248
M.e. Ball and J.B. Passioura
vations in the intertidal zone, the soil is typically saturated with water so
that the suction in the soil is close to zero and the soil water potential is
therefore essentially equal to the osmotic potential. During transpiration,
which, if it is to be sustained, requires the flow of water from roots to
leaves, the leaf water potential must be lower than the soil water potential,
and the osmotic potential of the leaves must be lower still if the leaves are to
maintain turgor. The frictional losses of water potential associated with the
passage of the transpiration stream through the plant may lower the water
potential of the leaves to several hundred kilopascals below that of the soil.
Typical values of leaf water potential in field-grown mangroves range from
-2.5 to -6MPa (Scholander et al. 1964; Scholander 1968; Rada et al. 1989;
Smith et al. 1989). These water potentials are necessarily much lower than
those of well-watered plants growing in freshwater, and possibly pose a
threat of embolism in the plants' xylem vessels, with a consequent runaway
increase in the overall hydraulic resistance, a point to which we will return.
The roots of the plants must, when they extract water from the soil,
exclude most of the salt from the transpiration stream, for otherwise there
would be a rapid and lethal buildup of salt at the evaporating surfaces in the
leaves. Some mangrove species secrete salt from their leaves. These species
allow more salt into the xylem than do the nonsecretors, but even secretors
typically exclude 90% of the salt in the soil water as it enters the root
(Scholander et al. 1962, 1966; Moon et al. 1986; Ball 1988b). The salt that
does gain entry largely accumulates (if not secreted) in the vacuoles of the
leaf cells, where it is the main contributor to the low osmotic potentials that
are required there (Popp 1984a,b; Popp et al. 1984). The great longevity of
the leaves of many mangrove species (Saenger and Moverley 1985) implies
that the salt balance of the leaves is well controlled.
Given this background of the essentials of the salt and water relations of
the plants, what are the processes that seemingly require the plants to have
low transpiration rates despite the abundance of albeit salty water, and
despite the presumed advantage accruing to a species that could transpire
faster, and thereby gain carbon faster, than its conservative cousins?
One possibility is that the filters in the roots that exclude most of the salt
from the transpiration stream may be so constituted that they present a very
large resistance to the flow of water through them. Rapid transpiration
would then induce such a low water potential in the leaves that an impossibly high concentration of solutes in the cells would be required to
maintain turgor. However, the root density of mangroves is typically very
high (Komiyama et al. 1987) and increases with increasing salinity, albeit at
the expense of shoot growth (Ball 1988b). Such increase in root mass per
unit leaf area concomitant with decrease in transpiration rates implies that
the demands for water by the shoot are met by taking up water slowly over a
large root system (Ball 1988b).
A second possibility, related to the first, is that water potentials below
about 6 MPa may so tax the integrity of the xylem sap that embolism occurs
M.e. Ball and J.B. Passioura
vations in the intertidal zone, the soil is typically saturated with water so
that the suction in the soil is close to zero and the soil water potential is
therefore essentially equal to the osmotic potential. During transpiration,
which, if it is to be sustained, requires the flow of water from roots to
leaves, the leaf water potential must be lower than the soil water potential,
and the osmotic potential of the leaves must be lower still if the leaves are to
maintain turgor. The frictional losses of water potential associated with the
passage of the transpiration stream through the plant may lower the water
potential of the leaves to several hundred kilopascals below that of the soil.
Typical values of leaf water potential in field-grown mangroves range from
-2.5 to -6MPa (Scholander et al. 1964; Scholander 1968; Rada et al. 1989;
Smith et al. 1989). These water potentials are necessarily much lower than
those of well-watered plants growing in freshwater, and possibly pose a
threat of embolism in the plants' xylem vessels, with a consequent runaway
increase in the overall hydraulic resistance, a point to which we will return.
The roots of the plants must, when they extract water from the soil,
exclude most of the salt from the transpiration stream, for otherwise there
would be a rapid and lethal buildup of salt at the evaporating surfaces in the
leaves. Some mangrove species secrete salt from their leaves. These species
allow more salt into the xylem than do the nonsecretors, but even secretors
typically exclude 90% of the salt in the soil water as it enters the root
(Scholander et al. 1962, 1966; Moon et al. 1986; Ball 1988b). The salt that
does gain entry largely accumulates (if not secreted) in the vacuoles of the
leaf cells, where it is the main contributor to the low osmotic potentials that
are required there (Popp 1984a,b; Popp et al. 1984). The great longevity of
the leaves of many mangrove species (Saenger and Moverley 1985) implies
that the salt balance of the leaves is well controlled.
Given this background of the essentials of the salt and water relations of
the plants, what are the processes that seemingly require the plants to have
low transpiration rates despite the abundance of albeit salty water, and
despite the presumed advantage accruing to a species that could transpire
faster, and thereby gain carbon faster, than its conservative cousins?
One possibility is that the filters in the roots that exclude most of the salt
from the transpiration stream may be so constituted that they present a very
large resistance to the flow of water through them. Rapid transpiration
would then induce such a low water potential in the leaves that an impossibly high concentration of solutes in the cells would be required to
maintain turgor. However, the root density of mangroves is typically very
high (Komiyama et al. 1987) and increases with increasing salinity, albeit at
the expense of shoot growth (Ball 1988b). Such increase in root mass per
unit leaf area concomitant with decrease in transpiration rates implies that
the demands for water by the shoot are met by taking up water slowly over a
large root system (Ball 1988b).
A second possibility, related to the first, is that water potentials below
about 6 MPa may so tax the integrity of the xylem sap that embolism occurs