1. Absorption of water in plants

2. The soil-plant atmosphere continuum
Plants absorb water from soil through roots.
Water is taken up by vascular tissue system through shoots and transpires into atmosphere in the form of vapors.
This system which starts from roots and ends with atmosphere is called continuum.
Whole mechanism can be divided into two parts
a) Absorption of water or movement of water across roots or radial movement of water
b) Ascent of sap – when water enters in plants it becomes sap because of having solutes in it or conduction of water or translocation of water

3. Absorption of water
Root is the major absorption organ except hydrophytes
The zone of maximum water absorption (about 20-folds) is located about mm from the root tip
This region is full of root hairs
Length of root hairs is 0.1 mm to 10 mm
Density of root hairs is 2500 hairs per cm2
Root hairs are unicellular while hairs of stem or leaf are multi-cellular
But the rhizome carries adventitious roots without hairs
Plants growing in solution culture and aquatic plants lack root hairs
The older parts of roots are suberized and no longer are absorbing

4. Pathways for water movement
There are three pathways for water movement
1. Apoplastic pathway
2. Trans-membrane pathway
3. Symplastic pathway
it is nonliving, discontinuous and separated into two regions
a) Outside the casparian strip (deposition of suberin and waxes)
b) Initially water moves through intercellular spaces but “casparian strips” in endodermis is main barrier in water flow so water moves symplastically here and again moves it can move apoplastically through intercellular spaces
This pathway is discontinuous pathways due to casparian strips barrier

5. Continuo-----
2. Trans-membrane pathway: There are number of membranes in cytoplasm i.e. membranous system of different cellular organelles
3. Symplastic pathway:
a) continuous- because connected from cell to cell via plasmodesmata.
b) symplast consists of total protoplasts of cells excluding vacuole.
c) movement of water through symplast is slow due to high viscosity of cytoplasm
Whether the pathway is taken, water has to move through cytoplasm because of casparian strips impregnated with subrin (barrier to water)
Opposite and outside xylem certain cells of endodermis are thin walled which are called passage cells.
Subrin is wax like hydrophobic substance

6. Driving forces for symplastic pathway
Active absorption or Osmotic movement of water occurs in slowly transpiring plant
Roots as osmometer-solutes (ions) become highly in roots because roots absorb ions from the dilute soil solution and transport them into the xylem due to which solute potential of xylem decreases thereby decreasing xylem water potential
Water enters the roots due to positive pressure – guttation or exudation
Osmotic potential -0.1 to -0.2 MPa – main factor
Turgor potential 0.1 to 0.15 MPa but up to 0.5 MPa
Passive absorption (mass flow) in rapidly transpiring plants
Forces originate at evaporating surface i.e. leaves in the shoots and transmitted to the roots
Roots act as absorbing organs

7. Ascent of xylem water or sap
Three theories
Cohesion
Root pressure
Capillarity or capillary rise

8. Ascent of Sap Translocation – conduction – transport
Water is in the form of sap because solutes are dissolved in it - xylem sap
Rate of water movement 3-45 m h-1
Transpiration is driven by cohesion tension theory proposed by Dixon (1914)
Water has high cohesive forces when present in small tubes it becomes subject to tension of 3.0 to 30 or more bars
One bar can raise the water column up to 10 m
Water forms a continuous column
Evaporation at surface causes tension
The DY is transmitted from evaporating surface to the root and is controlled by the rate of transpiration
Cohesion tension theory is objected because
Living cells around tracheids and vessels may be involved
Bubble formation may occur and can cause blocking of the column

9. Root pressure Root pressure is not linked with transpiration
Maximum root pressure is 1-2 bars – can raise water column up to 20 m
Mostly occurs during night
Guttation:
Due to root pressure
Pores on the margins of certain leaves and cause drop of water at margins of leaves

10. Capillary rise Forces involved
The rise of a fluid in a capillary tube is dependent on two balancing forces.
Lifting force – adhesive forces + surface tension due to cohesive,
Force of gravity which pulls the column downward.
Adhesion results from H–bonding of water molecules and polar groups on the capillary wall. The attraction of water molecules to the capillary wall causes the H2O to rise up the wall. Normally this would stretch the water column, but the high surface tension of water resists stretching and pulls the column up. Because of this curved surface (meniscus) results.

11. Tracheids and Vessels Vascular tissue – xylem and phloem
Xylem – conducting tissues are xylem vessels and tracheids
Vessels – small pipe like structure united end to end to make a long line, range from 40 to 60 mm in diameter and 300 mm to 10 m in length, present in angiosperms
Tracheids – elongating structures lie side by side and are interconnected, the diameter of tracheids (they are single cells) ranges from mm. Their length is 1.0 cm but in some cases it is 3.0 cm, present in gymnosperms