Absorption of water in plants

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

Absorption of water Root is the major absorption organ except hydrophytes The zone of maximum water absorption (about 20-folds) is located about 20-200 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

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

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

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

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

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

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

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.

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 10-15 mm. Their length is 1.0 cm but in some cases it is 3.0 cm, present in gymnosperms