11th Class Biology Transport In Plants Absorption Of Water

Water is absorbed from soil by root system and specially by younger parts (i.e., root tips). In higher plants water is absorbed through root hairs.

Soil water : The chief source of soil water is rain. In soil water is found in different forms. The total amount of water present in the soil is called holard, of this the available to the plant is called chresard and the water which cannot be absorbed by the plants is called echard.

Water occurs freely deep in the soil and above the parent rock, it is called ground water. These are briefly described below :

Gravitational water : When the water enters the soil and passes the spaces between the soil particles and reaches the water table, the type of soil water is called gravitational water.

Capillary water : It is the water which is held around soil particles in the capillary space present around them due to force like cohesion and surface tension. This is the water which can be utilised by the plants. It is also called growth water. It occurs in the form of films coating smaller soil particles.

The availability of capillary water to the plant depends upon its diffusion pressure deficit which is termed as the soil moisture stress. The plant cells have a DPD much more than the soil moisture stress for proper absorption of water.

Hygroscopic water : This is the form of water which is held by soil particles of soil surfaces. The water is held tightly around the soil particles due to cohesive and adhesive forces. Cohesive and adhesive forces greatly reduce the water protential (yw) and thus this type of water in soil is not available to plants.

Run-away water : After the rain, water does not enter the soil at all, but drained of along the slopes. It is called run-away water. Plants fail to avail this water.

Chemically combined water : Some of the water molecules are chemically combined with soil minerals (e.g., silicon, iron, aluminium, etc.). This water is not available to the plants.

Water vapour : That portion of the pore space in a soil which is not occupied by liquid water contain a soil atmosphere that always includes water vapour.

Water holding capacity : The amount of water actually retained by the soil is called field capacity or water holding capacity of the soil. It is about \[2535%\] in common loam soil. The excess amount of water beyond the field capacity produces water logging.

Soil atmosphere : In moderately coarse soils as well as in heavy soils (fine textured soil) that are with aggregated particles; there exists large interstitial spaces which facilitate the diffusion of gases. As a result the \[C{{O}_{2}}\]produced in a soil by respiration of soil organisms and roots is able to escape rather easily and oxygen used up in this process diffuses into the soil with corresponding case.

Soil organisms : The soil fauna includes protozoans, nematodes, mites, insects, earthworms, rats. Protozoans alone are approximately 1 million per gram of soil. Blue green algae and bacteria increase nitrogen content by nitrogen fixation in soil.

Water absorbing organs : Plants absorb water mostly from the soil by their roots, but in some plants even aerial parts like stem and leaves also do the absorption of atmospheric water or moisture. Some important examples of such plants are Vitis, Solanum, Lycopersicon, Phaseolus, Kochia baosia and Beta.

However, maximum absorption of water is done by the roots.

This area is usually characterized by the presence of root hairs which serve to increase the area of contact between the root surface and soil.

The root hairs develop mainly at the tip just above the zone of elongation (cell maturation). A root hair is the unicellular tubular prolongation of the outer wall of the epiblema.

During water absorption the plasma membrane of root hair, the cytoplasm and the vacuole membrane (tonoplast) behave together as a single differentially permeable membrane. Root hairs are at the most 1.25 cm in length and never more than 10 mm in diameter. 

The root-hairs of plants increase the absorption surface of a root system about 5 to 20 times and because they extend so widely through the soil they make available a supply of water that the plant could not otherwise obtain. Water potential of root hair cells is generally \[1\text{ }\,to\text{ }4\text{ }atm.\]

Pathway of water movement in root : Water in the root moves through three pathways. Munch coined the term apoplast and symplast.

Apoplast pathway : The apoplastic movement of water occurs exclusively through the cell wall without crossing any membrane.

Symplast pathway : The symplastic movement of water occurs from cell to cell through the plasmodesmata.

Transmembrane pathway : Water after passing through cortex is blocked by casparian strips present on endodermis. The casparian strips are formed due to deposition of wax like substance, suberin. In this pathway, water crosses at least two membranes from each cell in its path. These two plasma membranes are found on entering and exiting of water. Here, water may also enter through tonoplast surrounding the vacuole i.e., also called as vacuolar pathway.

Mechanism of water absorption : Two distinct mechanisms which are independently operated in the absorption of water in plants. These mechanisms are :

(1) Active absorption                      (2) Passive absorption

Renner (1912, 1915) coined the term active and passive water absorption.

(1) Active absorption : Active absorption takes place by the activity of root itself, particularly root hairs. The factor responsible for water absorption is present with in the roots. It utilizes metabolic energy. There are two theories of active absorption :

Osmotic theory : It was proposed by Atkins (1916) and Priestley (1922). It is purely a physical process, which does not directly require expenditure of energy.

