WATER UPTAKE BY ROOT.pptx

this presentation explains the process of water uptake by plant roots

1. WATER UPTAKE BY ROOTS ANKIT DHILLON DEPTT. OF G&PB

2. Water Absorption System in Plants Plants have the potentiality to absorb water through their entire surface right from root, stem, leaves, flowers, etc. However, as water is available mostly in the soil, only the underground root system is specialized to absorb water. Roots are often extensive and grow rapidly in the soil. Each root hair has a central vacuole filled with osmotically active cell sap and a peripheral cytoplasm. The wall is thin and permeable with pectic substances in the outer layer and cellulose on the inner layer. Root hairs pass into capillary micropores, get cemented to soil particles by pectic compounds and absorb capillary water. 2

3. SOIL WATER S oil is the upper weathered, humus, mineral water and air containing layer of the earth’s crust which supports plant life. Water is an important constituent of soil because all land  plants depend upon it for their requirement of water. Deep in the soil and above the impermeable stratum, water occurs freely in the pervious rocky matter. It is called the  Ground water. 3

4. Soil water, important to most plants, is the one present in 1-2m of soil because their roots are generally restricted to this region. Water is present in the soil in five forms capillary water, gravitational water, hygroscopic water, combined water and water vapours . The ultimate source of all soil water is rain or irrigation. A part of rain water does not enter the soil but is drained away from soil-surface along the slope. It is called run-away water or run-off. 4

5. Absorption of water

6. Site of water absorption Water is mainly absorbed through root hairs. Located in a group just above the root cap. This area rich in root hairs is called root hair zone. 6

7. Typical Root Hair Billions of root hairs are present in the root system of a plant. Root hairs are tubular hair like projections of the epidermal cells. Each root hair is single celled. Root hairs are 0.5 to 1.0 cm in length and 10µ in diameter. Root hair is modified epidermal cell The wall of the root is permeable to water it is made up of cellulose and pectic substances which are strongly hydrophilic (water loving in nature) Next to cell wall there is plasma membrane enclosing cytoplasm, nucleus and vacuole. Vacuole is filled with cell sap whose water potential is more negative than the soil solution.

8. ROOTS Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it's important to understand plant root systems because they have a pronounced effect on a plant's size and vigor, method of propagation, adaptation to soil types, and response to cultural practices and irrigation. Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, support the stem, and store food. In some plants, they can be used for propagation.

9. STRUCTURE OF ROOTS Internally, there are three major parts of a root : The meristem is at the tip and manufactures new cells; it is an area of cell division and growth. Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorption. As they grow, they push the root through the soil. The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex, or vascular tissue. A root's epidermis is its outermost layer of cells . These cells are responsible for absorbing water and minerals dissolved in water. Cortex cells are involved in moving water from the epidermis to the vascular tissue (xylem and phloem) and in storing food. Vascular tissue is located in the center of the root and conducts food and water.

10. Externally, there are two areas of importance: the root cap and the root hairs. The root cap is the root's outermost tip. It consists of cells that are sloughed off as the root grows through the soil. Its function is to protect the root meristem. Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root's growing tip. They generally appear as fine down to the naked eye. Their function is to increase the root's surface area and absorptive capacity. Root hairs usually live 1 or 2 days. When a plant is transplanted, they are easily torn off or may dry out in the sun

11. PATH OF ABSORBED WATER It move into the cortical cells. After crossing the cortical cells it reaches the epidermis. The endodermal cells lying opposite to root hairs are especially modified to transport the absorbed water. These endodermal cells are called passage cells. The passage cells are permeable to water because they lack of casparian thickening in their wall. Other endodermal cells are provided with casparian thickening are impermeable to water. The water passage cells pass into the pericycle cells. From the pericycle cells, the water pass into the xylem, through the xylem tube, the water move up through stem to reach the leaves. The water movement from one cell to another is brought about by turgor pressure.

12. WATER MOVEMENT MECHANISM IN PLANTS In plants, following two pathways are involved in the water movement. They are (1) Apoplastic pathway (2) Symplastic pathway (3) Transmembrane pathway 1. Apoplastic pathway : The apoplastic movement of water in plants occurs exclusively through the cell wall without crossing any membranes. The cortex receive majority of water through apoplastic way as loosely bound cortical cells do not offer any resistance. But the movement of water in root beyond cortex apoplastic pathway is blocked by casparian strip present in the endodermis.

13. Symplastic Pathway The movement of water from one cell to other cell through the plasmodesmata is called the symplastic pathway of water movement. This pathway comprises the network of cytoplasm of all cells inter-connected by plasmodermata .

