UNIT 18 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 J/kg. 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 J/kg. Typical values for mesophytic species are in the range - 1000 to -2000 J/kg. 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/kg 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 J/kg. 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 J/kg. 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 10⁵ J/kg, 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 J/kg.

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.

Define the following words and word combinations

Blood, sweat, cell sap, mesophytic species, transpiration, turgor, the osmotic potential, to drain.

Continue the sentences

1. It is useful for future computations…
2. The water potential of leaves…
3. If the soil is wet,…
4. When soils are saturated,…
5. The water content corresponding to this water potential…
6. As plants extract water from the soil,…
7. The water content below which minimal water extraction by plant roots occurs…
8. At the lower limit of water potential…