Absorption Transport and Water Loss in Plants: Absorption of Water by Plants and Root Pressure Theory

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Absorption of Water by Plants

Mainly plants absorbed water by roots but sometimes water may be absorbed by leaves and stems. A root hair cell has a large surface area that is ideal for absorbing nutrients, and its sap has a higher salt concentration than the surrounding soil water. The plasma membrane and the vacuolar membrane (tonoplast) act as semipermeable membranes and water is absorbed by osmosis. Root hairs are delicate tubular prolongations of the epiblema (piliferous layer) and the cell wall acts as permeable membrane which encloses cytoplasm, nucleus and vacuole and vacuole is large in size and contains cell sap. The cell sap is of higher osmotic potential. The movement is purely dependent on water potential gradient. The concept of apoplast and symplast pathways was introduced by Munch in 1980 to explain the flow of water and minerals in plants from root hair through root cortex to the xylem elements. Apoplast comprises of cell wall of all cells excluding casparian strips, whereas symplast is the network of cytoplasm of all the cells interconnected by plasmodesmata. The water absorbed through the roots is transferred radially to the xylem, from where it reaches to all the other plants of the plant by vertical conduction of water through the xylem vessels.

Image showing various pathways of water movement.

Pathways of Water Movement.

Image showing various pathways of water movement.

Image showing water path from root hairs to cortex and xylem vessel.

Water Path from Root Hairs to Cortex and Xylem Vessel.

Image showing water path from root hairs to cortex and xylem vessel.

Conduction of Water through the Xylem

The content of xylem vessels is known as xylem sap. Various theories have been postulated to describe the lifting of the xylem sap or ascent of sap in the xylem.

Root Pressure Theory

If a stem is cut near its base or incisions or boring are made into a plant, xylem sap is seen to flow out through them. The phenomenon is known as exudation or bleeding. Priestly was one of the first to explain the process of upward flow of water in exudation to be due to hydrostatic pressure (root pressure) developed in the roots due to the accumulation of water absorbed by them. Root pressure of magnitude up to 5 atmospheres has been recorded. Root pressure causes the movement of water through the xylem vessels. No root pressure has been observed in Gymnosperms.

Physical Force Theory or Cohesion Theory

According to physical force theories ascent of sap takes place by some physical forces developed in the dead cells of xylem. This theory is based on three important physiological aspects- adhesive force of water, cohesive of water and transpiration pull. Transpiration pull exerted on the water column and Cohesive and adhesive properties of water molecules to form a continuous water column in the xylem. Water forms an unbroken column starting from the intercellular space of the leaf mesophyll to the xylem of the leaf, through stems and roots to the water in the soil. A water potential gradient exists between the leaf to the root and transpiration causes a pull of the entire water column. During transpiration a stress is caused, and water is drawn by the leaves along the xylem of the stem and the root tips in contact of the soil water. It thus, generates a pull or tension called transpiration pull. This tension or pull results upward movement of water.

Image showing demonstration of pull due to evaporation and transpiration.

Pull Due to Evaporation and Transpiration.

Image showing demonstration of pull due to evaporation and transpiration.

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