Absorption and transport of water in plants

1. Absorption and transport of water in plants Dr. Harsh Manchanda Assistant Professor P. G. Govt. College for Girls Sector -11 Chandigarh

2. Water is highly essential for plants for various metabolic activities. Land plants get water supply from soil which serves as the source of water and minerals to them. The way in which water from soil enters roots, particularly to the root xylem, is called mechanism of water absorption. Both Active and Passive absorption have been proposed for mechanism of water absorption.

3. Structure of Root Involved in Absorption of water

4. Active Absorption It is absorption of water by roots with the help of metabolic energy generated by the root respiration. The force for water absorption originates from the cells of root due to root respiration. As the root cells actively take part in the process so it is called Active absorption. According to Renner, active absorption takes place in low transpiring and well-watered plants and 4% of total water absorption is carried out in this process. The active absorption is explained by two theories which are, Active osmotic water absorption and Active non-osmotic water absorption.

5. Active osmotic water absorption (Atkins and Priestley) According to this theory, the root cells behave as ideal osmotic system through which water moves up from soil solution to root xylem along an increasing gradient of D.P.D(suction pressure which is the real force for water absorption).If solute concentration is high and water potential is low in the root cells, water can enter from soil to root cells through endosmosis.

6. Nutrients are absorbed actively by the root cells due to utilisation of ATP.As a result, the concentration of ions in the xylem vessels is more in comparison to the soil water. A concentration gradient is established between the root and the soil water. Hence, the solute potential of xylem water is more in comparison to that of soil and correspondingly water potential is low than the soil water. This gradient of water potential causes endosmosis. The endosmosis of water continues till the water potential both in the root and soil becomes equal. It is the absorption of minerals that utilise metabolic energy, but not water absorption. Hence, absorption of water is indirectly an active process.

7. Active non-osmotic water absorption (Thimann and Kramer) According to the theory, sometimes water is absorbed against concentration gradient.This requires expenditure of metabolic energy released from respiration of root cells. There is no direct evidence, but some scientists suggest involvement of energy from respiration. In conclusion it is said that, the evidences supporting active absorption of water are themselves poor.

8. PASSIVE ABSORPTION This mechanism is carried out without utilisation of metabolic energy.Here only the root act as an organ of absorption or passage .Hence, sometimes it is called water absorption 'through roots', rather 'by' roots. It occurs in rapidly transpiring plants during daytime, because of opening of stomata and the atmospheric conditions. The force for absorption of water is created at the leaf end i.e., the transpiration pull.

9. Transpiration pull is responsible for dragging water at the leaf end, the pull or force is transmitted down to the root through water column in the xylem elements. The continuity of water column remains intact due to the cohesion between the molecules and it act as a rope. Root simply act as a passive organ of absorption. As transpiration proceeds, simultaneously water absorption also takes place to compensate the water loss from leaf end. Most volume of water entering plants is by means of passive absorption.

10. Movement of Water Through Cells - Two Routes, the Symplast and the Apoplast Symplastic Movement • Movement of water and solutes through the continuous connection of cytoplasm (though plasmodesmata) • No crossing of the plasma membrane (once it is in the symplast - however, if the solute was initially external to the cell, then it must have crossed one plasma membrane to enter the symplast) Apoplastic Movement • Movement of water and solutes through the cell walls and the intercellular spaces • No crossing of the plasma membrane • More rapid - less resistance to the flow of water

12. Ascent of Sap The upward movement of water through stem is called ascent of sap PATH OF ASCENT OF SAP The water absorbed by the root hairs is moves upwards via root tissues (cortex, endodermis, and pericycle) and finally enters the xylem. After entrance it starts its upward movement until it reaches mesophyll of leaves. Bulk of water enters mesophyll cells and finally evaporates through stomata. Only a small amount of water is utilized by plants for metabolism, rest is lost in transpiration.

13. Ascent of sap

14. Mechanism of Ascent of Sap • Various theories proposed to explain the mechanism of ascent of sap are: • Root pressure theory • Vital theories • Transpiration pull theory

15. Root Pressure Theory In, 1956 Stocking defined root pressure as “Pressure developing in tracheary elements of xylem as a result of metabolic activities of roots”. Experiment: If a well watered plant is cut near its base, the xylem sap is seen to flow out through cut end with a pressure. This pressure is actually the hydrostatic pressure developed in the root system called root pressure.

16. Vital Theories According to vital theories living cells are required for ascent of sap. These have been proposed by many workers namely Godlewski(relay pump theory), Bose (pulsation theory) and Molish. These theories were discarded because it was discovered that some poisons like picric acid and carbolic acid can also be translocated, thus fundamental basic of vital theories fails.

17. Transpiration pull theory (Cohesion – tension theory) proposed by Dixon and Jolly in 1894

18. This theory is based on following features. Cohesion and adhesion properties of water molecule to form unbroken continuous water column in xylem. 2. Transpiration pull or water tension is exerted on xylem column According to this theory, evaporation of water from the cells of leaf is responsible for raising water from the root. Evaporation results in a reduced water potential in the cells next to the xylem. Water therefore enters these cells from the xylem sap which has a higher water potential, passing through the moist cellulose cell walls of the xylem vessels at the ends of the vein. The xylem vessels are full of water and, as water leaves them, a tension is set up in the columns of water. This is transmitted down the stem all the way to root by cohesion of water (water molecules have high cohesion) and they also tend to stick to the vessel walls, a force called adhesion.

