V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
301
nondesalting mangroves it is even ten times smaller (Scholander et al.,
1962, p. 728). This indicates that some tissue of the root acts as a semipermeable membrane which excludes most of the salt and gives passage
to a very dilute salt solution only. Upward water movement in the xylem
is still possible since water potential in the leaves is lower than that of
seawater (Scholander, 1968, p. 260). This situation leads to high tension
of the water columns in the xylem vessels. The low water potential of
leaves is the result of salt accumulation in the vacuoles of mesophyll cells.
The high salt concentration in the leaves develops soon after seeds germinate. The viviparous seedlings germinating on the mother plant have
a very high osmotic potential (e.g., — 13 to— 18 atm for Rhizophora
mucronata) (Walter and Steiner, 1936, p. 168) and a low NaCl concentration. The mother plant supplies the seedlings with water by the glandular
action of the cotyledonar body. As soon as the seedlings separate from
the mother plant and root in the seawater-saturated mud, they accumulate
salt very fast. This accumulation indicates an appreciable salt permeability
of the root at this stage of development. Soon the leaves of seedlings reach
the same low osmotic potential and the high chloride fraction of expressed
leaf sap as the mother plants. When this stage is reached the root system
becomes highly impermeable for salts, since only small quantities of salts
are taken up to supplement the amount of salt needed to maintain a constant salt concentration while the total volume of the vacuome increases
in the growing plant.
The nondesalting mangrove regulates the salt concentration of cell sap
of leaf cells by (1) salt translocation from senescing leaves into young
leaf primordia, (2) increase of leaf succulence with age of the leaf (which
counteracts to some extent a too high salt concentration, since succulence
involves water uptake), and (3) removal of excess salt from the plant
by abscission of senescent salt-containing leaves.
The regulation of salt content in desalting halophytes can be visualized
to be less complicated than in nondesalting species. The roots of desalting
halophytes generally absorb more salts than the roots of the nondesalting
species.
Mechanisms similar to those of the mangrove for regulation of the salt
content in the plant seem to operate in the halophytes of arid zones, where
desalting and nondesalting species also are found. The seeds of halophytes
of different species have a low salt content (Schratz, 1936, p. 186; Joshi,
1971, p. 8) and the seedling roots take up salts for a limited time period
after germination only. Later only small amounts of salts are taken up and
the xylem sap has a relatively low salt concentration.
Steiner (1934, p. 185ff) distinguishes three types of halophytes with
regard to regulation of their salt concentration: (1) regulation by increase
301
nondesalting mangroves it is even ten times smaller (Scholander et al.,
1962, p. 728). This indicates that some tissue of the root acts as a semipermeable membrane which excludes most of the salt and gives passage
to a very dilute salt solution only. Upward water movement in the xylem
is still possible since water potential in the leaves is lower than that of
seawater (Scholander, 1968, p. 260). This situation leads to high tension
of the water columns in the xylem vessels. The low water potential of
leaves is the result of salt accumulation in the vacuoles of mesophyll cells.
The high salt concentration in the leaves develops soon after seeds germinate. The viviparous seedlings germinating on the mother plant have
a very high osmotic potential (e.g., — 13 to— 18 atm for Rhizophora
mucronata) (Walter and Steiner, 1936, p. 168) and a low NaCl concentration. The mother plant supplies the seedlings with water by the glandular
action of the cotyledonar body. As soon as the seedlings separate from
the mother plant and root in the seawater-saturated mud, they accumulate
salt very fast. This accumulation indicates an appreciable salt permeability
of the root at this stage of development. Soon the leaves of seedlings reach
the same low osmotic potential and the high chloride fraction of expressed
leaf sap as the mother plants. When this stage is reached the root system
becomes highly impermeable for salts, since only small quantities of salts
are taken up to supplement the amount of salt needed to maintain a constant salt concentration while the total volume of the vacuome increases
in the growing plant.
The nondesalting mangrove regulates the salt concentration of cell sap
of leaf cells by (1) salt translocation from senescing leaves into young
leaf primordia, (2) increase of leaf succulence with age of the leaf (which
counteracts to some extent a too high salt concentration, since succulence
involves water uptake), and (3) removal of excess salt from the plant
by abscission of senescent salt-containing leaves.
The regulation of salt content in desalting halophytes can be visualized
to be less complicated than in nondesalting species. The roots of desalting
halophytes generally absorb more salts than the roots of the nondesalting
species.
Mechanisms similar to those of the mangrove for regulation of the salt
content in the plant seem to operate in the halophytes of arid zones, where
desalting and nondesalting species also are found. The seeds of halophytes
of different species have a low salt content (Schratz, 1936, p. 186; Joshi,
1971, p. 8) and the seedling roots take up salts for a limited time period
after germination only. Later only small amounts of salts are taken up and
the xylem sap has a relatively low salt concentration.
Steiner (1934, p. 185ff) distinguishes three types of halophytes with
regard to regulation of their salt concentration: (1) regulation by increase