A root hair cell functions as an osmotic system. Water is absorbed by the root hair due to osmotic differences between soil water and cells sap. The osmotic pressure of soil water remains below 1 atm, but that of cell sap is usually \[28\] atms. Thus, there exists a great difference in the osmotic pressures of the two sides or in other words there exists, water potential gradient between the soil solution and cell sap. The soil solution having less OP, has higher water potential than the cell sap with more OP (i.e., the cell sap has more negative water potential). Thus, water moves from the region of higher water potential towards the region of lower water potential.

Non-osmotic theory : It was proposed by Thimann (1951) and Kramer (1959). It has been observed that absorption of water still occurs, if the concentration of cell sap in the root hair is lower than that of the soil water, or water is absorbed against concentration gradient (i.e., from higher DPD to lower DPD). Such type of water absorption occurs on the expense of energy obtained from respiration.

Following evidences support the view that energy is utilized during active absorption of water :

(i) Rate of water absorption is directly proportional to the rate of respiration.

(ii) Respiratory inhibitors such as KCN, which inhibit the absorption of water.

(iii) Auxins (growth hormones), which increase respiration also promote water absorption.

(iv) Wilting of plants occur in non-aerated soils such as water logged soils, as roots fail to absorb water in absence of respiration.

(2) Passive absorption : It is the most common and rapid method of water absorption. The factor responsible for water absorption is present some where else than roots. It accounts for about 98% of the total water uptake by plant.

According to this theory, the forces responsible for absorption of water originate not in the cells of roots but in the cells of transpiring shoots. The root cells remain passive.

Due to transpiration, the DPD of mesophyll cells in the leaves increases which causes absorption of water by these cells from the xylem vessels of leaves. As the water column is continuous from leaves to roots, this deficit is transmitted to the xylem elements of roots and finally to root hairs through pericycle, endodermis and cortex. In this way water is continuously absorbed due to transpiration pull created in the leaves. This type of water transport occurs mainly through the apoplast in cortex but through the symplast in endodermis and pericycle.

The path of water from soil upto secondary xylem is :

Soil\[\to \]Root hair cell wall\[\to \]Cortex\[\to \]Endodermis\[\to \]Pericycle\[\to \]Protoxylem\[\to \]Metaxylem.

Factors affecting rate of water absorption : The different factors which influence the rate of water absorption by a plant can be divided into external or environmental and the internal factors.

External or Environmental factors

The amount of soil water : It is optimum at field capacity. Water absorption decreases above it. It begins to decline and stops at PWP.

Concentration of the soil solution : If the concentration of solutes increases in the soil water, its OP also increases which slows down or even inhibits the absorption of water. It happens due to addition of enough fertilizers in the soil increasing its salinity. This is popularly called as physiological dryness.

Soil aeration : Water absorption is done more efficiently in well aerated soil. Any deficiency of oxygen stops the respiration of roots and causes accumulation of \[C{{O}_{2}}\] thus the protoplasm becomes viscous and the permeability of plasma membrane decreases. Due to all these factors the rate of water absorption is reduced. This is the reason for death of plants in flooded areas.

Soil temperature : The optimum temperature for maximum rate of water absorption ranges between \[20{}^\circ C\] and \[30{}^\circ C.\] Too high temperature kills the cells. At very low temperatures \[(4{}^\circ C)\] water absorption is reduced or stopped and about \[O{}^\circ C\] it is almost checked.

Transpiration : The rate of absorption of water is almost directly proportional to the rate of transpiration. A higher rate of transpiration increases the rate of water absorption.

Internal factors

Efficiency of the root system : A plant with deep and elaborate root system can absorb more water. The number of root hairs will be more in a highly branched and elaborate root system, thus its more surface area will be in contact with water.

In gymnosperms, the root hairs are absent, even then they are able to absorb water due to presence of mycorrhizal hyphae.

In epiphytes (orchid), the roots develop a special type of hygroscopic tissue called as velamen which can absorb atmospheric moisture.

Metabolic activity of roots : The poor aeration or use of metabolic inhibitors (e.g., KCN) inhibits the rate of water absorption. The metabolic activities help in proper growth of root system and generation of energy for absorption of certain vital minerals.

Absorption of water through leaves : Many species of plants can absorb at least limited amounts of water through the leaves. Temporary immersion of aerial organs in flood waters takes place in some cases. Also the aerial organs of plants frequently become wet as a result of fog, dew or rain. Most of the water enters through the epidermal cells, although in some species hairs and specialized epidermal cells provide regions of high permeability. In general water absorption is more rapid in young leaves than in old leaves of the same plant.