V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
233
hydric, including all gymnosperms, almost all angiosperms, and most of
pteridophytes. In homoiohydric plants the hydrature of the protoplasm depends on the hydrature of the vacuome, i.e., the cell sap concentration
(for further discussion, see Walter and Stadelmann, 1968).
With optimum water supply, the cells of a particular tissue or organ have
an osmotic potential (φ 8 ) which may be called optimal (ψ 8 opt ). This value
is characteristic of a certain tissue of a species. It has been shown that the
same species in America and in Europe has the same ψ 8 0P t for cells of the
same organ (Walter, 1960, p. 225).
Normally, leaf samples collected in the field will not show the optimum
value, but a somewhat lower one, since an optimal water supply is relatively seldom realized in nature. Values higher than φ 8 opt can be found
only in immature organs or in plants in deep shade where the C 0 2 balance
is temporarily negative. Under high water stress φ 8 can be very much
lower than the optimal value. The lowest value of φ 8 at which the first
visible tissue damage from drying may be directly seen is called the minimum osmotic potential (ψ 8 min ). An example of the range of the variation
of the osmotic potential is given in Fig. 11 for leaves of Artemisia tridentata
calculated from the results of Harris (1934). Unfortunately, the author
did not specify the growth stage of the plant and the actual environmental
conditions. Therefore only statistical evaluation is possible. There are
208 individual determinations of ψ 8 . The values range from —13.8 to
—72.9 atm.
The histogram is asymmetric, and most values of ψ 8 are in the range
of —25 to —29.9 atm. Values over —15 atm and under —40 atm are few.
Since the optimum value is only seldom reached under natural conditions,
it must be sought near —14 to —15 atm. The highest ψ 8 values recorded
50
Ξ 40
o
>
^ r 30
1 M
E 10
z
0
-10 -15 -20 -25 -30 -35 -40 -45 -50 -55 -60 -65 -70 -75
Osmotic potential ψ5 (atm)
Fig. 11. Osmotic potential of leaves of Artemisia tridentata calculated from 208
samples. Abscissa: ranges of osmotic potential φ 8 in atm. Ordinate: number of
values of φ 8 found in the particular range. (Width of the range 5 atm. First range
from - 7 . 5 to -14.9 atm.) (After Walter, 1936, p. 196, 1960, p. 238.)
233
hydric, including all gymnosperms, almost all angiosperms, and most of
pteridophytes. In homoiohydric plants the hydrature of the protoplasm depends on the hydrature of the vacuome, i.e., the cell sap concentration
(for further discussion, see Walter and Stadelmann, 1968).
With optimum water supply, the cells of a particular tissue or organ have
an osmotic potential (φ 8 ) which may be called optimal (ψ 8 opt ). This value
is characteristic of a certain tissue of a species. It has been shown that the
same species in America and in Europe has the same ψ 8 0P t for cells of the
same organ (Walter, 1960, p. 225).
Normally, leaf samples collected in the field will not show the optimum
value, but a somewhat lower one, since an optimal water supply is relatively seldom realized in nature. Values higher than φ 8 opt can be found
only in immature organs or in plants in deep shade where the C 0 2 balance
is temporarily negative. Under high water stress φ 8 can be very much
lower than the optimal value. The lowest value of φ 8 at which the first
visible tissue damage from drying may be directly seen is called the minimum osmotic potential (ψ 8 min ). An example of the range of the variation
of the osmotic potential is given in Fig. 11 for leaves of Artemisia tridentata
calculated from the results of Harris (1934). Unfortunately, the author
did not specify the growth stage of the plant and the actual environmental
conditions. Therefore only statistical evaluation is possible. There are
208 individual determinations of ψ 8 . The values range from —13.8 to
—72.9 atm.
The histogram is asymmetric, and most values of ψ 8 are in the range
of —25 to —29.9 atm. Values over —15 atm and under —40 atm are few.
Since the optimum value is only seldom reached under natural conditions,
it must be sought near —14 to —15 atm. The highest ψ 8 values recorded
50
Ξ 40
o
>
^ r 30
1 M
E 10
z
0
-10 -15 -20 -25 -30 -35 -40 -45 -50 -55 -60 -65 -70 -75
Osmotic potential ψ5 (atm)
Fig. 11. Osmotic potential of leaves of Artemisia tridentata calculated from 208
samples. Abscissa: ranges of osmotic potential φ 8 in atm. Ordinate: number of
values of φ 8 found in the particular range. (Width of the range 5 atm. First range
from - 7 . 5 to -14.9 atm.) (After Walter, 1936, p. 196, 1960, p. 238.)