Factors affecting water absorption

 The plants get two types of environment for water absorption, the external and internal. The external environment includes factors such as a) available soil water, b) concentration of soil solution. c) soil temperature and d) soil aeration. The internal factors include, i) transpiration, ii). absorbing root system and iii) metabolism.

I. External Environmental Factors

a. Available Soil Water

A sufficient amount of water should be present in the soil in such a form, which can readily and easily be absorbed by the plants. Usually, the plants absorb capillary water. Other forms of water in the soil (hygroscopic, gravitational etc.) are not easily available to plants.

b. Concentration of Soil Solution

The increased concentration of soil solution (due to the presence of more salts in the soil) results in a higher OP. If the OP of soil solution becomes higher than the OP of cell sap in root cells, the water absorption will be greatly suppressed. Therefore, the absorption of water is poor in alkaline soils and marshes. This is popularly known as physiological dryness.

C. Soil Temperature

An increase in soil temperature up to about 300C favours water absorption. At higher temperature, water absorption is decreased mainly due to the death of root cells on account of protein denaturation. At low temperature also water absorption decreases so much so that at about 100C, it is almost checked. This is probably because at low temperature :

1. Viscosity of water and protoplasm is increased.

2. permeability of cell membranes is decreased.

3. Metabolic activities of root cells are decreased, and

4. Growth and elongation of roots are checked.

d. Soil Aeration

Water is absorbed more efficiently in the well-aerated soil than in poorly-aerated soils, maybe because of the fact that roots fail to respire anaerobically and plants die in flooded areas. The deficiency of O2 inhibits the growth and metabolism and an accumulation of CO2 increases the viscosity of protoplasts and decreases the permeability of the cell membrane. Both of these factors affect and reduce the rate of water absorption severely.

II. Internal Environmental Factors

i. Transpiration :

The rate of absorption of water is directly proportional to that of transpiration. A higher rate of transpiration produces tension or pull called Transpiration Pull, transmitted to roots through the hydrostatic system of plants creating a favourable condition for the entry of water.

ii. Absorbing Root System

The efficiency of water absorption depends on the absorbing system or root system. The presence of more number of root hairs accounts for the increased rate of water absorption.

iii. Metabolism

Metabolism and absorption are closely related. Factors inhibiting respiration such as poor aeration, application of anaesthetics and KCN reduce the absorption rate. Thus, metabolic activities are expected to participate indirectly by forming a constantly elongated root system and always providing newer contacts with soil water.

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