06 Plant Water Relation - part 08 - Mechanism of absorption of water

06 Plant Water Relation - part 08 - Mechanism of absorption of water


Mechanism of absorption of water : 
  • Mainly, there are two ways/ modes of absorption of water viz - 
  1. Passive absorption and 
  2. Active absorption.
1. Passive absorption : 
  • It is the main way of absorbing water through the roots and not by the roots from soil into the plant. 
  • The driving force is transpiration pull and it thus proceeds through DPD gradient. 
  • There is no expenditure of energy (ATP) as water moves in accordance to the concentration gradient. Hence, it is passive absorption. 
  • About 98% of the total water absorbed in plants, occur passively. 
  • Passive absorption occurs during day time when transpiration is in progress. 
  • It stops at night when transpiration stops. 
  • Rapid transpiration creates a tension in the xylem vessel due to negative water potential.This tension is transmitted to xylem in the roots. 
  • Consequently water is pulled upwards passively. 
  • During passive absorption, no ATP is utilized. Obviously, the rate of respiration is not affected. 
  • In plants, water is mainly absorbed passively.
2. Active absorption : 
  • Here, water is absorbed due to activity of roots. 
  • Root cells play active role in the absorption of water. 
  • The driving force is the root pressuredeveloped, in the living cells of root. 
  • Active absorption occurs usually at night when transpiration stops due to closure of stomata. 
  • As water absorption is against the DPD gradient, there is expenditure of ATP (energy) generated through the respiratory activity of cells. 
  • Active absorption may be of two kinds viz - 
  1. Osmotic and 
  2. Non-osmotic 
I. Osmotic absorption : 
  • Atkins and Priestly (1922) proposed that water is absorbed from soil into xylem of the root according to the osmotic gradient. 
  • To create osmotic conditions, there is an expenditure of energy. 
  • But such absorption does not directly require an expenditure of energy. 
  • A gradient of DPD develops from cell of epiblema to pericycle due to activity of living cells of root. 
  • As the process is continuous, a hydrostatic pressure, called root pressure, is developed in root cells. 
  • This root pressure forces water from pericycle to xylem and then upwards to the stem. 
II. Non-osmotic absorption : 
  • Kramer and Thimann (1959) proposed this theory. 
  • Sometimes, water is absorbed from soil against the concentration gradiant. 
  • Such absorption requires an expenditure of energy released during respiration, directly. 
  • Poor supply of oxygen retards water absorption. 
  • Moreover low temperature retards water uptake because of decrease in the rate of respiration. 
  • Use of metabolic inhibitors also retards the rate of respiration and thus the water uptake.

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