Monday, 19 January 2015

Water Absorption and Pathway of Water Across the Root

Water absorbing structure of the plant is root hair zone. Root hair is tubular prolongation of epiblema cells. Root hairs are unicellular, short lived and arranged in an acropetal manner. Root hairs are found in zone of cell maturation. During transplantation the root hairs are removed, that is why, the plant remains wilted in the new habitat. The cell wall of the hair is made up of two layers. The outside wall is of pectin which dissolves in water, so that root hair surface becomes slimy and sticky. The inner wall is made up of cellulose. They are about 10 micro m. There OP is higher (3-8 atm) as compared to soil solution (less than 1 atm). Many forest trees, shrubs and some conifers have scantly root hairs so they make association with the fungi, called mycorrhiza. Orchid roots have a specific type of tissue for absorbing environmental moisture; this tissue is called velaman tissue.

Movement of water from root hair cell to xylem may occour by two possible paths :

1. Apoplast pathway: In this method, water passes from root hair cell to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The apoplastic movement of water beyond cortex is blocked due to the presence of casparian strips in the endodermal cells. Major movements of water through cortical cells occur by this method, as cortical cells offer least resistance.

Figure: Pathway of water movement in the root

2. Symplast Pathway: In this method, water passes from cell to cell by crossing plasma membrane; therefore it is also known as transmembrane pathway. This may occur by two methods:

(i) Non Vacuolar Symplast Pathway: In this method, water passes adjacent cells through plasmodesmata. It does not enter into the vacuoles.

(ii)  Vacuolar Symplast Pathway: In this method, water passes the tonoplast, surrounding the vacuole.
 This pathway offers a lot of resistance. Beyond cortex (through endodermis and pericycle) water is forced to move through symplast pathway. Terms Apoplast and Symplast were proposed by "Munch".

Mechanism of Water Absorption

(1) Passive absorption of water: In actively transpiring plants, absorption of water takes place due to the forces developed at the transpiring surface of the plant ( i.e., transpiration pull). In this type, the cells of the root do not play any part, and it does not consumes energy, hence it is known as passive absorption. Thus in passive absorption, water is just pulled through the roots. This is the most common (96%) and rapid method of water absorption. Generally, water is absorbed by the root hairs when the osmotic concentration of their sap is high. This is made possible by transpiration taking place in the aerial parts of the plant. It continuously removes water from the sap of the root hairs which, in turn, are in contact of the soil water.
In actively transpiring plants, water loss from mesophyll cells occurs and increases their osmotic concentration. It also results in the increase of their DPD. As a result, water from neighboring cells enters in them by osmosis. These cells in turn have now increased their osmotic concentration or lowered their water potential. Hence, water enters into them by osmosis from other adjacent cells. In this way mesophyll cells draw water from one another along the suction pressure gradient or DPD till it reaches the xylem of the leaf. Once water is drawn from xylem of the leaf, the entire water column in the xylem of the leaf, stem and the root is lifted. The movement of water is apoplastic. In this way, water is absorbed by the root hair due to illusion pressure deficit gradient produced by transpiration that develops in the leaf. Root simply acts as a path of water.

(2) Active absorption of water: Although a very small amount of water (4 %) is absorbed by active mechanism, it involves an expenditure of metabolic energy which comes from the respiring cells of the root. Roots are actively involved in this method, so it is absorption by the roots. Water absorption from soil and its inward movement is OP dependent or independent (OP of the root hairs is higher than soil solution, OP of cortical cells is higher than root hairs). Passage of water from living cells to xylem channel requires accumulation of solute in xylem which is an energy dependent process. Hence, pumping of water in xylem channel is active. This creates a positive pressure in xylem called root pressure. Certain evidences also favors non-osmotic absorption of water, requiring energy.

Factors affecting water absorption

(1) Available soil water: Absorption of water is more, if the amount of available water is more. Rate of water absorption decreases, if the amount of soil water is below permanent wilting percentage or beyond field capacity.
(2) Soil air: Absorption of water takes place at a rapid rate in well aerated soil. Oxygen deficiency retards the growth of roots, thus inhibiting of water. In the soil, if all the air spaces are filled with water the condition is known as water logging of soil. Such soil is physiologically dry soil.
(3) Concentration of soil solution: If the soil solution is highly concentrated due to the presence of salts, it will inhibit the water absorption. It is also kind of physiological dryness.
(4) Soil temperature: An increase in soil temperature up to about 30 Celsius favors water absorption. At higher temperatures, water absorption is decreased and at 0 Celsius it is almost checked. 

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