14. Transmembrane pathway In plant roots, water movement from soil till the endodermis occurs through apoplastic pathway i.e. only through cell wall. The casparian strips in the endodermis are made-up of wax -like substance suberin which blocks water and solute movement through the cell wall of the endodermis. As a result water is forced to move through cell membranes and may cross the tonoplast of vacuole. This movement of water through cell membranes is called transmembrane pathway 18

15. MECHANISM OF WATER ABSORPTION: Active absorption of water : In this process the root cells play active role in the absorption of water and metabolic energy released through respiration is consumed active absorption may be of two kinds. Steps involved in the active osmotic absorption of water : First step in osmotic the osmotic absorption of water is the imbibition of soil water by the hydrophilic cell walls of root hairs. Osmotic pressure of the cell sap of root hairs is usually higher than the OP of the soil water. Therefore, the DPD and suction pressure in the root hairs become higher and water from the cell walls enters into them through plasma membrane by osmotic diffusion. 20

16. As a result, OP, suction pressure and DPD of root hairs become lower, while their turgor pressure is increased. Now the cortical cells adjacent to root hairs have high OP, SP & DPD in comparison to the root hairs. Therefore, water is drawn into the adjacent cortical cells from root hairs by osmotic diffusion. In the same way, by cell to cell osmotic diffusion gradually reaches the inner most cortical cells and the endodermis. Osmotic diffusion of water into endodermis takes place through special thin walled passage cells because the other endodermis cells have casparian strips on thin walls which are impervious to water. Water from endodermis cells is down into the cells of pericycle by osmotic diffusion which now become turgid and their suction pressure in decreased. 21

17. In the last step, water is drawn into xylem from turgid pericycle cells (In roots the vascular bundles are radical and protoxylem elements are in contact with pericycle). It is because in the absence of turgor presume of the xylem vessels, the SP of xylem vessels become higher than SP of the cells of the pericycle when water enters into xylem from pericycle a pressure is developed in the xylem of roots which can raise the water to a certain height in the xylem. This pressure is called as root pressure. 22

18. Osmotic theory : According to osmotic theory absorption of water takes place due to osmotic reason. According to this theory accumulation of salt occurs in xylem sap, which is responsible for making water potential more negative in xylem sap with respect to water potential of soil solution. Consequently, absorption of water takes place under influence of water potential gradient . 23

19. Non osmotic theory: According to this theory absorption of water occurs non osmotically. Non osmotic flow of water is energized from coupling of the water flow to a metabolically driven flow of solutes. The non-osmotic active water-uptake can also be supported by the fact that there is a correlation between the rate of water absorption and respiration. Water absorption rate is lowered by low temperature, oxygen tension, and respiratory inhibitors. Sometimes, it has been observed that absorption of water takes place even when the O.P. of the soil water is higher than the O.P. of cell-sap. This type of absorption which is non- osmotic and against the osmotic gradient requires the expenditure of metabolic energy prob­ably through respiration. 24

20. Passive Absorption of Water: Passive absorption of water takes place when rate of transpiration is usually high. Rapid evaporation of water from the leaves during transpiration creates a tension in water in the xylem of the leaves. This tension is transmitted to water in xylem of roots through the xylem of stem and the water rises upward to reach the transpiring sur­faces. As a result, soil water enters into the cortical cells through root hairs to reach the xylem of roots to maintain the supply of water. The force for this entry of water is created in leaves due to rapid transpiration and hence, the root cells remain passive during this process. 25

21. It is mainly due to transpiration, the root cells do not play active role and remain passive. STEPS: Transpiration creates tension in water in the xylem of the leaves Tension is transmitted to water in xylem of root through xylem of stem and water rises upward to reach transpiring surface Hence soil water enters cortical cells through root hairs to reach xylem of roots to maintain the supply of water. The force for entry of water in leaves is due to rapid transpiration and root cells remain passive 26

22. During absorption of water by roots, the flow of water from epidermis to endodermis may take place through three different pathways: ( i ) Apoplastic pathway (cell walls and intercellular spaces), (ii) Trans-membrane pathway (by crossing the plasma membranes) and (iii) Symplast pathway (through plasmodesmata). 27

23. Difference between active and passive absorption 28

24. External factors affecting absorption of water Available soil water : increased amount of water in the soil beyond a certain limit results in poor aeration of the soil which retards metabolic activities of root cells like respiration and hence, the rate of water absorption is also retarded. Concentration of soil solution: I ncreased concentration of soil solution (due to presence of more salts in the soil) results in higher OP. If OP of soil solution will become higher than the OP of cell sap in root cells, the water absorption particularly the osmotic absorption of water will be greatly suppressed. Soil air : Absorption of water is retarded in poorly aerated soils because in such soils deficiency of O 2 and consequently the accumulation of CO 2 will retard the metabolic activities of roots like respiration. Soil temperature : Increase in soil temperature up to about 30°C favours water absorption. At higher temperature water absorption is decreased 29

25. Internal factor

26. SYMBOL OF TRUST