234
H. WALTER AND E. STADELMANN
were probably found in immature shoots. Frequently, samples might have
been taken during suboptimal water conditions. The extremely low values
may result from samples taken from plants under high water stress.
The difference ψ 8 — ψ 8 opt indicates the condition of the actual water
supply of a plant. It measures the deviation of the actual value of the
relative activity of water in the living protoplasm from its optimum value.
Usually the physical aspects of plant water relations, the water absorption, water movement, and the transpiration, have been emphasized, and
studies of water activity of protoplasm have been neglected (cf. Gates,
1964, p. 4ff; Stadelmann, 1971, p. 341); however, the protoplasm is the
site of all metabolic activity on which growth and development of the plant
depend, and protoplasmic hydration controls to a great extent metabolic
activity and therefore plant growth and production. This ecophysiological
aspect of plant-water relations has been almost totally neglected and is
hence especially emphasized here.
The osmotic potential of the cell sap is a measure for the hydrature of
protoplasm and, therefore, the osmotic potential (ψ 8 ) and its variations will
indicate the changes occurring in the protoplasmic hydration.
VIII. Diurnal Fluctuation of the Osmotic Potential (ψ 8 )
The osmotic potential reaches the value which is characteristic for a
particular plant species only under optimum environmental conditions.
Sometimes considerable differences also exist in the ψ 8 value of the different
plant organs and between different parts of the plant organs (Table II).
As soon as leaf transpiration exceeds water replacement from the roots,
the plant dehydrates. Concurrently, the concentration of the cell sap
increases with resulting decreasing osmotic potential and hydrature of protoplasm. Since transpiration increases faster than water uptake and conduction during the first part of each day, the osmotic potential exhibits diurnal
fluctuations. On sunny days the ψ 8 values of leaves are maximal at sunrise
and minimal during the afternoon. The amplitude depends upon the water
supply and the intensity of transpiration (see example in Table III). The
diurnal fluctuations in leaves are primarily caused by the midday decrease
of the water content and subsequent increased cell sap concentration;
hence the product (osmotic potential X water content) does not change
significantly. Only during the afternoon a small accumulation of sugar may
temporarily cause a very slight increase of cell sap concentration (Pisek
and Cartillieri, 1932, p. 225; Biebl, 1962, p. 128ff). The amplitude of the
diurnal change of ψ 8 varies even among species in the same habitat, since
plants differ in root formation, water-conducting system, transpiring sur-
H. WALTER AND E. STADELMANN
were probably found in immature shoots. Frequently, samples might have
been taken during suboptimal water conditions. The extremely low values
may result from samples taken from plants under high water stress.
The difference ψ 8 — ψ 8 opt indicates the condition of the actual water
supply of a plant. It measures the deviation of the actual value of the
relative activity of water in the living protoplasm from its optimum value.
Usually the physical aspects of plant water relations, the water absorption, water movement, and the transpiration, have been emphasized, and
studies of water activity of protoplasm have been neglected (cf. Gates,
1964, p. 4ff; Stadelmann, 1971, p. 341); however, the protoplasm is the
site of all metabolic activity on which growth and development of the plant
depend, and protoplasmic hydration controls to a great extent metabolic
activity and therefore plant growth and production. This ecophysiological
aspect of plant-water relations has been almost totally neglected and is
hence especially emphasized here.
The osmotic potential of the cell sap is a measure for the hydrature of
protoplasm and, therefore, the osmotic potential (ψ 8 ) and its variations will
indicate the changes occurring in the protoplasmic hydration.
VIII. Diurnal Fluctuation of the Osmotic Potential (ψ 8 )
The osmotic potential reaches the value which is characteristic for a
particular plant species only under optimum environmental conditions.
Sometimes considerable differences also exist in the ψ 8 value of the different
plant organs and between different parts of the plant organs (Table II).
As soon as leaf transpiration exceeds water replacement from the roots,
the plant dehydrates. Concurrently, the concentration of the cell sap
increases with resulting decreasing osmotic potential and hydrature of protoplasm. Since transpiration increases faster than water uptake and conduction during the first part of each day, the osmotic potential exhibits diurnal
fluctuations. On sunny days the ψ 8 values of leaves are maximal at sunrise
and minimal during the afternoon. The amplitude depends upon the water
supply and the intensity of transpiration (see example in Table III). The
diurnal fluctuations in leaves are primarily caused by the midday decrease
of the water content and subsequent increased cell sap concentration;
hence the product (osmotic potential X water content) does not change
significantly. Only during the afternoon a small accumulation of sugar may
temporarily cause a very slight increase of cell sap concentration (Pisek
and Cartillieri, 1932, p. 225; Biebl, 1962, p. 128ff). The amplitude of the
diurnal change of ψ 8 varies even among species in the same habitat, since
plants differ in root formation, water-conducting system, transpiring sur-