucrose breakdown, may be
linked to respiration, given its response to adenylate balance toward conservation of ATP and its lack of apparent light activation Kalt-Torres and Huber, 1987; Koch and
Nolte, 1995. Moreover, during periods of low carbon availability, localization of SS activity within specific cells and or tissues may be a mechanism for providing sink
priority to the most essential cells.
In seagrasses, research on sucrose metabolism has been limited mostly to studies of Zostera marina, a species that stores most of its carbon reserves in belowground
rhizomatous tissues as mentioned. Root SS activity in Z . marina decreases with tissue
age SS activity in distal root tissues—for example, declined by two-fold in roots from the first to the eighth node; Kraemer et al., 1998. Nonetheless, increased SS activity has
been observed in very old root systems root bundle 10 and beyond; Kraemer et al., 1998. Based on this information, it has been hypothesized that there are two separate SS
isozymes in Z
. marina, one of which is highly sensitive to low sucrose levels; and that the elevated SS activity in the oldest root tissues may be in response to low sucrose
levels Kraemer et al., 1998. The activity of this SS isozyme may enhance plant survival under periods of carbon limitation by maximizing sucrose utilization during
periods when concentrations are low, and during periods of replenishment Kraemer et al., 1998. In aboveground leaf tissue, SS activity also has been reported to decrease
with age, a trend that may reflect a change in function from carbon sink to source as would be expected Kraemer et al., 1998. But even the oldest leaves maintain
substantial SS activity, suggesting that high levels of carbon metabolism can still occur in these tissues.
As in terrestrial angiosperms, SS activity in Z . marina does not appear to be
light-activated Zimmerman et al., 1995b although, in belowground tissues, SS activity has been reported to vary inversely with H
Alcoverro et al., 1999. Moreover,
sat
root-rhizome SS activity in many plants including this seagrass tends to increase when
194 B
.W. Touchette, J.M. Burkholder J. Exp. Mar. Biol. Ecol. 250 2000 169 –205
dissolved oxygen concentrations are low, indicating increased carbon metabolism in response to anoxic hypoxic conditions Sachs et al., 1996; Germain et al., 1997;
Touchette, 1999. In contrast, a positive relationship has been shown between below- ground tissue SS activity and temperature in Z
. marina Touchette, 1999. This temperature-associated increase in SS activity and, thus, higher sucrolysis, may be
associated with increased respiratory demand Touchette, 1999. 5.3. Starch metabolism
Starch synthesis occurs in plastids and, like sucrose production, is activated by an accumulation of triose-P and yields fructose 1,6-bisP as an initial product Godwin and
Mercer, 1983; Fig. 3. It differs from sucrose synthesis in that an ADP-glucose rather than UDP-glucose is involved, from which starch is formed by the enzyme starch
synthase.
The data on starch storage in seagrasses are highly variable. Low starch content has been reported in Thalassia testudinium Jagels, 1983 and Zostera marina ,5 of the
total carbohydrates, US Pacific Coast, Alcoverro et al., 1999; Touchette, 1999. In contrast, Burke et al. 1996 reported that starch could form more than 65 up to 140
21
mg g dry weight of the total nonstructural carbohydrate content in Z
. marina from Chesapeake Bay. In Z
. noltii, starch levels can approach one-third of the total nonstructural carbohydrates in belowground tissues Pirc, 1989; Vermatt and Verhagen,
1996. Halophila ovalis has also been found to accumulate up to 90 of its total nonstructural carbohydrates as starch in leaf and root tissue Longstaff et al., 1999.
Moreover, in plants with low shoot and root-rhizome starch, there may be substantial starch accumulation in fruit tissues e.g., in Halodule spp., Halophila ovalis, Phyllos-
padix iwatensis, P. japonicus, Thalassia hemprichii — Bragg and McMillan, 1986; Kuo et al., 1990; Kuo et al., 1991; Kuo and Kirkman, 1992.
6. Respiration