Water Potential

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It is difference in the free energy or chemical potential of water molecules in the solution and that of pure water at the same temperature and pressure. It is indicated by the Greek latter (PSI). The chemical potential of pure water at normal temperature and pressure is zero. In solutions, its value is always negative, i.e., less than zero. The water potential is always measured in bars (a unit of pressure); a bar is a pressure unit, which equals to The concept of water potential was proposed by Slatyer and Taylor. According to this concept water potential of protoplasm of the cell is equal but opposite in sign to the diffusion pressure deficit (DPD) or suction pressure (SP). Thus the water potential of a solution can be determined with the help of pure or distilled water. The water potential of pure water at atmospheric pressure is zero. Presence of solute particles induces the free energy of water thereby decreasing its water potential (negative value). Therefore, the water potential of a solution is always negative, i.e. less than zero. Water potential determines the water status in plant cells and tissues. The lower the water potential in a plant cell or tissue, the greater is its ability to absorb water. Conversely, the higher the water potential, the greater is the ability of the tissue to supply water to other more desiccated cell or tissues.

Turgor Pressure

If a plant cell is immersed in water, it swells due to absorption of water. As a result of entry of water into the cell sap, a pressure is developed in the protoplasm which presses against the cell wall. The actual pressure exerted by the protoplasm against the cell wall is the turgor pressure (TP) which is always less than osmotic pressure (OP) unless the cell is distilled water. Cell wall is rigid and elastic, therefore, it exerts an equal and opposite pressure against the expanding protoplasm. This pressure is called wall pressure (WP). At a given time TP becomes equal to , or

A plant cell with the vacuole pushing out on the cell wall is said to be turgid and the vacuole exerts turgor pressure on the cell wall. Turgor pressure develops in plant cells only because of the presence of cell wall. Turgidity plays an important role in the plants;

  • The turgor pressure helps in maintaining the shape and form of the plant,

  • The stems of herbaceous plants and the ones with non-woody tissue like maize, sugarcane and banana are held straight by fully turgid cells packed tightly together,

  • Turgor pressure holds the leaves in flat and horizontal position,

  • Turgor pressure helps in cell enlargement and consequently in stretching of the stems,

  • Opening and closing of stomata is governed by turgidity of the guard cells,

  • Certain plants like bean and touch me not plant (Mimosa pudica) show quick response of leaves by controlling the turgidity.

Image showing a turgid cell showing osmotic pressure, turgor pressure and wall pressure.

Turgid Cell Showing Pressures

Image showing a turgid cell showing osmotic pressure, turgor pressure and wall pressure.

Availability of Water in the Soil

The plants absorb water through the roots hairs from the soil. The soil contains water in three forms;

Gravitational water: when water enters the soil from the surface either by irrigation or rain, it passes through the spaces between the soil particles reaches the water table. This is called gravitational water. It is not available to plants because it lies far below the reach of the roots. Rains may provide water temporarily for absorption by plants. When rain water remains along the root for a long time, it is called logging. Gravitational water causes washing out of minerals and nutrients from the soil called leaching.

Hygroscopic Water: The soil particles are surrounded by a film of water called the hygroscopic water. This water is not available to the plant as it is not free but remains around the soil particles due to hygroscopic action. In the clay soils, it amounts to about and in the sandy soils to about .

Capillary water: This is the only water available to the plants. This water is held in between the soil particles by capillary force. It also contains a number of mineral salts in the solution. This form of water is most suited for absorption by roots. This very form of water is available to plants for absorption. Finer the texture of soil, greater the capillary water held. When a soil is watered, it retains good amount of capillary water and thus condition is known as field capacity.

Image showing types of soil water.

Image Showing Types of Soil Water.

Image showing types of soil water.