(i) Gravitational Water.
- It is the water that drains downwards through the soil.
The level to which it drains is called the water table. The water table of a
place differs in depth due to rainfall.
- The gravitational water lies far below and is generally not available to plant
roots. It is of extreme importance as it causes washing out of minerals and
nutrients from the soil called leaching.
- Part of water that is retained by soil could be hygroscopic water and/or
(ii) Hygroscopic Water.
- It is the water that is retained as a thin film around the individual soil particles.
- Strong attractive forces between the soil particles and the water molecules hold this water tightly. This is the water least available to the plant and is generally the water left in the dry soils.
- In the clay soils,it amounts to about 15% and in the sandy soils to about 0.5%.
(iii) Capillary Water.
- The soil particles always have very fine pores in between, forming a very fine capillary system As the water spreads, it fills the finer pores and is held round the soil particles by capillary forces against the force of gravity, due to high surface tension of water.
- It is this water, which is readily available and is easily utilized by the plant roots. The clay soil being very
fine textured holds much more water than sandy soil. When a soil is watered, it retains good amount of capillary water and thus condition is known as field capacity.
ABSORPTION OF WATER BY PLANTS
- Major portion of water required by plants in absorbed by roots but in some cases
water may be absorbed by leaves and stems also.
- Root hair is a specially modified epidermal cell meant for absorption of capillary
water of the soil.
- The plasma membrane and the vacuolar membrane (tonoplast) act as semipermeable membranes and water is absorbed by osmosis.
- Soil solution should have a higher water potential as compared to root hair cell, then only water will enter the root hair cell. Once into the root hair, water will pass into cortical cells, endodermis, pericycle and into the xylem vessel. The movement is purely dependent on water potential gradient.
- Water movement into the plant follows two pathways – symplast and apoplast
- Cytoplasm of the entire plant is connected through plasmodesmata stand forming the symplast system. Water movement through the cells take this symplast pathway by osmosis.
- The cell wall and the intercellular spaces form the apoplast pathway which allows water movement inside the plant by the phenomenon of capillarity and adsorption.
- The water absorbed through the roots is transferred radially to the xylem, from where it reaches to all the other plants of the plant by vertical conduction of water through the xylem vessels
Conduction of water through the xylem
The content of xylem vessels is known as xylem sap. Various theories have been postulated to describe the lifting of the xylem sap or ascent of sap in the xylem.
Root Pressure Theory
If a stem is cut few inches above from its base with a sharp knife, xylem sap is seen flowing out through the cut end. This phenomenon is known as exudution and this is due to the positive pressure developed within the root system due to continuous absorption of water by osmosis which develops a positive pressure
known as root pressure. This pressure can be measured and ranges from 3 to 5 atmospheres. But this pressure is enough to raise water to small heights in herbaceous plants.
Physical Force Theory or Cohesion Theory
This theory takes into account the physical forces which act in case of very tall trees and water lifted up to great heights. The three forces that act together are force of cohesion (attraction between water molecules), force of adhesion (attraction between water and lignocellulose walls of xylem) and transpiration pull which lifts the water column by creating a tension inside the xylem vessel. Water forms an unbroken column starting from the intercellular space of the leaf mesophyll to the xylem of the leaf, through stem and root to the water in the soil. A water potential gradient exists between the leaf to the root and transpiration causes a pull of the entire water column. So long as the column is an unbroken one from the outer atmosphere, through the plant upto the soil, water is lifted up by the force of transpiration pull.