V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
243
where ψ is the water potential, ψ 8 is the osmotic potential, and ψ ρ is the
pressure potential (cf. Knipling, 1969).
The term water potential was first introduced by Stocker (1947, p. 363).
The water potential is highest with pure water and this value is arbitrarily
set at zero. Therefore, the values for the water potential of the cell as well
as those for the osmotic potential are always negative. Thus, when the
absolute value of ψ 8 increases (this absolute value is identical with the
potential osmotic pressure π*) the real value of φ 8 decreases, i.e., becomes
more negative, and vice versa.
The pressure potential for cells with positive turgor is positive.
Since
energy
force
Pressure = —
=
(6)
volume
area
potential values measured in erg/cm
3
are equal to dyn/cm
2
and therefore
can be expressed as pressure in atm or bar (1 atm = 1.013 bar).
From the above the following identities result:
Water potential = — D P D = —suction tension
(7)
Osmotic potential = —OP = —osmotic value
= — potential osmotic pressure (π*)
(8)
Pressure potential = T P = wall pressure or turgor pressure
(absolute value) (9)
This thermodynamic terminology will be followed here. Occasionally osmotic value or potential osmotic pressure will be used instead of the osmotic potential.
The three potentials describe quite different features of the cell water
relations and should not be confused:
1. The water potential (—DPD) is the decisive factor for water movement in the plant.
2. The osmotic potential [—OP = —osmotic value = —potential osmotic pressure (?r*)] is an indicator of hydrature of the protoplasm.
3. The pressure potential [TP = (absolute value of) turgor pressure]
causes stretching of the cell wall and therefore gives the cells of nonlignified tissues and herbaceous organs mechanical strength and turgor.
Routine determination of the water potential frequently is difficult and
time consuming (cf. Ursprung, 1939; Barrs, 1968, p. 263ff), while the
osmotic potential φ 8 can be measured more easily. The pressure potential,
in general, can only be calculated from the difference ψ 8 — ψ, but under
rare circumstances direct measurement is possible (Barrs, 1968, p. 336ff).
243
where ψ is the water potential, ψ 8 is the osmotic potential, and ψ ρ is the
pressure potential (cf. Knipling, 1969).
The term water potential was first introduced by Stocker (1947, p. 363).
The water potential is highest with pure water and this value is arbitrarily
set at zero. Therefore, the values for the water potential of the cell as well
as those for the osmotic potential are always negative. Thus, when the
absolute value of ψ 8 increases (this absolute value is identical with the
potential osmotic pressure π*) the real value of φ 8 decreases, i.e., becomes
more negative, and vice versa.
The pressure potential for cells with positive turgor is positive.
Since
energy
force
Pressure = —
=
(6)
volume
area
potential values measured in erg/cm
3
are equal to dyn/cm
2
and therefore
can be expressed as pressure in atm or bar (1 atm = 1.013 bar).
From the above the following identities result:
Water potential = — D P D = —suction tension
(7)
Osmotic potential = —OP = —osmotic value
= — potential osmotic pressure (π*)
(8)
Pressure potential = T P = wall pressure or turgor pressure
(absolute value) (9)
This thermodynamic terminology will be followed here. Occasionally osmotic value or potential osmotic pressure will be used instead of the osmotic potential.
The three potentials describe quite different features of the cell water
relations and should not be confused:
1. The water potential (—DPD) is the decisive factor for water movement in the plant.
2. The osmotic potential [—OP = —osmotic value = —potential osmotic pressure (?r*)] is an indicator of hydrature of the protoplasm.
3. The pressure potential [TP = (absolute value of) turgor pressure]
causes stretching of the cell wall and therefore gives the cells of nonlignified tissues and herbaceous organs mechanical strength and turgor.
Routine determination of the water potential frequently is difficult and
time consuming (cf. Ursprung, 1939; Barrs, 1968, p. 263ff), while the
osmotic potential φ 8 can be measured more easily. The pressure potential,
in general, can only be calculated from the difference ψ 8 — ψ, but under
rare circumstances direct measurement is possible (Barrs, 1968, p. 336ff).