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Water Absorption by Plants

Water absorption

Uptake of water by plants more specifically by plant root hair is called water absorption.

  • Main organ for absorbing water is root hair.
  • The absorbed water crosses a variety of cells such as cortical cells, endodermis (pass cells), pericycle cells and xylem tubes to reach the leaves.

  • Passage cells being present in endodermis permits water to pass.
  • Except xylem, all are living cells.
  • Dry wood is a dead cell and there is no osmotic potential. So, it absorbs water by imbibition. Dry wood is similar to cell wall.
  • Casperian strip is made up of suberin or wax.

Mechanism of absorption of water

In higher plants, water is absorbed through root hairs which are in contact with soil water and form a root hair zone a little behind the root tips. Root hairs are tubular hair like prolongations of the cells of the epidermal layer of the roots.

  • The walls of the root hairs are permeable and consists of pectic substances and cellulose which are strongly hydrophilic in nature.
  • Root hairs contain vacuoles with cell sap.
  • When root hairs elongate, the older hairs die and new root hairs are developed so that they are in contact with fresh supplies of water in the soil.

Mechanism of water absorption is of two types:

  • Active absorption
  • Passive absorption

Active absorption

In this process the root cells play active role in the absorption of water and metabolic energy releases through respiration is consumed.

  • Need ATP.
  • Root hairs play active role.
  • When water is absorbed by roots by their own effort, it is called active absorption.
  • It takes place when transpiration is low and the quantity of water in the soil is high.
  • The force for water absorption comes from the cells of root due to respiration.
  • 4% of total absorption is carried out by the active absorption.
  • Active absorption involves symplastic movement of water in root hairs.
  • The osmotic process involves osmotic/diffusion/water potential of root cells (Ψπ).
  • Evidences in support of active absorption are root pressure, bleeding and guttation (in hydathode).
  • It is not the main method of water absorption.

(Figure)

Active absorption may be of two types:

1. Active osmotic absorption

When water is absorbed from the soil into the xylem of the roots according to the osmotic gradient.

First step in the active absorption of water ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

2. Active non-osmotic absorption

When water is absorbed from the soil into the xylem of the roots against the osmotic gradient.

  • Abnormal condition.
  • Require expenditure of metabolic energy probably through respiration.

Following evidences support this view:

  • The factors which inhibit respiration also decrease water absorption. (No respiration …………………………………..)
  • Poisons which retard metabolic activities of the root cells also retard water absorption.
  • Auxins (growth hormones) which increases metabolic activities of the cells stimulate absorption of water. (Auxin given……………………………………..)

Passive absorption

  • Without utilisation of metabolic energy.
  • Root hairs play passive role. Thus called absorption ’through roots’ rather than ‘by roots’.
  • Occurs when rate of transpiration is usually high because of opening and closing of stomata.
  • Driving force for water absorption is transpiration.
  • In passive absorption, water moves probably through the free spaces or apoplast (cell wall + intercellular space) of root.
  • Passive absorption occurs due to tension created in the xylem cells of the leaves by rapid transpiration or evaporation. This tension is transmitted to the xylem of roots through the xylem of stem and water rises upward to reach the transpiring surface. As a result, soil water enters into cortical cells through root hairs.
  • Evidence in support of passive absorption can be given by

Cutting the roots under water.

The absorption of water still occurs if all the roots are removed.

  • Figure

 

Proof of active absorption

Proof of passive absorption

During absorption of water by roots, the flow of water from epidermis to endodermis may take place through two different pathways.

  1. Symplastic (Sym means cytoplasm)
  • Through plasmodesmata.
  • No hindrance.
  1. Apoplastic (Apo means without)
  • Through cell wall and intercellular space.
  • Hindrance is present which is casperian strip.
  • Water passes from cell wall of one cell to the cell wall of another.

From apopastic pathway, water may enter into symplastic root and thus water passes is regulated i.e. less water passes.

External factors affecting absorption of water

1. Available soil water

2. Concentration of the soil solution

 

3. Soil air (O2)

  • Absorption of water is retarded in poorly aerated soils because in such soils the deficiency of O2 and consequently the accumulation of CO2 will retard the metabolic activities of the roots like respiration. This also inhibits rapid  ……………………………
  • Water logged  ……………………………………….

4. Soil temperature

There are three kinds of water

  • Hygroscopic water
  • Capillary water – only this water is taken up by root hairs.
  • Gravitational water

Passage cells

Osmotic diffusion of water into endodermis takes place through special thin walled passage cells because the other endodermal cells have casperian strips in their walls which are impermious to water.

Stele is the vascular region.

About Abulais Shomrat

Abulais Shomrat
Currently in 4th year (Hons) in Department of Botany, University of Dhaka. Planning to have multiple careers one by one but promised to be with 'Plantlet' as long as it's primary stage remains unfinished.

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