Soil Water


Soil Water, Water is present in the soil, Water Absorption system, Mechanism of water absorption ,

 Soil Water 

Soil  is the upper weathered, humus, mineral water and air conditaining  layer  of the earth crust. Deep in the soil and above the impermeable  stratum, water occurs freely in the previous  rocky matter. It is called ground water. Ground water bearing  stratum is known as aquifer. The upper layer of ground water is called water table. In the region of water table soil is completely saturated with water. Air is excluded. Tube wells and hand pumps bring ground water to the surface. Very few plants can send their roots upon fringe of the water table due to deficiency  of air. The plants which do so, are called phreatophytes,e.g., Populus. They have been  used  to locate underground  water. Plant roots also go deep in the arid soils in order to absorb maximum  amount of water,e.g., Aerva persica (60 cm high plant with 6m deep tap root). Here shallow rooted plants cannot  survive dry periods. Soil water, important to most plants, is the one present in 1_ 2m of soil because  their roots are generally restricted to this region. Water is present in the soil in five forms:_ 




1: Capillary  Water : It is  present in soil     micropores  having a diameter  of 20 mm or below. It is held in the soil by capillary  forces. It, therefore,  does not fall down to water table by gravity. Only capillary water is available  to plant roots for absorption. Roots are unable to absorb water from capillaries below the diameter  of 0.2 mm. 

2 : Gravitational  Water : It is not a permanent  constituent of soil water. It occurs only after a good rain or irrigation. Excess water goes down under the influence of gravity through soil macropores larger than  50  mm. The phenomenon  is called percolation. After percolation, the water joins ground  water. 

3. Hygroscopic Water : It is the water kept absorbed by soil colloids. This is not available to plant roots since th water is held with very strong attractive forces. 

4. Combined Water : It is th water combined with chemicals and is, therefore, not available 

5. Water Vapours : They occur in soil macropores.They are unimportant. In certain seasons they may condense near soil  surface when they are a means of night recovery. 

  Excess water can be harmful, as it would expel soil air. This happens when flow of gravitational water is prevented and soil is covered with water for longer periods. The maximum  amount of water retained per unit dry  weight of soil after the stoppage  of gravitational flow is called field capacity. It is 25 _ 35% in common  loam soil. Soil moisture beyond field capacity  produces water logging. If soil water is not replenished, a stage is reduced when the plants growing in ot become permanently wilted and die. It is  known as permanent Wilting  percentage ( PWP) Or Permanent  wilting coefficient (PWC). At permanent wilting  percentage  the soil contains about 10% of water which is either present in extremely  fine micropores or in the non_ available  form. 

Available  Water : The total water content present in the soil is called hoard. The  water available  to plants is chesard. It consists of 75% of capillary  water. The rest of soil water ( hygroscopic, combined, water vapour and 25% of capillary water) is called echard or unavailable  water. 




WATER ABSORBING SYSTEM: 

Plants have the potentiality to absorb water through their entire surface right from root, stem, leaves, flowers, etc. As water is available mostly in the soil,  only the underground root system is specialized to absorb water. In roots, the most efficient  region of water absorption  is the root hair zone. Each root hair zone has thousands  of root  hairs. Root hairs pass into capillary micropores, get cemented to soil particles  by pectic compounds and absorb capillary water. 

Path of water movement  through root cells: 

They are three possible routes of water through the individual cells. In order of increasing resistance, they are 

(a) Apoplast Pathways (Cell wall only ): Apoplast consists  of non_ living continuum made of cell walls, intercellular spaces and water filled xylem channels. Apoplastic movement is dependent  upon the gradient. The apoplast does not provide any barrier to water movement  and water movement  is through mass flow. 


(b) Symplast Pathways (Cell wall plus cytoplasm): Symplast is the living continuum of the plant which is made up of protoplasts ( excluding vacuoles) interconnected by Plasmodesmata. Movement  of water is relatively  slower and is down a potential . Symplastic movement  may be aided by cytoplasmic streaming.  

(c) Vacuoles  Pathway (Cell wall, cytoplasm and central vacuole) . 

The apoplastic movement  of water occurs exclusively  through the cell wall without crossing any membranes, while th symplastic movement  occurs from cell to cell through the Plasmodesmata. 

Major proportion of water flow in the root cortex via the apoplast, as the cortical cells are loosely  packed and thus, the corte offers no resistance. The apoplastic water movement beyond the cortex is blocked by the  casparian strip present in the epidermis. The casparian strip is composed of  suberin which blocks water and solute movement through the cell wall of the endoplasmis. Beyond enoddermis, water is forced to move through  the cell membrane and it is called transmembrane pathway. In this pathway, water may also cross through the tonoplast surrounding  the vacuole. 



Mechanism  of water absorption:

Water absorption  is of two types, passive and active. 

Passive Water Absorption:

The force originates in the aerial parts of the plant due to los of water in transpiration. This creates a tension in the xylem channels. Creation of tension in the xylem  is evident from: 

(i) A negative  pressure  is found in the xylem sap. It is because  of it that water does not spill out if a cut is given to a shoot.

(ii) Water can be absorbed  by a shoot in the absence  of the root system.

(iii) The rate of water absorption is approximately equal to the rate  of transpiration.  

Because  of the central vacuole a root hair cell has a water potential  of _ 3  to _ 8 bars. Water potential  of the soil water  is _ 0.1 to _ 0.3 bars . As a result water of the soil passes into the root hair cell. The root hair cell loses water to cortical cell lying inner to it because  the latter has lower water potential. This cortical cell loses water to the next inner cell and so on. Water will, thus, move along the gradient of water potential. Ultimately  it enters into xylem vessels.  This, traceheary elements of xylem have lower water potential due to tension under which water is present in them, caused by transpiration in the aerial parts. 



Active Water Absorption: 

It is due to force present in the root. Living cells are ess for  this. Auxins are known to enhance water absorption (even from hypertonic solution) while respiratory inhibitors reduce it. Energy is involved in this process. Water absorption  from soil and its inward movement may occur due to osmosis. Passage of water from living cells to the xylem channels can occur by, 

(i) Accumulation of sugars or salts in the tracheary elements of xylem due to either secretion by the nearby living cells or left there during decay of their protoplasts.

(ii) Development of bioelectric potential favourable for movement of water into xylem channels.

(iii) Acitve pumping  of water by the surrounding  living cells into tracheary elements.  

Root  pressure  is a manifestation of active water absorption. 

Mycorrhizal Water Absorption: In mycorrhiza a large number of funal hyphae are associated  with the young roots. The fungal  hyphae extend to sufficient distance into the soil. They have a large surface area. The hyphae are specialized to absorb both water and mineral. The two are handed over to the root which provides the fungus with both sugars and N_ containing  compounds. Mycorrhizal association between  fungus and root is often obligate. Pinus and orchid seeds do not germinate and establish themselves into plants without mycorrhizal association.


  






























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