Liquid Water in Organisms and their Environment
(water potential of the leaf equal to zero), the pressure would be almost
twice this value. These are typical values for plant leaves and illustrate
the amazingly high pressures that routinely exist in living systems.
4.2 Water Potentials in Organisms and their
Surroundings
It is useful for future computations to have some feeling for the range of
water potentials that exist in organisms and their environment. Human
blood has an osmotic potential around -700 Jkg. Fresh sweat is about
half this concentration and urine is two to three times as concentrated as
blood. Osmotic potentials of blood and other body fluids of most mammals are similar to these. The osmotic potentials of cell sap in plant leaves
ranges from -500 to -7000 Jkg. Typical values for mesophytic species
are in the range - 1000 to -2000 Jlkg. The water potential of leaves approaches that of the soil at night when transpiration rates are very low. If
the soil is wet, the maximum leaf water potential is near zero. In the day,
with high transpiration rates, the turgor pressure is close to zero, and the
leaf water potential is about equal to the osmotic potential. The variation
in leaf water potential, for a plant growing in wet soil, may therefore vary
from - 100 to -2000 J k g over a diurnal cycle.
When soils are saturated, their water potential is near zero, but gravity
quickly drains them to potentials between -10 and -30 Jlkg. The water
content corresponding to this water potential is called field capacity. It
is an approximate, but useful upper limit for available water in soil. As
plants extract water from the soil, the water potential decreases until all
remaining water is so tightly held that root water potentials cannot drop
low enough to withdraw additional water. The water content below which
minimal water extraction by plant roots occurs is called the permanent
wilting point, and it corresponds roughly to the water content when soil
has a water potential of - 1500 Jkg. Again, this point is not exact, but sets
a useful lower limit for water available to the plant. Soil near the surface
is further dried by the air, and may reach potentials of -3 x lo5 Jkg, but
this drying only affects the top few decimeters of soil. The remainder of
the soil profile is not likely to dry below about -2000 Jkg.
At the lower limit of water potential for living systems, some fungi are
able to live at water potentials in the range -50 to -70 kJ/kg, and there
are reports of both plants and insects taking up water from environments
which are this dry (though their internal water potentials are probably
much higher). These, however, are very unusual situations.
With these water potentials in mind, we now consider the effect of
water potential on the vapor pressure at the liquid-vapor interface.
4.3 Relation of Liquid- to Gas-Phase Water
For every computation of evaporation rate the vapor concentration at the
evaporating surface needs to be known. This surface is the interface be-
(water potential of the leaf equal to zero), the pressure would be almost
twice this value. These are typical values for plant leaves and illustrate
the amazingly high pressures that routinely exist in living systems.
4.2 Water Potentials in Organisms and their
Surroundings
It is useful for future computations to have some feeling for the range of
water potentials that exist in organisms and their environment. Human
blood has an osmotic potential around -700 Jkg. Fresh sweat is about
half this concentration and urine is two to three times as concentrated as
blood. Osmotic potentials of blood and other body fluids of most mammals are similar to these. The osmotic potentials of cell sap in plant leaves
ranges from -500 to -7000 Jkg. Typical values for mesophytic species
are in the range - 1000 to -2000 Jlkg. The water potential of leaves approaches that of the soil at night when transpiration rates are very low. If
the soil is wet, the maximum leaf water potential is near zero. In the day,
with high transpiration rates, the turgor pressure is close to zero, and the
leaf water potential is about equal to the osmotic potential. The variation
in leaf water potential, for a plant growing in wet soil, may therefore vary
from - 100 to -2000 J k g over a diurnal cycle.
When soils are saturated, their water potential is near zero, but gravity
quickly drains them to potentials between -10 and -30 Jlkg. The water
content corresponding to this water potential is called field capacity. It
is an approximate, but useful upper limit for available water in soil. As
plants extract water from the soil, the water potential decreases until all
remaining water is so tightly held that root water potentials cannot drop
low enough to withdraw additional water. The water content below which
minimal water extraction by plant roots occurs is called the permanent
wilting point, and it corresponds roughly to the water content when soil
has a water potential of - 1500 Jkg. Again, this point is not exact, but sets
a useful lower limit for water available to the plant. Soil near the surface
is further dried by the air, and may reach potentials of -3 x lo5 Jkg, but
this drying only affects the top few decimeters of soil. The remainder of
the soil profile is not likely to dry below about -2000 Jkg.
At the lower limit of water potential for living systems, some fungi are
able to live at water potentials in the range -50 to -70 kJ/kg, and there
are reports of both plants and insects taking up water from environments
which are this dry (though their internal water potentials are probably
much higher). These, however, are very unusual situations.
With these water potentials in mind, we now consider the effect of
water potential on the vapor pressure at the liquid-vapor interface.
4.3 Relation of Liquid- to Gas-Phase Water
For every computation of evaporation rate the vapor concentration at the
evaporating surface needs to be known. This surface is the interface be-