V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
283
a particular ion species [e.g., cap plasmolysis no longer develops when the
plasmolyzing solution is made up of 1 part of an 0.39 M CaCl 2 solution
and 8 parts of an 0.6 M KN0 3 solution (Höfler, 1928, p. 80)]. An equilibrated salt solution with a minimum of harmful effects for plant cells is
the Brenner mixture (Brenner, 1920, p. 284): 100 gm H 2 0, 1.82 gm
NaCl, 0.06 gm KC1, 0.47 gm MgCL · 6H 2 0, 0.28 gm MgS0 4 · 7H 2 0,
1.6gmCaCl 2 -6H 2 0.
Except for saline soils, the salt concentration in the soil of natural habitats of higher plants is always very low and other than the essential ions
taken up for incorporation into membranes, enzymes, and other macromolecules, only small amounts of nonessential salts are usually taken up
as ballast elements (see Levitt, 1969, p. 132). Therefore, salt concentration
of such soils cannot significantly affect colloidal qualities and metabolic
activity of protoplasm. The hydrature and the water balance remain the
major factors determining protoplasmic imbibition.
However, when saline soils are considered, the effect of the ions absorbed by the protoplasm must be considered, since the salts (primarily
NaCl) will be taken up in considerable amounts as ballast materials. Since
NaCl is highly soluble, it cannot be precipitated inside the cell. It accumulates in many cell elements and in the vacuome of the transpiring organs
especially (see Ziegler and Lüttge, 1967, p. 14). Only a few plant species
have developed salt glands, which excrete (recrete) the unused salts.
The most frequently found salt in arid zones is NaCl. However, Na 2 S0 4
and MgS0 4 are very common and sometimes may predominate (e.g., in
soils around the lake of Neusiedel, Austria; Walter, 1936, p. 185). To
decide whether the effect of NaCl on protoplasm is caused mainly by the
Na ion or the Cl ion, their specific effects have to be considered. A high
degree of absorption of NaCl may disturb the cation equilibrium in the
cell. The plant does not take up Na ions and Cl ions in equivalent amounts,
but frequently replaces the Na ion partially with potassium ions. In contrast to animals, plants have a high requirement for the potassium ion
whereas sodium is generally not essential (p. 281 ). However, under certain
conditions some glycophytes may be able to replace potassium partially
by sodium (see Stiles, 1958, p. 600).
The Na ion concentration in the cell sap of halophytes usually is considerably higher than of glycophytes (see the analysis of salt marsh plants,
Steiner, 1934, p. 152). However, in even low concentrations, other cations
(K
+
, Ca
2+
, Mg
2+
) can nullify possible damaging effects of an excess Na ion
content by their antagonistic action on the protoplasm. Such action of the
K
+
ion is considered by Joshi (1973, p. 139; 1971, p. 25) for the protoplasmic salt resistance of halophytes. Atriplex halimus plants synthesize
considerably more dry matter in a one to one mixture of KC1 and NaCl
283
a particular ion species [e.g., cap plasmolysis no longer develops when the
plasmolyzing solution is made up of 1 part of an 0.39 M CaCl 2 solution
and 8 parts of an 0.6 M KN0 3 solution (Höfler, 1928, p. 80)]. An equilibrated salt solution with a minimum of harmful effects for plant cells is
the Brenner mixture (Brenner, 1920, p. 284): 100 gm H 2 0, 1.82 gm
NaCl, 0.06 gm KC1, 0.47 gm MgCL · 6H 2 0, 0.28 gm MgS0 4 · 7H 2 0,
1.6gmCaCl 2 -6H 2 0.
Except for saline soils, the salt concentration in the soil of natural habitats of higher plants is always very low and other than the essential ions
taken up for incorporation into membranes, enzymes, and other macromolecules, only small amounts of nonessential salts are usually taken up
as ballast elements (see Levitt, 1969, p. 132). Therefore, salt concentration
of such soils cannot significantly affect colloidal qualities and metabolic
activity of protoplasm. The hydrature and the water balance remain the
major factors determining protoplasmic imbibition.
However, when saline soils are considered, the effect of the ions absorbed by the protoplasm must be considered, since the salts (primarily
NaCl) will be taken up in considerable amounts as ballast materials. Since
NaCl is highly soluble, it cannot be precipitated inside the cell. It accumulates in many cell elements and in the vacuome of the transpiring organs
especially (see Ziegler and Lüttge, 1967, p. 14). Only a few plant species
have developed salt glands, which excrete (recrete) the unused salts.
The most frequently found salt in arid zones is NaCl. However, Na 2 S0 4
and MgS0 4 are very common and sometimes may predominate (e.g., in
soils around the lake of Neusiedel, Austria; Walter, 1936, p. 185). To
decide whether the effect of NaCl on protoplasm is caused mainly by the
Na ion or the Cl ion, their specific effects have to be considered. A high
degree of absorption of NaCl may disturb the cation equilibrium in the
cell. The plant does not take up Na ions and Cl ions in equivalent amounts,
but frequently replaces the Na ion partially with potassium ions. In contrast to animals, plants have a high requirement for the potassium ion
whereas sodium is generally not essential (p. 281 ). However, under certain
conditions some glycophytes may be able to replace potassium partially
by sodium (see Stiles, 1958, p. 600).
The Na ion concentration in the cell sap of halophytes usually is considerably higher than of glycophytes (see the analysis of salt marsh plants,
Steiner, 1934, p. 152). However, in even low concentrations, other cations
(K
+
, Ca
2+
, Mg
2+
) can nullify possible damaging effects of an excess Na ion
content by their antagonistic action on the protoplasm. Such action of the
K
+
ion is considered by Joshi (1973, p. 139; 1971, p. 25) for the protoplasmic salt resistance of halophytes. Atriplex halimus plants synthesize
considerably more dry matter in a one to one mixture of KC1 and NaCl