V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
279
The examples given above show that the chlorides of saline soils directly
or indirectly are always of marine origin. Sea salts have accumulated gradually over the periods of earth history. Most probably the chlorine originates from the volcanic emissions which contain HC1 as well as S0 2
(Dauvillier, 1965, p. 86).
XVIII. Halophytes and Salt Uptake
Halophytes are plants which are able to grow on salt soils. In reviews
of desert plants the halophytes were frequently discussed together with
xerophytes and succulents of nonsaline soils (Ruhland, "Encyclopedia of
Plant Physiology," Volume III, 1956). Such a grouping, however, does not
seem to be adequate from a physiological or ecological point of view and
goes back to Schimper's theory of "physiological dryness" of saline soils.
This hypothesis assumes that the osmotic concentration in saline soils makes
water uptake by the plants more difficult, in a way similar to that of physical dryness of soil in arid regions. Such an assumption would be correct
only when the plants do not absorb salts. However, all halophytes accumulate salts in the cell sap (Steiner, 1934, p. 152), and especially in cells
of transpiring tissues (Walter and Steiner, 1936, p. 163ff). Glycophytes
which grow on saline soils also accumulate salts. The concentration
of salt in the cell sap of leaf cells is mostly as high as that of the soil
solution or even higher. Thus, the osmotic effect of the soil solution is compensated for and absorption of water is not impeded.
There are basic differences in response to salt of the protoplasm of halophytes and glycophytes. The protoplasm of the glycophytes is sensitive to
toxic effects of the salts and the salts soon reach a lethal concentration.
The protoplasm of the halophytes is very salt tolerant. True halophytes
show a furthering of growth in salt solutions (Mozafar et al., 1970, p.
478 for Atriplex halimus) whereas glycophytes or salt-tolerant plants always grow best on nonsaline soils.
The different effects of electrolytes (NaCl, KN0 3 ) and nonelectrolytes
(saccharose, mannitol) in hydroponic culture is shown clearly in experiments with Solarium lycopersicum (Slatyer, 1961). These plants were
grown first in normal nutrient (^ s = —0.7 atm) and later transferred
to NaCl solutions (with
36
C1) or mannitol solutions (with
14
C). Each
solution had an osmotic potential ψ 8 = —10 atm. In both solutions, the
plants after the transfer became strongly wilted, but after 28 hours those
in the NaCl solution were turgid again whereas those in the mannitol
solution were not.
The recovery of plants in the NaCl solution was caused by salt uptake.
279
The examples given above show that the chlorides of saline soils directly
or indirectly are always of marine origin. Sea salts have accumulated gradually over the periods of earth history. Most probably the chlorine originates from the volcanic emissions which contain HC1 as well as S0 2
(Dauvillier, 1965, p. 86).
XVIII. Halophytes and Salt Uptake
Halophytes are plants which are able to grow on salt soils. In reviews
of desert plants the halophytes were frequently discussed together with
xerophytes and succulents of nonsaline soils (Ruhland, "Encyclopedia of
Plant Physiology," Volume III, 1956). Such a grouping, however, does not
seem to be adequate from a physiological or ecological point of view and
goes back to Schimper's theory of "physiological dryness" of saline soils.
This hypothesis assumes that the osmotic concentration in saline soils makes
water uptake by the plants more difficult, in a way similar to that of physical dryness of soil in arid regions. Such an assumption would be correct
only when the plants do not absorb salts. However, all halophytes accumulate salts in the cell sap (Steiner, 1934, p. 152), and especially in cells
of transpiring tissues (Walter and Steiner, 1936, p. 163ff). Glycophytes
which grow on saline soils also accumulate salts. The concentration
of salt in the cell sap of leaf cells is mostly as high as that of the soil
solution or even higher. Thus, the osmotic effect of the soil solution is compensated for and absorption of water is not impeded.
There are basic differences in response to salt of the protoplasm of halophytes and glycophytes. The protoplasm of the glycophytes is sensitive to
toxic effects of the salts and the salts soon reach a lethal concentration.
The protoplasm of the halophytes is very salt tolerant. True halophytes
show a furthering of growth in salt solutions (Mozafar et al., 1970, p.
478 for Atriplex halimus) whereas glycophytes or salt-tolerant plants always grow best on nonsaline soils.
The different effects of electrolytes (NaCl, KN0 3 ) and nonelectrolytes
(saccharose, mannitol) in hydroponic culture is shown clearly in experiments with Solarium lycopersicum (Slatyer, 1961). These plants were
grown first in normal nutrient (^ s = —0.7 atm) and later transferred
to NaCl solutions (with
36
C1) or mannitol solutions (with
14
C). Each
solution had an osmotic potential ψ 8 = —10 atm. In both solutions, the
plants after the transfer became strongly wilted, but after 28 hours those
in the NaCl solution were turgid again whereas those in the mannitol
solution were not.
The recovery of plants in the NaCl solution was caused by salt uptake.