Components Of Plant Water Relations

The flow of water from the soil to the leaves not only cools the canopy through transpiration but also supplies water to all living cells in the plant. Cells have direct contact with films of water from the xylem. The water films have a measurable "tension" that is called the water potential (0w), and this tension is developed when the transport of water does not meet the environmental demand for evaporation.

Cells are bathed in water at a 0w that ranges from approximately zero under well-watered conditions and no water stress to a negative pressure (less than zero) when water stress is developing. The cell, however, must maintain a positive pressure, or turgor, for expansion and normal biochemical function. Cell turgor pressure (0p) is the result of three primary factors:

1. Solute concentration, or solute potential (0s): The solutes in the cell draw water into the cell via osmosis. The 0s of the cell is less than zero, and the more negative its value, the greater is the potential influx of water.

2. Effect ofwater-binding colloids and capillary attraction for water, or matric potential (0m): Water is held by electrostatic forces to charged surfaces in the cell, such as proteins and nucleic acids, and the capillary channels within the cell wall also bind water. The 0m in the cell is generally of little significance in maintaining turgor because the volume of water related to it is very small.

3. Effect of gravity (0g): This effect is generally negligible except when comparing water potentials at different heights in a tree.

Cell water relations can be expressed algebraically as:

Assuming that matric and gravitational potentials are negligible and cell volume does not change, the following illustrates the relationships between the components of cell water:

Condition of the cell

I w

0

+

0

Units

No water stress

and the cells are fully turgid

0 =

-2

+

2

MPa

Moderate water stress

and the cells are partly turgid

-1 =

-2

+

1

MPa

Severe water stress

and the cells are flaccid

-2 =

-2

+

0

MPa

Severe water stress

and the cells are partly turgid

-2 =

-3

+

1

MPa

In this example, if the concentration of solutes in the cytoplasm (0s) increases, then turgor pressure will increase, at any constant negative 0w . This is a common adaptation in plants called osmoregulation that is one of many ways plants adapt to their environment. Water stress and plant water relations are very complex phenomena, and this brief explanation only highlights some general trends on a whole-plant basis.

Yield and plant growth are reduced more by water deficits than by any other limiting factor in a plant's environment. Daily water deficits that occur during hot periods of the day, as well as seasonal deficits of water, alter a plant's morphology, physiology, productivity, and quality as a food product. Water deficits also increase a plant's susceptibility to insect and disease damage. An understanding of plant water relations aids in diagnosing conditions that limit plant growth and development. Plant breeders utilize knowledge of how plants morphologically, biochemically, and physiologically adjust to water stress in order to adapt new cultivars to their environment.

Related Topics: CARBOHYDRATE PARTITIONING AND PLANT GROWTH; IRRIGATION; TEMPERATURE RELATIONS

SELECTED BIBLIOGRAPHY

Faust, M. (1989). Physiology of temperate zone fruit trees. New York: John Wiley and Sons.

Jones, H. G. (1992). Plants and microclimate, Second edition. Cambridge, UK:

Cambridge Univ. Press. Kramer, P. J. and J. S. Boyer (1995). Water relations ofplants and soils. New York: Academic Press.

Nobel, P. S. (1991). Physiochemical and environmental plant physiology. New York: Academic Press.

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