234
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
(Invers et al., 2002). Very little is known about the
form in which phosphorus storage occurs.
Storage can take place at different time-scales. In
the short term, seagrasses can take up nutrients, and
use them several hours to days later. For example,
Z. marina has the ability to reduce nitrate regardless
of time of day, thus taking advantage of infrequent,
short-term night pulses of nitrate. This short-term
storage capacity also allows the species to reduce
nitrate during storms, in which nitrate availability increases but the turbidity of the water severely limits
light reaching the canopy (Roth and Pregnall, 1988;
Burkholder et al., 1994). Such behavior would require redirection of carbohydrates from storage reserves to the production of the energy and carbon
skeletons needed to reduce nitrate and form amino
acids (Turpin, 1991; Ferrario-M´ ery et al., 1997).
However, probably the best documented storage
behavior is that related to the seasonal changes in nutrient availability. In most temperate areas, nutrient
availability varies seasonally, and is usually uncoupled to light availability and to optimal temperature
conditions. Thus, during the nutrient ‘rich’ period,
plants take up nutrients, probably assimilate them
(at least in the case of N), and store them in leaves
and/or in rhizomes (Pellikaan and Nienhuis, 1988;
Alcoverro et al., 1995, 1997; Kraemer and Mazzella,
1999), to be used during the nutrient ‘poor’ period.
Less is known about possible interannual storage
mechanisms, although the minimum value of N content in rhizomes, i.e. that reached after the use of reserves, changes from year to year (Alcoverro et al.,
2000); there is thus the possibility that some stored
N remains in the rhizomes after a ‘good’ year, to be
used the year after.
D. Nutrient Transport
Given the participation of both leaves and roots in
nutrient acquisition, it could be supposed that transport of nutrients would not be as important as in terrestrial plants. However, internal nutrient demand is
by no means coupled to nutrient acquisition (neither
spatially nor temporally), and there are a number of
processes requiring nutrient transport between different parts of the plant, both over relatively short
distances (for example, from cell to cell, from the
leaf blade to the basal meristem, etc.) and over relatively long distances (relocation of nutrients from
senescent leaves before abscission, mobilization of
nutrients stored in rhizomes, transport from intercalary shoots to apical shoots, etc.). Transport should
therefore be considered as an essential part of seagrass nutrient economy, and a key aspect in its functional clonal integration.
Surprisingly, the research effort devoted to the
study of nutrient transport in seagrasses is notoriously sparse, especially when compared to the
advances made in terrestrial plants. Issues such
as the physiological basis of transport, the factors determining source/sink shifts within the plant,
the biochemistry of the transported substances, or
how resources are shared between ramets have
been addressed only preliminarily or remain simply
unexplored.
Cell-to-cell (symplastic) transport, or transport by
diffusion in the extracellular space (apoplastic) can
account for short-distance movements of solutes.
For example, in Syringodium isoetifolium no connections (i.e. plasmodesmata) between adjacent epidermal cells nor between epidermal and mesophyll
cells were observed, suggesting exclusively apoplastic transport (Kuo, 1993). However, in other species
such as Zostera muelleri and Phyllospadix sp. plasmodesmata are present connecting epidermal cells
among them and with mesophyll cells (Kuo, 1993;
Kuo and Stewart, 1995), allowing symplastic nutrient transport.
These cell-to-cell mechanisms are excessively
slow for long-distance transport (i.e. above 10 cm),
at least in terrestrial plants (Mengel and Kirkby,
2001), where long-distance transport is achieved via
the vascular system, basically through the xylem
system (acropetal transport) but also through the
phloem system (multidirectional transport). It is obvious that water pressure gradients created by water
evaporation at the leaves surfaces do not exist in
seagrasses, and xylem transport, if any, should be
driven differently. To date, the only known alternative is through the active accumulation of solutes in
the plasma membrane of xylem parenchyma cells,
which induces a pressure (root pressure) from the
water tending to enter the xylem (e.g. Steudle and
Peterson, 1998). Although this mechanism seems to
have a certain role in terrestrial plants whose transpiration is inhibited, it seems unlikely that it plays
an effective role in seagrasses, due to the high salt
content of seawater. Moreover, it has to be remembered that xylem is generally reduced in seagrasses
(Kuo, 1993; Kuo and Stewart, 1995). Thus it would
seem that long-distance transport should be predominantly achieved via the phloem system. The phloem,
although classically described as the vascular system responsible for the distribution of assimilates
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
(Invers et al., 2002). Very little is known about the
form in which phosphorus storage occurs.
