WATER MOVEMENT BY PLANTS

WATER MOVEMENT IN PLANTS:

 

·         Long distance of water movement is crucial to the survival of terrestrial plants. Although plants vary considerably in their tolerance of water deficits, they all have their limits beyond which survival is no longer possible. About 85% of the fresh weight of leaves can be water. On a dry warm, sunny day a leaf can evaporate 100% of its water weight in just one hour.

·         Water loss from the leaves must be compensated by the uptake of water from the soil.

·         Water transport is also important for the uptake of essential mineral nutrients from the soil.

·         Shortages of mineral nutrients such as nitrogen, phosphorous, potassium are often limiting to plant growth that’s why fertilizers are often added to the soil to improve plant productivity and appearance.

Cohesion-Tension theory:

·         The major mechanism for long distance water transport is explained by this theory, whereby the driving force of transport is transpiration i.e. the evaporation of water from the leaf surfaces.

·         Water molecule cohere (stick together) and pulled up the plant by the tension or pulling force exerted by evaporation at the leaf surface.

·         Water will move away towards a site with lower water potential which is the measure of the chemical free energy of water.

·         Pure water has a water potential of zero MPa. In contrast, 20% relative humidity, the water potential of the atmosphere is approx 500 MPa.  The difference signifies the water will tend to evaporate into the atmosphere.

·         The water within plants also has the negative potential, indicating water will tend to evaporate into the air from the leaf.

·          The leaves of crop plants function at 1 MPa and some desert plants can tolerate leaf water potential as low as approx 10 MPa.

·         The water in plants can exist at such water potential due to the cohesive forces of water molecule. The chemical structure of water molecule in such that they cohere very strongly.

·         By the cohesion tension theory, when sunlight strikes as leaf, the resultant evaporation first causes a drop in leaf water potential. This causes water to move from stem to leaf, lowering the water potential in the stem, which in turn causes water to move from root to stem and soil to root.  This serves to pull water up through the xylem tissue of the plant.

 

 

 

Absorption of water by plants:

·         Uptake of water is called water absorption. Water is absorbed in the soil mainly through root hairs. The absorbed water crosses a variety of cells such as cortical cells, passage cells (endodermis), pericycle and xylem tubers to reach the leaves.

·         In 1949, Kramer proposed that water is absorbed by two mechanism- active absorption and passive absorption.

 

1)      Active absorption:

 Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root such as respiration. Absorption takes place against concentration gradient, even when the concentration of cell sap is lower than that of soil water.

Active absorption involves symplast movement of water in root hairs. The water first enters the cell sap and then passes from one cell to another. Such type of movement where living protoplasm is involved is called symplast.

2)      Passive absorption:

 It takes place along the concentration gradient, when the concentration of cell sap is higher than that of soil water. Water absorbed when transpiration rate is high or soil is dry. Due to high transpiration rate, water deficit is created in transpiring cells. Rapid transpiration removes water and reduces turgor pressure in living cells of root. The suction force then develops is transmitted to root xylem. It pulls water from surrounding roots cells to make up water deficit.

In passive absorption, water moves probably through the free spaces or apoplast of root. The apoplast path of water movement includes cell wall and intercellular spaces which are fully permeable. The water can reach up to endodermis through apoplast but it moves through the endodermis by symplast.

Pathways for root absorption:

·         Some herbicides will be absorbed by roots and will tend to remain in membrane and lipid bodies in epidermis, while herbicides with some water solubility can move by three major pathways:

                    I.            Apoplast (non-living) pathway:

The apoplastic (non-living) pathway provides a route toward the vascular stele through free spaces and cell wall of the epidermis and cortex. An additional apoplastic route that allows direct access to the xylem and phloem is along the margins of secondary roots. Secondary roots develop from the pericycle, a cell layer just inside the endodermis. The endodermis is characterized by the casparian strip, a suberized layer that forces all herbicides to move in the symplast in order to enter the vascular system. Since secondary route grow through an endodermis, a direct pathway to the xylem and phloem is available that bypasses the casparian strip and allows herbicides to enter the vascular system without moving into the symplast.

 

                  II.            Symplast  (living) pathway:

In this pathway movement of water molecule is from cell to cell through plasmodesmata. They are channels of cytoplasm lined by plasma membrane that transverse cell wall. These channels allow herbicides to move from cell to cell without passing through the cell wall. Plasmodesmata represent direct cytoplasmic connections from one cell to the next.  The casparian strip separates the cortex and endodermis. It is composed of wax like substance called suberin, which blocks water and solute molecule through the cell wall of endodermis. Now the water is forced to go through the cell membrane of different cells leading to transmembrane pathway.

 

                III.            Trans-membrane  Pathway:

When water moves from cytoplasm of one into the other, crossing the plasma membrane of the cell; it said to be traveling through the trans-membrane pathway. Water can also pass through tonoplast into the vacuole and out of it, while crossing the cell. This movement of water across cellular membrane in many tissues mostly takes place via aquaporins. The trans- membrane route involves movement across cells and cell wall combining both symplastic and apoplastic movement. Herbicides move by symplastic and trans-membrane pathway are already moving in such a way that casparian strip represent the significant barrier into the vascular system.

Herbicides moving in the apoplast, however are forced by the casparian strip to cross the plasma membrane and under the cytoplasm of epidermal cells. Once inside the endodermis, these herbicides must cross the plasma membrane, a second time to reach the vascular stele. This process can restrict herbicide movement to the vascular system and subsequently translocation to the shoot.

WATER ABSORPTION BY PLANTS:

·         Plants absorbed water through the entire surface – root, stem and leaves. However mainly the water is absorbed by roots.  The area of young roots where most absorption takes place is the root hair zone. Roots have numerous unicellular root hairs to increase the surface area for efficient water absorption.

·         Root hairs lack cuticle and provide a large surface area. They are extensions of epidermal cells which have sticky walls to facilitate adherence of soil particles.

·         The entry of water into the root hairs is facilitated by the water potential gradient that exits as root cells have relatively low water potential owing to the presence of inorganic ions and organic substances. Osmosis is the process which helps in uptaking of water from the intervening spaces.

·         After entering root hair, water can take the following three pathways, Apoplastic, symplastic and trans- membrane pathways to move across different tissues of root to reach the vascular system.

·         Water enters plant roots along a concentration gradient through fine root hairs, then make its way across the root tissues (cortex, endodermis, pericycle) towards the xylem that part of the vascular system which distribute water and dissolved mineral salts throughout the plant.

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