In this article we will discuss about how do plants absorb water from soil.
Soil is the main provider of water to the roots. The soil has particles and the size of these particles increase with increase in depth. Soil is produced by weathering process of rocks, which could be chemical, physical or biological.
When the soil receives water (by rain) some water flows away due to the sloping gradient, this water is called run away water.
Water filters down between the soils particles due to gravitational attraction and accumulates over the rock; this deeply located water is called ground water. The upper part of the ground water is called water table.
Quite a few plants have roots that reach the water table and these plants like Tamarix, Prosopsis are called phreatophytes. These plants indicate availability of ground water in the area.
Types of Soil Water:
(a) Gravitational Water:
It is found in the water table. It is available to plants that develop deep tap roots. For shallow rooted plants this water is not available.
(b) Hygroscopic or Imbibitional Water:
This water is unavailable to the roots because the water molecules tightly adhere to the surface of soil particles.
(c) Combined Water:
It is found as water of hydration of alumina, iron and silicates in the soil and this water is also unavailable to the roots.
(d) Capillary Water:
This water is readily available to the roots. It is held in the fine capillary spaces between the soil particles and when capillary water is exhausted, some amount of water can rise up by capillary forces from deeper parts of the soil.
The height to which capillary water can rise is called capillary fringe.
In soil with more clay the ascent of capillary water can be up to ten feet. This is because the capillary spaces between clay particles are very fine. In sandy soil the capillary ascent is only around three feet, this is because capillary spaces are larger.
The best soil for plant growth is loam. Loamy soil has equal amount of clay and sand, clay holds water and sand holds air and roots need both water and air for growth and respiration.
The amount of water retained by the soil is called field capacity or water holding capacity. This water is not affected by gravity. Loamy soils have up to 30% of water and it is a good field capacity. If field capacity is exceeded then it results in water logging and such soil is not good for plant growth due to absence of air.
If the soil becomes dry and not replenished with water the leaves of the plant wilt. This temporary wilting enters a stage of permanent wilting if the soil is not watered by rain or artificially.
The total amount of water in the soil is the holard value. The water available to the plants is called chresard value and three fourths (75%) of chresard value is capillary water. The total unavailable water is the echard value; it includes 25% of capillary water, hygroscopic water, combined water and soil water vapour.
Soil Water Absorbing System:
Aquatic plants can absorb water from the entire body surface. The rooted land plants absorb the water from the soil system. Land plants absorb vast amounts of soil water and most of the water is lost by the process of transpiration, some water is retained to maintain cellular turgidity and other physiological functions.
Most of the water absorption is by the millions of root hairs of the root tips. The root hair zone lies just above the zone of cell elongation. In this zone the endodermis is not fully developed so water can be absorbed fast. Just above the root hair zone there is suberization and lignification of root cells which does not favour water absorption.
The Root System and Root Hairs:
The root system has the following zones and the structural features of the roots are correlated to soil penetration and absorption of water and minerals:
(1) Rootcap:
It is a zone of cells that protect the apical meristem.
(2) Region of Apical Meristem:
It is the zone of dividing cells which is essential for root growth.
(3) Region of Cell Elongation:
The cells elongate to a great degree. This zone is few millimetres from the tip.
(4) Region of Cell Maturation:
In this zone different types of tissues like epiblema, cortex and stele develop.
Root Hair Zone:
This region produces root hairs and it is just above the region of cell elongation.
Structure of a Root Hair:
The root hairs account for 85 to 90% of the total soil water absorbed. Root hairs are unicellular tubular extensions of root epiblema cells. Epiblema cells producing root hairs are called trichoblasts. Root hairs reach a length of 0.05 to 1.5 mm and 0.01 mm in diameter. Each root hair has a vacuole filled with osmotically active cell sap and a peripheral cytoplasm.
There is nucleus in the cytoplasm. The root hair cell wall is very thin with an inner thin layer of cellulose and outer covering of highly hydrophilic pectic substance. Root hairs immerse themselves in the capillary water and pectic substances help the root hair to adhere to soil particles.
Roots hairs function for a few days or few weeks, in growing root tips they are constantly replaced.
Mechanism of Water Absorption:
There are two possible pathways encompassing the entire root system through which the absorbed water can move into deep parts of the root like endodermis and pericycle, reach the xylem tissue for onward transport to the plant parts via the extensive and continuous tracheary elements of xylem.
The two pathways are:
(a) Apoplastic Pathway:
The apoplast in all plants is a continuous system of nonliving parts of plants like cell walls of living cells, dead cells and the intercellular spaces. This is also called free space. Apoplast does not offer any resistance to movement of substances like water with solutes dissolved in it (Apoplastic pathway does not include plasma membrane, cytoplasm and vacuolar membrane). Capillary action plays a major role; mass flow also has a role in apoplastic pathway of water movement. Osmosis does not have any direct role.
(b) Symplastic Pathway:
The numerous protoplasts of cells all interconnected by cytoplasmic strands of plasmodesmata forming a continuous symplastic system which is enclosed by the plasma membrane of cells. (The plasma membrane also surrounds the plasmodesmata). Symplastic movement of water occurs along a gradient of favourable water potential. Cytoplasmic streaming also helps in symplastic movement of water.
Cortical cells of the root offer no resistance to water movement, but the innermost layer of the cortex called endodermis has suberized casparian strips that block water movement, hence water molecules are directed to parts of endodermal cells without suberin and water enters endodermal cells via plasma membrane, this is the transmembrane pathway. As soon as the water enters the stele it accesses the tracheary element of xylem, these xylem cells are dead with an empty lumen and offer no resistance to movement of water.
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