Storage can take place at different time-scales. In
the short term, seagrasses can take up nutrients, and
use them several hours to days later. For example,
Z. marina has the ability to reduce nitrate regardless
of time of day, thus taking advantage of infrequent,
short-term night pulses of nitrate. This short-term
storage capacity also allows the species to reduce
nitrate during storms, in which nitrate availability increases but the turbidity of the water severely limits
light reaching the canopy (Roth and Pregnall, 1988;
Burkholder et al., 1994). Such behavior would require redirection of carbohydrates from storage reserves to the production of the energy and carbon
skeletons needed to reduce nitrate and form amino
acids (Turpin, 1991; Ferrario-M´ ery et al., 1997).
However, probably the best documented storage
behavior is that related to the seasonal changes in nutrient availability. In most temperate areas, nutrient
availability varies seasonally, and is usually uncoupled to light availability and to optimal temperature
conditions. Thus, during the nutrient ‘rich’ period,
plants take up nutrients, probably assimilate them
(at least in the case of N), and store them in leaves
and/or in rhizomes (Pellikaan and Nienhuis, 1988;
Alcoverro et al., 1995, 1997; Kraemer and Mazzella,
1999), to be used during the nutrient ‘poor’ period.
Less is known about possible interannual storage
mechanisms, although the minimum value of N content in rhizomes, i.e. that reached after the use of reserves, changes from year to year (Alcoverro et al.,
2000); there is thus the possibility that some stored
N remains in the rhizomes after a ‘good’ year, to be
used the year after.
D. Nutrient Transport
Given the participation of both leaves and roots in
nutrient acquisition, it could be supposed that transport of nutrients would not be as important as in terrestrial plants. However, internal nutrient demand is
by no means coupled to nutrient acquisition (neither
spatially nor temporally), and there are a number of
processes requiring nutrient transport between different parts of the plant, both over relatively short
distances (for example, from cell to cell, from the
leaf blade to the basal meristem, etc.) and over relatively long distances (relocation of nutrients from
senescent leaves before abscission, mobilization of
nutrients stored in rhizomes, transport from intercalary shoots to apical shoots, etc.). Transport should
therefore be considered as an essential part of seagrass nutrient economy, and a key aspect in its functional clonal integration.
Surprisingly, the research effort devoted to the
study of nutrient transport in seagrasses is notoriously sparse, especially when compared to the
advances made in terrestrial plants. Issues such
as the physiological basis of transport, the factors determining source/sink shifts within the plant,
the biochemistry of the transported substances, or
how resources are shared between ramets have
been addressed only preliminarily or remain simply
unexplored.
Cell-to-cell (symplastic) transport, or transport by
diffusion in the extracellular space (apoplastic) can
account for short-distance movements of solutes.
For example, in Syringodium isoetifolium no connections (i.e. plasmodesmata) between adjacent epidermal cells nor between epidermal and mesophyll
cells were observed, suggesting exclusively apoplastic transport (Kuo, 1993). However, in other species
such as Zostera muelleri and Phyllospadix sp. plasmodesmata are present connecting epidermal cells
among them and with mesophyll cells (Kuo, 1993;
Kuo and Stewart, 1995), allowing symplastic nutrient transport.
These cell-to-cell mechanisms are excessively
slow for long-distance transport (i.e. above 10 cm),
at least in terrestrial plants (Mengel and Kirkby,
2001), where long-distance transport is achieved via
the vascular system, basically through the xylem
system (acropetal transport) but also through the
phloem system (multidirectional transport). It is obvious that water pressure gradients created by water
evaporation at the leaves surfaces do not exist in
seagrasses, and xylem transport, if any, should be
driven differently. To date, the only known alternative is through the active accumulation of solutes in
the plasma membrane of xylem parenchyma cells,
which induces a pressure (root pressure) from the
water tending to enter the xylem (e.g. Steudle and
Peterson, 1998). Although this mechanism seems to
have a certain role in terrestrial plants whose transpiration is inhibited, it seems unlikely that it plays
an effective role in seagrasses, due to the high salt
content of seawater. Moreover, it has to be remembered that xylem is generally reduced in seagrasses
(Kuo, 1993; Kuo and Stewart, 1995). Thus it would
seem that long-distance transport should be predominantly achieved via the phloem system. The phloem,
although classically described as the vascular system responsible for the distribution of assimilates