06 Plant Water Relation - part 07 - Path of water across the root

06 Plant Water Relation - part 07 - Path of water across the root


Path of water across the root (i.e. from epiblema upto xylem in the stelar region) : 
  • Water is absorped by root hair cell through imbibition diffusion osmosis, sequentially. 
  • Consequently the cell becomes turgid. 
  • Its turgar pressure increases, but its DPD value decreases. 
  • However, the immidiately adjacent cortical cell inner to it, hasmore DPD value, because its O. P. is more. 
  • Therefore, cortical cell will suck water from the turgid root hair cell. 
  • It then becomes turgid. 
  • The flaccid root hair cell now absorbs water from soil. 
  • Water from the turgid cortical cell is sucked by inner cortical cell and the process goes on. 
  • Thus, a gradient of suction pressure (DPD) is devloped from cells of epiblema to the cortex of the root. 
  • Consequently water moves rapidly across the root through loosely arranged living cells of cortex, followed by passage cells of endodermisand finally into the cell of pericycle. 
  • Protoxylem is in the close proximity with pericycle.
  • Absorption of water being a continous process, a sort of hydrostatic pressure is developed in living cells of root. This is called root pressure. 
  • It is due to root pressure, water from pericycle is not only forced into the xylem, but also conducted upwards against the gravity. 
  • Pathway of water across the root essentially occurs in two ways viz - 
  1. Apoplast and
  2. Symplast.
Apoplast pathway :  
  • When some amount of water passes across the root through the cell wall and the intercellular spaces of cortical cells of root, it is then called apoplast pathway. 
  • This pathway occurs up to endodermis.
Symplast pathway :
  • When water passes across from one living cell to other living cell through plasmodesmata, then it is called symplast pathway. 
  • It is also called transmembrane pathway. 
Additional Information :
  • The apoplastic (non-living) pathway provides a route toward the vascular stele through free spaces and cell walls of the epidermis and cortex. 
  • An additional apoplastic route that allows direct access to the xylem and phloem is along the margins of secondary roots. 
  • Secondary roots develop from the pericycle, a cell layer just inside the endodermis. 
  • The endodermis is characterized by theCasparian strip, a suberized layer that forces all to move in the symplast in order to enter the vascular system. 
  • Since secondary roots grow through the endodermis, a direct pathway to the xylem and phloem is available that bypasses the Casparian strip and allows to enter the vascular system without moving into the symplast (living tissue). 

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