V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
281
was considerably slower. On the other hand, when osmotic potential (NaCl
solution with a much lower osmotic potential) was applied, the seeds most
probably compensated the increase in water stress by uptake of salts and
germination was not impeded. Sodium chloride (0.5 osmolal) was also
found to have toxic effects on germination of Phaseolus vulgar is (Prisco
and O'Leary, 1970, p. 182). Thus, with regard to seed germination there
is no summation of the effect of matric and osmotic potentials. Correspondingly, seeds of Suaeda depressa, S. linearis, Salicornia europaea,
and Spergularia marina showed relatively high germination percentage up
to a salt content of 1-2% of the germination medium. At higher salt percentage germination decreased sharply, caused by osmotic action (Ungar,
1962, p. 764).
The problem of halophytes is primarily one of salt balance and not
water balance, since the salts which are absorbed by plants have specific
effects on protoplasm and may cause damage when the protoplasm is not
salt-resistant (Repp, 1958, p. 556.)
Therefore, not only the external conditions of the plants have to be considered for the halophyte problem but especially the conditions inside the
cells which are decisive for normal functioning of living protoplasm. It
is not sufficient to know the salt concentration in the soil or the nutrient
solution; rather the salt concentration in the cell sap should be determined.
However, salts in the soil may have indirect effects on the water balance
of plants by changing soil structure. For instance, the heavily soluble sodium salts caused a considerable decrease in porosity of soil and an unfavorable change of its structure. These effects of salts on soils are of great
importance for agricultural use but are beyond the scope of this chapter.
XIX. The Effects of Ions on Protoplasm
The salts which are absorbed by halophytes accumulate particularly in
transpiring organs. The presence of salt in cell vacuoles of these organs
causes a decrease of osmotic potential. It can be assumed that some kind
of equilibrium will be established between ion concentration in the vacuole
and in the cytoplasm. Therefore, the decrease in the osmotic potential of
the vacuoles in those cells will not affect the hydration of the protoplasm
in the same way as will a decrease of osmotic potential from water
stress. Such conditions cause a decrease in hydrature and dehydration of
protoplasm.
The ions have to pass through the plasmalemma, mesoplasm, and
tonoplast in order to accumulate in the vacuole. The passive permeability
of the plasmalemma and tonoplast for ions is very low (Stadelmann, 1969,
281
was considerably slower. On the other hand, when osmotic potential (NaCl
solution with a much lower osmotic potential) was applied, the seeds most
probably compensated the increase in water stress by uptake of salts and
germination was not impeded. Sodium chloride (0.5 osmolal) was also
found to have toxic effects on germination of Phaseolus vulgar is (Prisco
and O'Leary, 1970, p. 182). Thus, with regard to seed germination there
is no summation of the effect of matric and osmotic potentials. Correspondingly, seeds of Suaeda depressa, S. linearis, Salicornia europaea,
and Spergularia marina showed relatively high germination percentage up
to a salt content of 1-2% of the germination medium. At higher salt percentage germination decreased sharply, caused by osmotic action (Ungar,
1962, p. 764).
The problem of halophytes is primarily one of salt balance and not
water balance, since the salts which are absorbed by plants have specific
effects on protoplasm and may cause damage when the protoplasm is not
salt-resistant (Repp, 1958, p. 556.)
Therefore, not only the external conditions of the plants have to be considered for the halophyte problem but especially the conditions inside the
cells which are decisive for normal functioning of living protoplasm. It
is not sufficient to know the salt concentration in the soil or the nutrient
solution; rather the salt concentration in the cell sap should be determined.
However, salts in the soil may have indirect effects on the water balance
of plants by changing soil structure. For instance, the heavily soluble sodium salts caused a considerable decrease in porosity of soil and an unfavorable change of its structure. These effects of salts on soils are of great
importance for agricultural use but are beyond the scope of this chapter.
XIX. The Effects of Ions on Protoplasm
The salts which are absorbed by halophytes accumulate particularly in
transpiring organs. The presence of salt in cell vacuoles of these organs
causes a decrease of osmotic potential. It can be assumed that some kind
of equilibrium will be established between ion concentration in the vacuole
and in the cytoplasm. Therefore, the decrease in the osmotic potential of
the vacuoles in those cells will not affect the hydration of the protoplasm
in the same way as will a decrease of osmotic potential from water
stress. Such conditions cause a decrease in hydrature and dehydration of
protoplasm.
The ions have to pass through the plasmalemma, mesoplasm, and
tonoplast in order to accumulate in the vacuole. The passive permeability
of the plasmalemma and tonoplast for ions is very low (Stadelmann, 1969,