19. Transpiration • Transpiration is the loss of water from plant leaves. Water exits the leaf through stomata, which are tiny pore spaces in the leaf. The rateof transpiration depends on air temperature and solar radiation. • A typical foliage leaf consists of a large, flat leaf blade (lamina), a petiole (leaf stalk) and a leaf base with which the leaf is attached to the stem.Most monocots have simple leaves, while dicots can have simple or compound leaves.

20. Depending on the plant surface involved, transpiration is categorised into three types: Cuticular transpiration - Transpiration taking place through the cuticle -outermost layer of stems, leaves is called cuticular transpiration. It accounts for 0.1% water loss. Lenticular transpiration - Lenticels are small openings present in woody stems, twigs and fruits. Loss of water vapour through lenticels is called lenticular transpiration. It accounts for 1% water loss. Stomatal transpiration - Stomata are minute pores present in the epidermis of leaves, young stems, etc. The loss of water vapour through stomata is called stomatal transpiration. About 93% of water loss takes place through the stomata only.

21. Structure of the Stomata: A stoma or pore is formed by a pair of bean-shaped guard cells. The guard cells have the ability to open and close the stoma. The inner walls of the guard cells are thick and the outer walls thin. Guard cells differ from the translucent epidermal cells in that they contain chloroplasts. Stomata communicate with the air chambers in the spongy mesophyll. There are more stomata on the lower epidermis of the leaf than the upper epidermis. Stomata with guard cells open and closed

22. Functions of stomata • The stomata are responsible for the interchange of gases for respiration and photosynthesis. • The stomata allow for the loss of excess water in the form of water vapour, which also allows for cooling.

23. Mechanism of Stomatal Movement Stomatal movement depends upon the turgor pressure in the guard cells. When the guard cells are turgid, the stoma opens and when the guard cells lose water, stoma closes.

24. Stomatal Movement in Dicot Plants

25. Stomatal Movement in Monocot Plants

26. Active Potassium Theory (Levitt 1974) It was that opening of stomata occurs due to the influx of K+ ions into the guard cells. The source of K+ ions are the neighbouring subsidiary and epidermal cells, there by increasing the concentration from 50mM to 300mM in guard cells. The increase in K+ ion concentration increases the osmotic concentration of guard cells thus leading to stomatal opening. ATP helps in entry of K+ ions into the guard cells.

27. Role of ions in opening of stomata

28. Levitt (1974) observed that proton (H+) uptake by guard cells, chloroplasts takes place with the help of ATP. This leads to increase in value of pH in guard cells. Rise in pH converts starch into organic acid like malic acid. • The uptake of K+ ions is balanced by: • Uptake of chloride (Cl-) ions • Transport of H+ ions released from organic acid (malic acid) • By negative charges of organic acids when they lose H+ ions • Thus all these factors lead to the opening of stomata.

31. Factors affecting the rate of transpiration: External factors: 1.Temperature Higher the temperature more is the transpiration. 2.Light Light causes stomata to open and hence increase the water loss from plant.

32. 3. Availability of soil water When the soil gets dry, soil solution becomes more concentrated and the rate of absorption by cells decreases. This leads to reduction in transpiration and stomata close quickly to keep the water loss to minimum. 4.Atmospheric humidity High humidity means high water vapour pressure outside and it results in lower rate of transpiration and as the humidity decreases rate of transpiration increases.

33. 5.Wind The wind removes water vapour and thus increases the rate of transpiration. High winds lead to stomatal closure to stop the rapid water loss and hence bring a drop in rate of transpiration. Moderate winds may also reduce transpiration by lowering the temperature of leaf. 6.Atmospheric pressure Lower the atmospheric pressure, higher is the rate of transpiration.

34. Significance of transpiration: Transpiration is described as a necessary evil for the plant as it has both advantages and disadvantages Advantages : Absorption of water Transpiration influences the rate of absorption of water from the soil. Water movement By transpiration, water moves upwards and as it passes into the cell vacuoles, it makes the cells turgid. This gives form and shape to cells and plant as a whole. Mineral salt transport The water stream moving upwards carries dissolved minerals with it. Transpiration also helps in distributing these minerals throughout the plant. Cooling The evaporation of water during transpiration cools the leaves. Protection from heat injury Some plants like cacti, retain water by reducing transpiration. This saves the plants from high temperatures and strong sunlight.

35. Disadvantages 1. Transpiration creates water deficit in the plant which results in wilting of plants. Persistent wilting can lead to death. 2. Higher rate of transpiration reduces the rate of growth. The metabolic activities of the plant are affected. Stunted growth is observed in plants if transpiration is very high. 3. Many xerophytes have to develop their structural modifications in order to reduce transpiration. These modifications cause extra burden on the plants.

36. Anti-transpirants These are the substances which are used to reduce the rate of transpiration . These also used for crop plants, which fetch good returns. Example of antitranspirants are phenyl mercuric acid, silicon oils, fungicides, absicic acid, etc. The antitranspirants bring about partial closure of stomata by forming a thin film over the transpiring surface.